Plant Maintenance, Proof of Performance and Signal Leakage Rev[1]. A

Plant Maintenance, Proofof Performance and SignalLeakage

Training Manual

[email protected]/broadband

Copyright © 2003 by Motorola, Inc. All rights reserved. No part of this publication may be reproduced in any form or by any means or used to make any derivative work (such as translation, transformation, or adaptation) without written permission from Motorola, Inc. Motorola reserves the right to revise this publication and to make changes in content from time to time without obligation on the part of Motorola to provide notification of such revision or change. Motorola provides this documentation without warranty of any kind, either implied or expressed, including, but not limited to the implied warranties of merchantability and fitness for a particular purpose. Motorola may make improvements or changes to the products described in this manual at any time. MOTOROLA and the stylized M logo are registered trademarks of Motorola, Inc.

Plant Maintenance, Proofing & Signal Leakage

Contents

Introduction Audience ........................................................................................................................... i Prerequisites ..................................................................................................................... i Objectives ......................................................................................................................... i Materials ............................................................................................................................ ii Agenda .............................................................................................................................. ii

Day 1 ............................................................................................................................ ii Day 2 ............................................................................................................................ iii Day 3 ............................................................................................................................ iii Day 4 ............................................................................................................................ iii Day 5 ............................................................................................................................ iv

Section 1 Plant Construction Objectives ......................................................................................................................... 1-1 Construction Practices .................................................................................................... 1-2

Safety ........................................................................................................................... 1-2 OSHA .................................................................................................................... 1-2 NESC ..................................................................................................................... 1-3 NEC ....................................................................................................................... 1-3

Aerial Construction-Strand/Messenger ......................................................................... 1-4 Pulling Tension ............................................................................................................ 1-4 Bending Radius ............................................................................................................ 1-5 Expansion Loops ......................................................................................................... 1-7

Damage to Cable .............................................................................................................. 1-10 Radial Cracking Due To Fatigue .................................................................................. 1-10 Mechanical Deformation of Cable ................................................................................ 1-11

Aerial Cable Placement .................................................................................................... 1-12 Aerial Drop Cable Placement .......................................................................................... 1-12 Underground Cable Placement ....................................................................................... 1-13

Pulling Tension ............................................................................................................ 1-13 Bending Radius ............................................................................................................ 1-13 Expansion Loops ......................................................................................................... 1-14 Construction Methods .................................................................................................. 1-14

ii Contents


Bonding and Grounding Aerial Plant ............................................................................. 1-14 Grounding and Bonding Components ......................................................................... 1-15

Ground Rods ......................................................................................................... 1-15 Ground Rod Clamps- ............................................................................................ 1-15 Ground Rod Wire .................................................................................................. 1-15 Ground Wire Molding- ........................................................................................... 1-15 Bonding Clamps- .................................................................................................. 1-16

Cable Plant Bonding & Grounding Guidelines- ........................................................... 1-17 Aerial Grounding Configuration ................................................................................... 1-17

Bonding and Grounding Underground Plant ................................................................ 1-18 General Underground Construction Considerations ................................................... 1-18 Coaxial Cable Storage and Handling ............................................................................. 1-20 Coaxial Cable Preparation, Connectorization, and Splicing ....................................... 1-21

Connector Types ......................................................................................................... 1-21 Pin Connectors ..................................................................................................... 1-21 Feed-Thru Connector ............................................................................................ 1-22 Splice Connector ................................................................................................... 1-22 Cable Terminator .................................................................................................. 1-22 Housing-to-Housing Connector ............................................................................ 1-23 Right Angle Connector .......................................................................................... 1-23 180 Degree Angle Connector ............................................................................... 1-23 F Connector .......................................................................................................... 1-24 Splice Connector ................................................................................................... 1-24 F-Series Terminator .............................................................................................. 1-24

Connector Installation .................................................................................................. 1-24 Connectorizing Trunk and Distribution Cable .............................................................. 1-25

Pin Connector ....................................................................................................... 1-26 Cable Preparation Tool ......................................................................................... 1-26 Main Nut Installation ............................................................................................. 1-27 Feed-Thru Connector ............................................................................................ 1-27 Splice Connector ................................................................................................... 1-28

Connectorizing Drop Cable ......................................................................................... 1-29 Cable Preparation ................................................................................................. 1-29

Splicing Practices ............................................................................................................ 1-31 Underground Plant .......................................................................................................... 1-32

Upright Pedestal .......................................................................................................... 1-32 Low Profile Trunk Amplifier Configuration ................................................................... 1-33 Upright Pedestal Tap Configuration ............................................................................ 1-33 Below Grade, Single Tap Configuration ...................................................................... 1-34 Below Grade Trunk Amplifier Configuration ................................................................ 1-34

Contents iii


Section 2 Fiber Optic Cable Fiber Optic Cable Design ................................................................................................. 2-2

Loose Tube Construction ............................................................................................. 2-2 Central-Core Tube Construction .................................................................................. 2-3 Tight Buffer Construction ............................................................................................. 2-4 Slotted Core Construction ............................................................................................ 2-5 Fiber Optic Cable Specifications .................................................................................. 2-5

Overview ............................................................................................................................ 2-6 Types of Cable Construction .......................................................................................... 2-7 Joining Optical Fibers ...................................................................................................... 2-8

Fusion Splicing ............................................................................................................. 2-8 Mechanical Splicing ..................................................................................................... 2-9 Connectorization .......................................................................................................... 2-10

Flat Connector ....................................................................................................... 2-12 PC Connector ........................................................................................................ 2-12 APC Connector ..................................................................................................... 2-13

Return Loss ...................................................................................................................... 2-14 Connector Termination ................................................................................................ 2-16 Causes of Light Loss ................................................................................................... 2-16

Lateral Misalignment ............................................................................................. 2-16 Angular Offset ....................................................................................................... 2-16 Core Deformation .................................................................................................. 2-17 Contamination/End Separation ............................................................................. 2-17 Core Size/Shape Differences ................................................................................ 2-17

Fiber Optic System Components ................................................................................... 2-18 Pre-Installation Field Testing .......................................................................................... 2-19 Test Methods and Equipment ......................................................................................... 2-19

Optical Loss Measurement or Attenuation by Substitution Method ............................. 2-19 Optical Time Domain Reflectometer (ODTR) .............................................................. 2-20

Fault Location and Restoration ...................................................................................... 2-22 Emergency Restoration of Fiber Cable System .......................................................... 2-23 Emergency Cable Restoration – Alternatives .............................................................. 2-23

Cable Cut with Retrievable Slack .......................................................................... 2-23 Cable Cut with No Slack ........................................................................................ 2-23 Typical Emergency Restoration Tool Kit ............................................................... 2-24

Section 3 Fiber Optic Transmitter and Node Maintenance Advantages of Fiber Optics ............................................................................................. 3-2

Transmission ................................................................................................................ 3-2 Physical ........................................................................................................................ 3-2 Non-Conductive ........................................................................................................... 3-2 Economical .................................................................................................................. 3-2

Basic Light Theory ........................................................................................................... 3-3 Fundamental Principle ................................................................................................. 3-3

iv Contents


Wavelength of Infrared Light, Compared to Frequency .............................................. 3-5 Optical Fiber Primary Parameters ............................................................................... 3-8

Attenuation ............................................................................................................ 3-8 Bandwidth ............................................................................................................. 3-8

Fiber Manufacturing ........................................................................................................ 3-9 Fiber/Coax Comparisons ................................................................................................ 3-12

Coax ............................................................................................................................ 3-12 Fiber ............................................................................................................................ 3-14 Fiber vs. Coax for 10 dB Attenuation .......................................................................... 3-15 Fiber vs. Coax System Comparison ............................................................................ 3-16

AM Fiber Path Engineering ............................................................................................. 3-17 Optical Couplers .......................................................................................................... 3-17 Path Loss Calculations ................................................................................................ 3-20

Calculations .......................................................................................................... 3-20 Example ................................................................................................................ 3-20

Typical Path Splitting Using an Optical Coupler .......................................................... 3-21 Path 1 .................................................................................................................... 3-21 Path 2 .................................................................................................................... 3-22 Transmitter Output Power ..................................................................................... 3-22

Laser Transmitter Technologies .................................................................................... 3-23 Laser Subassembly ..................................................................................................... 3-23 Semiconductor Lasers ................................................................................................. 3-24 Modulation Techniques ............................................................................................... 3-26

Direct ..................................................................................................................... 3-26 External ................................................................................................................. 3-29

AM Optical Transmitters .............................................................................................. 3-31 Optical Power Test Point ...................................................................................... 3-32 Converting Between dBm and mW ....................................................................... 3-33 RF Test Point Level vs. Channel Loading ............................................................ 3-34

Receiver Technologies .................................................................................................... 3-35 Diode Detectors ........................................................................................................... 3-35

Pin Diodes ............................................................................................................. 3-35 AM Optical Receivers .................................................................................................. 3-36

AM Fiber Network Performance ..................................................................................... 3-40 Major Contribution to Noise and Distortion in AM System .......................................... 3-40 Optimizing System Noise and Distortion Performance ............................................... 3-41 Headend Contribution in AM Fiber System Performance ........................................... 3-41 Combining AM Fiber and RF Plant Performance ........................................................ 3-42 Cascading Fiber Links ................................................................................................. 3-42 CNR/CTB/CSO Addition .............................................................................................. 3-43

Addition of Two CTB Values - Example ................................................................ 3-43 Performance Testing of CNR, CTB, and CSO ............................................................ 3-44 Impact of Noise on AM Fiber Optic Links .................................................................... 3-45 C/N Limits Due to Laser RIN ....................................................................................... 3-45

Contents v


C/N Limits due to Shot Noise ....................................................................................... 3-46 Detector Responsivity ........................................................................................... 3-46 Shot Noise ............................................................................................................. 3-46

Detector Amplifier Noise .............................................................................................. 3-47 Limits to C/N ................................................................................................................ 3-48 Link C/N Performance Summary ................................................................................. 3-48

Exercise ............................................................................................................................. 3-49

Section 4 Coax Plant Maintenance RF Amplifier Configuration ............................................................................................. 4-2

Mechanical Issues ....................................................................................................... 4-2 Housing Integrity ................................................................................................... 4-2 Module Integrity ..................................................................................................... 4-4 Module Cover Integrity .......................................................................................... 4-4

Basic Amplifier Components ........................................................................................ 4-6 Diplex Filters .......................................................................................................... 4-7 Input and Output Test Points ................................................................................ 4-9 Equalizer and Pad ................................................................................................. 4-10 Pre-amp Stage and Interstage Pad ....................................................................... 4-12 Bode Equalizer ...................................................................................................... 4-13 Interstage Equalizer .............................................................................................. 4-18 Post Amplifier ........................................................................................................ 4-20 Level Correction .................................................................................................... 4-21 Return Amplifier Kit ............................................................................................... 4-22 Power .................................................................................................................... 4-23 Mini-Bridger ........................................................................................................... 4-24

Unity Gain Concept ...................................................................................................... 4-28 RF Amplifier Bench Testing ............................................................................................ 4-37

Use of the Bench Sweep ............................................................................................. 4-37 Types of Bench Sweep Equipment .............................................................................. 4-38

Sweep and Balance/Maintenance ................................................................................... 4-39 Powering up the Network ............................................................................................. 4-39

Standby Power Supply .......................................................................................... 4-39 Voltage Drop (Coaxial Cable) ............................................................................... 4-40

Meter Balance .............................................................................................................. 4-42 System Sweep Equipment ........................................................................................... 4-42

BLE-75 SH/JH Set Up Procedure ......................................................................... 4-42 Forward and Low System Sweep ................................................................................ 4-45

Today’s Sweep Methods ....................................................................................... 4-45 Normalized ............................................................................................................ 4-47 High End ................................................................................................................ 4-48 Low End ................................................................................................................. 4-49 Ringing .................................................................................................................. 4-50

vi Contents


Section 5 System/FCC Proof of Performance Network Components ...................................................................................................... 5-2 FCC Customer Tap Proof of Performance .................................................................... 5-4

Total Test Points .......................................................................................................... 5-4 Channels to Test ......................................................................................................... 5-4 Aural Carrier Frequency .............................................................................................. 5-5 Visual Carrier Signal Level .......................................................................................... 5-6 Aural Carrier Signal Level ........................................................................................... 5-8 In-Band Channel Response ........................................................................................ 5-9 Carrier-to-Noise ........................................................................................................... 5-10 Coherent Disturbances ................................................................................................ 5-11 Low Frequency Disturbances ...................................................................................... 5-11 Terminal Isolation ........................................................................................................ 5-12

FCC Customer Tap Proof of Performance .................................................................... 5-13 Testing Methods .............................................................................................................. 5-15

Headend System RF Measurements .......................................................................... 5-15 Video Carrier Level and Video/Audio (V/A) Separation ........................................ 5-15 Video Carrier-to-Noise (C/N) Ratio ....................................................................... 5-17

VISUAL, AURAL CARRIER LEVEL: 24 HOUR VARIATION ...................................... 5-17 Definition ............................................................................................................... 5-17 Test Procedure ..................................................................................................... 5-18 Test Methodology ................................................................................................. 5-19 Notes, Hints and Precautions ............................................................................... 5-20

AURAL CARRIER CENTER FREQUENCY ................................................................ 5-21 Definition ............................................................................................................... 5-21 FCC §76.605 (a) (2) .............................................................................................. 5-21 Procedure ............................................................................................................. 5-21

VIDEO SWEEP OF MODULATORS OR PROCESSORS .......................................... 5-22 VISUAL CARRIER-TO-NOISE RATIO ........................................................................ 5-24

Definition ............................................................................................................... 5-24 Discussion ............................................................................................................. 5-27

Coherent vs. Non-Coherent ........................................................................................ 5-28 Second Order Distortions: Carrier-to-Second Order Beat Ratio ................................. 5-30

Discussion ............................................................................................................. 5-31 Composite Second Order Distortion: CW Carriers: Standard Frequency Plan

(not HRC) .............................................................................................................. 5-32 Third Order Distortion: CW Carriers ............................................................................ 5-35

Procedure ............................................................................................................. 5-35 Third Order Distortion: Modulated Carriers ................................................................. 5-38

Performance Objective ......................................................................................... 5-39 Discussion ............................................................................................................. 5-40 Discussion ............................................................................................................. 5-43

Modulation Distortion at Power Frequencies .............................................................. 5-44

Contents vii


Section 6 Spectrum Analyzer Basics Overview ............................................................................................................................ 6-2 Analyzer Functions .......................................................................................................... 6-3

Input Attenuator ........................................................................................................... 6-4 Preamp ........................................................................................................................ 6-4 Mixer ............................................................................................................................ 6-5 Resolution bandwidth (RBW) or IF Filter ..................................................................... 6-6 Data Carriers ................................................................................................................ 6-7 Log Amplifier ................................................................................................................ 6-7 Video Detector ............................................................................................................. 6-8 Video Bandwidth (VBW) Filter ..................................................................................... 6-10 Video Averaging ........................................................................................................... 6-10 Sweep Generator ......................................................................................................... 6-10 Trace Position .............................................................................................................. 6-11

Marker Noise Function ..................................................................................................... 6-12 Noise-to-Noise or Beat-to-Noise Correction .................................................................. 6-13 C/N Measurement ............................................................................................................. 6-15 CSO and CTB Measurement ............................................................................................ 6-16 Other Considerations ....................................................................................................... 6-17

Section 7 Signal Leakage What are Ingress and Egress? ........................................................................................ 7-2

Ingress ......................................................................................................................... 7-2 Egress .......................................................................................................................... 7-2 Ingress/Egress Effects ................................................................................................. 7-2

Ingress ................................................................................................................... 7-2 Egress ................................................................................................................... 7-2

FCC Requirements ........................................................................................................... 7-3 Rules ............................................................................................................................ 7-3 Signal Leakage Limits .................................................................................................. 7-4 Mandated Engineering Procedures ............................................................................. 7-4 Calculating Cumulative Leakage Index ....................................................................... 7-6 Frequency Offsets ........................................................................................................ 7-7 Converting Between dBmV and µV/m ......................................................................... 7-7

Good Engineering Practices ........................................................................................... 7-9

Appendix A Optical Transmitters AM750ATH ............................................................................................................ A-1 AM-750ATH Front Panel ....................................................................................... 1-1 AM-750ATH Operating Controls ........................................................................... 1-2 AM-750ATH Indicators .......................................................................................... 1-2 AM-750ATH Test Points ........................................................................................ 1-3 AM-750ATH Rear Panel Inputs ............................................................................. 1-3

viii Contents


AM-750ATH Power Up Procedure .............................................................................. 1-4 Laser RF Drive Level ............................................................................................ 1-4 Cooler Current ...................................................................................................... 1-4 Laser Bias Current ................................................................................................ 1-4 Laser Optical Power .............................................................................................. 1-4 Laser Temperature ............................................................................................... 1-5 Laser Key Switch .................................................................................................. 1-5 Modulation Bar Graph ........................................................................................... 1-5 Changing the RF Drive Level ................................................................................ 1-5 Maintenance ......................................................................................................... 1-5

Transmitter Performance Optimization ........................................................................ 1-6 Optimizing Modulation - AGC Mode On ............................................................... 1-6 Operation with Reduced Channel Loading - AGC Mode On ................................ 1-6 Planned Increase in Channel Loading - Set Mode On ......................................... 1-6 User Selected Operating Point ............................................................................. 1-6

OmniStar Laser Transmitter ........................................................................................ 1-6 AM OMNI LM* Laser Module ................................................................................ 1-7 Front Panel Indicators and Controls ..................................................................... 1-8 Module Operation ................................................................................................. 1-9 The RF Signal Path ............................................................................................... 1-10 Laser Circuit Board ............................................................................................... 1-11 Digital Board ......................................................................................................... 1-13

Status Monitoring ......................................................................................................... 1-13 Connect the Fiber ........................................................................................................ 1-14 Install the Module ........................................................................................................ 1-15 Check the Input Signal ................................................................................................ 1-16 Operation with 77 NTSC Channels ............................................................................. 1-17 Electrical Checks ......................................................................................................... 1-17 Operation ..................................................................................................................... 1-19 Operation with Reduced Channel Loading ................................................................. 1-19 Planned Increase in Channel Loading ........................................................................ 1-20 User Selected Operating Point .................................................................................... 1-20 Modulation Indicator .................................................................................................... 1-21 Changing the Modulation ............................................................................................ 1-21 Manual Operation ........................................................................................................ 1-22 Video/CW Mode of Operation ..................................................................................... 1-22 Laser Output Power ..................................................................................................... 1-22 Specifications .............................................................................................................. 1-23

Glossary Acronym List .................................................................................................................... 1 Definitions of Terms ........................................................................................................ 10


Introduction

Please take a few moments to fill out the Registration Form.

Audience This course was developed for outside plant craftsmen, technicians, and managers.

Prerequisites Before attending this course, participants should have completed the Design Basics course.

Objectives This five-day course is comprised of hands-on exercises pertaining to plant maintenance, operation, and performance testing. It provides in-depth coverage of operational theory, mechanical components, amplifier operation, test equipment operation, and testing methods. Topics include troubleshooting, signal leakage testing, splicing, and plant performance certification. After completing this course, you will be able to:

Understand the fundamentals of aerial and underground construction

Understand the various types of fiber optic cable design

Understand the advantages of fiber optic cable

Understand sweep and balance procedures

Understand network components and proof procedures

Understand spectrum analyzer functions

ii Introduction


Materials During the course, you will be given the following materials:

Participant Registration Form

Plant Maintenance and Performance Testing Training Manual

Various handouts

Training Course Evaluation Form

Course Completion Certificate

Agenda The duration for this course is five days. Note that the training materials are being continuously updated and improved, therefore this agenda is subject to change.

Day 1

Topic

Registration/Introduction/Agenda Review Coaxial Cable

Aerial Construction Aerial Cable Placement Underground Cable Placement Bonding, Grounding, & Drop Clearances Coaxial Cable Storage Coaxial Cable Prep., Connectorization, & Splicing

Fiber Optic Cable Fiber Optic Cable Construction Practices Fiber Optic Cable Prep., Connectorization and Splicing

Coaxial Cable Prep., Connectorization and Splicing Lab Fiber Optic Cable Prep., Connectorization and Splicing Lab Day 1 Review/Questions and Answers

Introduction iii


Day 2

Topic

Review of Day 1 Fiber Optic Transmitter and Node Maintenance Fiber Optic Transmitter and Node Maintenance Lab Day 2 Review/Questions and Answers

Day 3

Topic

Review of Day 2 Coax Plant Maintenance

RF Amplifier Configuration

RF Amplifier Bench Testing

Sweep and Balance/Maintenance

RF Amplifier Lab Day 3 Review/Questions and Answers

Day 4

Topic

Review of Day 3

Fiber Optic Node Proof of Performance

FCC Customer Tap Proof of Performance Proof of Performance Lab

Day 4 Review/Questions and Answers

iv Introduction


Day 5

Topic

Review of Day 4

Signal Leakage

Antenna Distance Correction by Ron Hranac

NCTA Paper on Headend Offsets by William T. Homiller

Signal Leakage Rules

FCC Form 320 Signal Leakage Measurements Lab

Complete Course Review/Evaluations


Section 1 Plant Construction

Objectives Upon completion of this section you will be able to:

Understand the basic safety requirements of the various national codes that apply to cable construction

Understand the fundamentals of aerial and underground construction practices

More construction practices detail can be found in the accompanying material provided by CommScope.

1-2 Plant Construction


Construction Practices Construction of a broadband cable system requires a substantial amount of resources such as manpower, tools and associated equipment for the specific type of construction.

Whatever type of construction is to be performed, both the employees and the general public will be exposed to safety hazards. Only qualified and trained employees should perform any work function that could result in personal injury and/or property damage.

Safety There are three national codes and standards that govern the construction of CATV and broadband systems:

• OSHA- Occupational Safety and Hazards Administration

• NESC- National Electrical Safety Code

• NEC- National Electrical Code

OSHA

The OSHA standards that are applicable to telecommunications systems can be found in section 1910.268.

These standards are federal regulations that were founded in 1970 to ensure workplace safety and enable employers and employees to recognize, understand and control workplace hazards. The outline of the OSHA standards for telecommunications is as follows:

• Application

• General

• Training

• Employee Protection in Public Work Areas

• Tools and Personal Protective Equipment

• Rubber Insulated Equipment

• Personal Climbing Equipment

• Ladders

• Other Tools and Personal Protection Equipment

• Vehicle-mounted Material Handling Devices and Other Mechanical Equipment

• Materials and Handling Storage

• Cable Fault Locating and Testing

• Grounding for Employee Protection-Pole Lines

• Overhead Lines

Plant Construction 1-3


• Underground Lines

• Microwave Transmission

• Tree Trimming Electrical Hazards

• Buried Facilities

• Definitions

NESC

The National Electrical Safety Code (NESC) typically identifies the construction techniques and materials necessary in outside plant construction of electrical supply or communication cable systems.

The NESC is an American National Standard written by a group of professionals concerned with the standard’s scope and provisions. The NESC has been adopted by the American National Standards Institute (ANSI).

The purpose of the NESC is identified in section one of the document as follows:

“The purpose of these rules is the practical safe-guarding of persons during the installation, operation or maintenance of electric supply and communication lines and associated hardware.”

“These rules contain the basic provisions that are considered necessary for the safety of employees and the public under the specified conditions. This code is not intended as a design specification or as an instruction manual.”

The scope of the NESC (1993) is identified in section one of the NESC as follows:

“These rules cover supply and communication lines equipment and associated work practices employed by a public or private electric supply, communications, railway or similar systems under the control of qualified persons, such as those associated with an industrial complex or utility interactive system.”

“NESC rules do not cover installations in mines, ships, railway, rolling equipment, aircraft or automotive equipment, or utilization wiring except as covered in Parts 1 and 3. For building utilization wiring requirements, see National Electrical Code, ANSI/NFPA 70-1990 [47].”

NEC

The National Electric Code (NEC) typically identifies the construction techniques and materials necessary in building wiring requirements, i.e., inside plant construction of fiber optic and coaxial cable systems.

The (NEC) has been developed by the National Fire Protection Association’s (NFPA) National Electric Code Committee. The committee is comprised of professionals in the electrical industry. The NEC addresses safety for fire and electrocution. The NEC has been adopted by the American National Standards Institute (ANSI).

“The purpose of this Code (NEC) is the practical safeguarding of persons and property from hazards arising from the end of electricity.”



The scope of the NEC (1993) is identified in Article 90-2 of the NEC, and covers the following:

“Installations of electrical conductors within or on public or private buildings or other structures, including mobile homes, recreational buildings and floating buildings; and other premises such as yards, carnival, parking and other lots and industrial substations.”

Installations and conductors that connect to the supply of electricity.”

Installations of other outside conductors and equipment on the premises.’

Installations of optical fiber cables.”

Other contractual agreements such as utility pole leases, etc may also govern the build of your system. These safety codes may be more stringent than those listed by the NESC or others. The more restrictive code or regulation must always be practiced.

Aerial Construction-Strand/Messenger The messenger used for coaxial cable support is commonly referred to as the strand. This section will review the strand-placing techniques typically used in the industry.

It is assumed that the make-ready function has been completed and proper clearances have been achieved. Make-ready is the preconstruction process performed to ensure proper clearance from other utilities at the aerial pole locations. This process also includes the verification of easement rights; utility location in underground installations; and the ability of all support structures to withstand the additional loads imposed by the new construction.

Two methods are used for the placement of strand: the back-pull method and the drive off method. Both of these methods are used when the intention is to first place the strand. The coaxial cable is to be placed at a later time, as a separate function.

Pulling Tension Aerial cable is pulled into place by applying force to the end of the cable. Excessive forces will cause the cable to permanently elongate. The maximum rated pulling tension for cable is supplied by the manufacturer and should not be exceeded. Good construction techniques and proper tension monitoring equipment should be used.

During cable placement, attention should be given to number and placement of cable blocks. The amount of sag between the blocks and the amount of bending at the blocks does affect the pulling tension.

Breakaway swivels should be placed on each individual cable to ensure that the maximum allowable pulling tension for that particular cable will not be exceeded. The swivel is placed between the cable puller and the pulling grip.

Dynamometers are used to measure the dynamic tension in the cable. These devices allow continuous review of the tension and accordingly, a realization can be made of any sudden increase in pulling tension. Sudden increases in pulling tension can be caused by numerous factors such as cable falling from a block or a cable binding against pole-line hardware.



Cable Type

Cable Size

Maximum Pulling Tension

QR .540 220 lbs.

QR .715 340 lbs.

QR .860 450 lbs.

QR 1.125 750 lbs.

P-3 .500 300 lbs.

P-3 .625 475 lbs.

P-3 .750 675 lbs.

P-3 .875 875 lbs.

P-3 1.000 1300 lbs.

Cable Type Maximum Pulling Tension59 Series 125 lbs.6 Series 150 lbs.7 Series 220 lbs.11 Series 330 lbs.

Dual Cables 270 lbs.

Bending Radius Mechanical stress causing damage to the cable during cable construction is of substantial concern. The cable may be greatly damaged by exceeding the maximum allowable pulling tension or exceeding the minimum bending radius.

Deformations to the geometry of the cable can affect the electrical characteristics and decrease the life expectancy of the cable. Coaxial cable is extremely affected by the geometric changes due to the concentricity of the conductors.

Cables are often routed around corners during cable placement. As cables are routed around the corners, pulling tension must be increased to apply adequate force to the cable to bend the cable around the corner. Tension is directly related to the flexibility of the cable. Flexibility is a function of cable size and design.

The static bending radius of coaxial cable is the minimum bending radius that cable can be formed without electrical or mechanical degradation of the cable. The minimum bending radius provided by the manufacturer is generally the static bending radius. In other words, the bending radius figure is in reference to cable that is not loaded.

Loaded means that the cable is under the maximum allowable pulling tension and is being bent simultaneously. Unloaded means that the cable is under no tension. Unloaded conditions occur at splice points, hand-holes, vaults or other termination points. The unloaded bending radius is the radius allowed for storage purposes.



The radius in the cable as the cable is being pulled is the dynamic bending radius. The dynamic bending radii of cables during the construction process are controlled by construction techniques and construction equipment. In aerial construction, corner blocks and set-up chutes have large radius bends and low friction surfaces that minimally contribute to the increase in pulling tension that is required to pull the cable through this equipment.

Basically, the bending radius dictates the minimum bend that can be applied to a piece of coaxial cable without causing mechanical deformations. The minimum bending radius is measured in inches and differs for various cable sizes. Following is an illustration of the definition of radius.

. .

R

Figure 1-1 Bending Radius

The following table lists the minimum bending radii for CommScope’s Parameter 3 and Quantum Reach products.

Type Size Jacketed Non-Jacket ArmoredQR .540 4.0 in. N/A 6.5 in.QR .715 5.0 in. N/A 7.5 in.QR .860 7.0 in. N/A 9.5 in.QR 1.125 8.0 in. N/A 10.5 in.

Type Size Standard Jacketed

Standard Non-Jacket

Standard Armored

P-3 .500 6.0 in. 6.5 in. 8.5 in.P-3 .625 7.0 in. 7.5 in. 9.5 in.P-3 .750 8.0 in. 9.0 in. 10.5 in.P-3 .875 9.0 in. 10.0 in. 11.5 in.P-3 1.000 10.0 in. 11.0 in. 12.5 in.

Type Size Bonded Jacketed

Bonded Non-Jacket

Bonded Armored

P-3 .500 3.5 in. 4.0 in. 6.0 in.P-3 .625 4.5 in. 5.0 in. 7.0 in.P-3 .750 6.0 in. 7.0 in. 8.5 in.P-3 .875 7.0 in. 8.0 in. 9.5 in.P-3 1.000 8.0 in. 9.0 in. 10.5 in.



Expansion Loops When installing coaxial cable in aerial applications, the difference in the expansion and contraction coefficients of the strand and cable must be considered.

Expansion loops are placed in the cable spans to provide excess cable; allowing for strand creep, differential thermal expansion, contraction of both the aluminum cable and the steel strand. In order to create a typical flat-bottom expansion loop, approximately 2 to 3 inches of excess cable is required.

Below is an illustration of the typical flat bottom expansion loop.

Messenger/StrandWire

6” Deep

12-15”Flat Bottom

Cable Spacers

Figure 1-2 Typical Expansion Loop

All metals have what is known as the coefficient of thermal expansion. This means that as the temperature rises the material expands. So the hotter it gets, the longer the cable becomes. The same holds true for the steel strand. The coefficient of thermal expansion for cable is on the order of 1.56 x 10-5. The coefficient for ¼” steel strand is 0.56 x 10-5. As shown, the coefficient of thermal expansion for steel and aluminum differ.

Example:

Strand and cable are installed at 70º F. At a length of 500 ft., add a temperature change of 30º F.

Strand:

(0.56 x 10-5 x 12) x 500 x 30 = 1.008 in.

Cable:

(1.56 x 10-5 x 12) x 500 x 30 = 2.808 in.



As the example shows, the cable will move 2.8”, in comparison to the strand which will move 1.08”. If the temperature change occurs in the summer months, the +30º change will cause the cable to increase 2.8” for the 500 ft. span in the example. If the temperature changes during the winter season, the cable would decrease 2.8” in the overall length. In either case, the expansion loops allow for the greater movement of the cable.

A properly designed and formed expansion loop is recommended at every pole location. In spans where there are multiple trunk and distribution cables, a loop is formed at each pole. Where extreme temperature swings are found or the span length exceeds 200 ft., it is recommended that two expansion loops be placed: one at each pole location of the span. See the following figure.

Figure 1-3 Two Loops at a Pole

Expansion loops for cable, up to and including 0.750” o.d., will be a 12-inch flat bottom design that is 6 inches deep and has an overall length of 42 inches.

When the cable size exceeds 0.750” o.d., the expansion loop design will be a 15-inch flat bottom that is 6 inches deep and has an overall length of 56 inches.



It is common to have one expansion loop placed on either the input or output side of the passive equipment at each pole line span for both the trunk and distribution cables.

At amplifier locations or other active equipment locations, two expansion loops are recommended regardless of the span length. The loops are placed on the input and output sides of the equipment.

6”

4” 14”

15”

Band & Spacer

Lashing WireClamp

50” min.

10-12”

Figure 1-4 Expansion Loop Dimensions

Figure 1-5 Expansion Loop Dimensions at Equipment Location



Damage to Cable

Radial Cracking Due To Fatigue

F

F

F’

F’

Expansion

Contraction

Radial Crack

Figure 1-6 Cable Cracking

Causes of radial cracking include

• Improperly formed expansion loops

• Poor construction techniques

• Expansion loop fatigue

Band and SpacerCable

Strand

Area of potential damage(Radial Crack)

Figure 1-7 Poorly Formed Expansion Loop



Mechanical Deformation of Cable Mechanical deformation includes:

• Impact

Figure 1-8 Impact Damage

• Stretching caused by exceeding maximum rated load (tension).

Figure 1-9 Stretching

• Flattening/scarring caused by:

• Trailer Brake Tension

• Roller/Chute Placement

• Unseen Points of Contact with Cable

• Exceeding Maximum Rated Load (Tension)

• Damage to reels during the unloading process (Dropping).

Figure 1-10 Flattening

Since the characteristic impedance of coaxial cable depends on conductor diameters,

,dDlog

E138Z

×=

each of these produces a change in the impedance of the cable at the point of damage.



Aerial Cable Placement Like the practices described earlier for steel strand installation, there are generally two methods for the placement of aerial coaxial cable: the back-pull method and the drive-off method. Many circumstances regarding the construction project will govern the method utilized for cable placement. More often than not with new strand, the back-pull method is more common.

Aerial Drop Cable Placement The following figure and table define clearances for aerial drop cable.

Figure 1-11 Aerial Drop Cable Clearances

Location Clearance

State highway 20 feet* Public street/alley (subject to truck traffic) 18 feet* Driveways (not accessible to trucks) 12 feet Signs, chimneys, and radio and TV antennas 3 feet Track rail of railroad 24 feet Water (no sailboats) 14 feet Earth 8 feet Flat roof buildings (<3% slope) 8 feet Lightning rod conductors 6 feet



Location Clearance

Power service wire Pole 40” (below) House 12” (below)

Telephone service Pole 12” (above) House 4” (above)

Underground Cable Placement Underground systems are generally made up of two categories. The categories are comprised of two cable placement methods. The first method is the direct buried cable system, and the second is the conduit (duct) cable system, where cables are placed into a duct structure.

The same concerns exist for underground placement as outlined for aerial plant.

Pulling Tension Underground cable is pulled into place by applying force to the end of the cable. Once again, excessive force will cause the cable to permanently elongate. The maximum pulling tension, recommended by the manufacturer, should never be exceeded.

Breakaway swivels should be placed on each individual cable to ensure that the maximum allowable pulling for that particular cable will not be exceeded. The swivel is placed between the pulling line and the pulling grip. A breakaway swivel is required for each cable when pulling multiple cables.

Dynamometers are used to measure the dynamic tension in the cable. These devices allow continuous review of the tension and, accordingly, any sudden increase in pulling tension can be seen.

Bending Radius Both the static and dynamic bending radii are of concern during the placement of underground cable.

An example of the static bending radius in underground construction is, after the cable is pulled, it must be stored until the splicing or termination process occurs. Splicing is a separate construction function and, most often, occurs at a later date. The cable must be mechanically reshaped to fit into the underground vault or pedestal. Care must be taken to ensure that the minimum bending radius is not exceeded.



The dynamic bending radius of cable is of concern during the play-off process during substructure cable placement. The cable is under the greatest tension at this time. Close attention must be paid to the arc in the coaxial cable from the reel to the cable shoe or plowshare.

Expansion Loops Expansion loops are not required for underground cable. Excess cable is commonly placed in the underground vault or pedestal locations during splicing and used mostly for future maintenance.

Construction Methods Once the type of underground system has been selected, the construction method is then determined. There are several factors governing the type of construction method utilized for underground excavation. These factors range from labor costs to soil conditions and the required depth.

Bonding and Grounding Aerial Plant The primary purpose for bonding and grounding broadband and CATV systems is to protect the personnel and public working on or around the system. A secondary purpose is to establish a reference ground potential (common reference voltage) for the electronics in the system.

The National Electric Code (NEC) sets the rules and regulations governing the grounding of both the inside and outside wiring of commercial and residential building and property.

The National Electric Safety Code (NESC) sets the rules and regulations concerning both aerial and underground plant in public rights-of-way or other areas exposed to the general public.

Grounding is a process to create a network that consists of many electrical grounds throughout the system. The grounds are established by various methods known to offer a low impedance path for current flow.

Bonding is the method by which all conductive cables and messengers are continuously connected. Current flow is enabled from the conductive cables and messengers to ground, by the continuous bonding of the plant to the grounding network.



Grounding and Bonding Components The following are common components used for the grounding and bonding of a system.

Ground Rods

Ground rods are electrical grounds and are also referred to as grounding electrodes. Ground rods are generally made of copper-clad steel. They should always be made from a conductive metal with a non-corrosive outer surface.

Ground rods must be at least 8 feet in length and 5/8 of an inch in diameter.

These rods are not to be installed within 10 feet of high-pressure pipelines that transport flammable materials.

Connections to ground rods should always be made with a corrosion resistant clamp.

Ground Rod Clamps-

A ground rod clamp is used to connect the ground wire to the ground rod. This drawing illustrates a typical bronze ground rod clamp.

Figure 1-12 Ground Rod Clamp

Ground Rod Wire

Ground wire provides a means for interconnection of the messenger and the cables to the ground rod.

Ground wire should be made of copper and of a size not less than AWG No. 6.

Insulation, or other non-conducting material, must be removed from the ground wire prior to any connection.

Ground wire should be installed in straight lines whenever possible. Any necessary bends should be made gradually.

Ground Wire Molding-

A nonmetallic ground wire molding is placed at the first 8 ft. of the grounding wire that is above ground level on the pole. The molding is generally held in place with galvanized staples.



Bonding Clamps-

Bonding clamps are made in various types and many styles. Corrosion at the bonding clamp is controlled by using a clamp that is made from a material that will not corrode when in contact with another, dissimilar metal.

Three types of bonds are generally found in a system. The types and hardware specific to each bond are as follows.

• Copper-to-Copper Bond: This bond is generally made with a bronze split bolt clamp. Following is a drawing of a split bolt clamp.

Figure 1-13 Copper-to-Copper Bond

• Copper to Galvanized Steel or Aluminum Bond: This bond is usually made with a bimetal clamp that has bonding surface compatible to the two different materials being bonded. Each side of the clamp is specific and must be matched to the correct material. Following is a drawing of a b-metal bonding clamp.

Figure 1-14 Copper to Galvanized Steel or Aluminum Bond

• Galvanized Steel to Galvanized Steel Bond: This clamp is used for standard bonding and may be easily confused with the bimetal bonding clamp. Following is a drawing of a bonding clamp.

Figure 1-15 Galvanized Steel to Galvanized Steel Bond



Cable Plant Bonding & Grounding Guidelines- The following are the general guidelines that are followed for grounding and bonding of a CATV cable plant.

1 Grounding of aerial plant must be done in accordance with the pole lease agreements, or other local regulations.

2 Cable bearing strand is grounded or bonded at least eight times per mile of plant.

3 The grounding standard is that the 1st, 10th and last poles are grounded in any given pole-line.

4 Grounding must be accomplished at the pole before and the pole after all amplifier and active locations.

5 Ground rods used for aerial applications are normally placed within 12” of the pole and the rod is driven totally into the ground and at least 4” to 6” below the surface.

6 Two or more strands running in the same horizontal or vertical plane must be bonded together.

7 Underground locations are normally grounded at all active and end-of-line locations.

Aerial Grounding Configuration Following is an illustration of an aerial grounding configuration.

Figure 1-16 Aerial Grounding Configuration



Bonding and Grounding Underground Plant The following are the general guidelines that are followed for grounding and bonding of CATV cable plant.

1 Grounding of underground plant must be done in accordance with all utility and local regulations.

2 All power supplies should be bonded to the power company ground.

3 All equipment placed in an underground metal enclosure should be bonded to that enclosure.

4 All trunk and transport cable should be grounded at all amplifier and end-of-line locations.

5 All distribution cable should be grounded at each line extender and end-of-line locations. (At least once for every 1000’ of trench line).

6 Bond fiber cable at a minimum of every 2400’

7 All CATV metal enclosures should be bonded to all other utility enclosures within 8’ of each other.

General Underground Construction Considerations As with aerial plant construction, OSHA regulations and all other local guidelines for underground construction must be followed.

Broadband and CATV cable construction is typically done with the right-of-way dedicated for the routing of municipal and utility pipes, wires, cables and conduits providing other services.

Excavation within any right-of-way will often reveal a multitude of underground infrastructure. The integrity of existing subsurface plant must be ensured during any excavation activity.

Most states have laws that require the responsible construction party to notify all sub-surface plant owners prior to the commencement of any excavation. This will allow the plant owners time to go to the construction site and locate their existing facilities, reducing the chance of damage.

Electronic transmitters and receivers, referred to as cable locators, are available to assist in locating the existing plant. Once the plant is located, it is marked using color-coded flags and/or paint, indicating the existing routing.



The color code for marking underground utility lines is as follows:

Red Electric Yellow Gas/oil Orange Communication/CATV Blue Water Green Sewer Pink Temporary survey markings White Proposed excavation



Coaxial Cable Storage and Handling Cable may be stored either indoors or outdoors and the reels may be stacked or stored upright on the rolling edge, flange to flange.

When cable is stored outside, the ground should be somewhat level and support timbers used to protect the reel wrapping and flanges from the elements of harsh weather.

Each end of the cable must also be protected by using cable caps, hose clamps or something comparable, to prevent moisture ingress and/or the migration of flooding compounds.

When cable is stored outdoors, a covering should be used to protect the packaging materials from deterioration. Plastic or canvas offers minimal protection while an open-air roof provides optimum low cost protection.

When coaxial cable is moved, shipped or stored in the stacked position, the maximum stacking height (outlined in the following table) must not be exceeded. To allow the use of a pallet jack or forklift, place boards between each layer and a pallet on the ground under the bottom reel.

ProductMaximum Storage

Height of StackMaximum Loading

Height of StackP3 500 6 Reels 4 ReelsP3 500 Messenger 5 Reels 3 ReelsP3 625 5 Reels 3 ReelsP3 625 Messenger 5 Reels 3 ReelsP3 750 5 Reels 3 ReelsP3 875 5 Reels 3 ReelsP3 1000 4 Reels 2 Reels

QR 540 5 Reels 3 ReelsQR 540 Messenger 5 Reels 3 ReelsQR 715 5 Reels 3 ReelsQR 860 5 Reels 3 ReelsQR 1125 4 Reels 2 Reels

F 59 Series 4 Cartons 3 CartonsF 59 Messenger 6 Cartons 5 CartonsF 6 Series 4 Cartons 3 CartonsF 6 Messenger 6 Cartons 5 CartonsF 7 Series 5 Cartons 5 CartonsF 7 Messenger 5 Cartons 5 CartonsF 11 Series 5 Cartons 5 CartonsF 11 Messenger 5 Cartons 5 CartonsComm/Pak 3 Cartons 3 Cartons

Care should be taken when unloading cable reels from delivery trucks. A forklift or other proper unloading technique should always be used. The cable reels should never be rolled off the truck and allowed to drop to the ground. Damage to the cable that effects return loss can occur if proper procedures are not followed.



Coaxial Cable Preparation, Connectorization, and Splicing In order to produce a high quality and technically sound CATV system or broadband communications system, the integrity of the splicing techniques and connectorization of the cables are crucial. Connectors provide continuity for both the center and outer conductors of the cable.

Connectors are used to join all components and cable spans in a CATV or broadband system. They carry various RF signals along with 60v and 90v AC power and must maintain a 75-ohm characteristic impedance.

Most connectors are made from corrosion resistant aluminum alloys and are protected from the hazards of weather with some form of weather-guard. Another important point about the connectors is that they also protect the cable and equipment from water migration and signal egress.

There are numerous cable sizes and different types of equipment found in a broadband system that are joined together by connectors in order to provide continuity.

Cable connectors are made with different cable entry sizes to accommodate all types of cable. The threaded portion of a connector body is 24 threads per inch by 5/8” in diameter. (This is the standard size found on all connector ports of system equipment or other multiple connector housings.)

Connector Types There are various types of connectors and several connector manufactures. The following connectors are commonly used in a CATV or broadband system.

Pin Connectors

The following drawing illustrates a basic pin connector. Pin connectors are used to connect the trunk and distribution cables to the active and passive components of the system plant.

Figure 1-17 Pin Connector



Feed-Thru Connector

A feed-thru connector is very similar to a pin connector without the pin. The center conductor of the coaxial cable is prepped to be long enough to be inserted into the seizure mechanism of the active or passive device.

Figure 1-18 Feed-Thru Connector

Splice Connector

Splice connectors are used to join ends of coaxial cable. Some splice connectors available to join two different cable sizes with the same characteristic impedance.

Figure 1-19 Splice Connector

Cable Terminator

This connector is used to provide a 75-ohm load and block 60 cycle AC on the coaxial cable at locations that do not require an in-line active or passive device.

75 Ohm Load

Figure 1-20 Cable Terminator



Housing-to-Housing Connector

This connector is used to join two pieces of equipment together.

Figure 1-21 Housing-to-Housing Connector

Right Angle Connector

This type of connector is typically used in conjunction with a pin or feed-thru connector. It is commonly used at active and passive locations when the coaxial cable cannot be formed to the equipment without exceeding the minimum bending radius.

Figure 1-22 Right Angle Connector

180 Degree Angle Connector

This type of connector is used at equipment locations when the cable cannot be formed without exceeding the minimum bending radius or when the cable is fed from a different direction than the entry port.

Figure 1-23 180˚ Connector



F Connector

This type of connector is used on the drop cable for all subscriber installations.

Figure 1-24 F Connector

Splice Connector

This type of connector is used to join drop cable.

Figure 1-25 Splice Connector

F-Series Terminator

This type of connector is generally used to terminate unused tap ports in the distribution system.

Figure 1-26 Terminator

Connector Installation Proper connector installation is crucial. A properly installed connector will prevent signal ingress and egress while also preventing corrosion and water migration into the system equipment and cable.

A connector must be installed in accordance with the manufacturer’s recommendations. Failure to do so will cause signal impairments. A poorly installed connector will cause impairments to the system such as signal egress (RF signal leakage), signal ingress (entry of unwanted signals), co-channel interference and interfering spurious signals. It will also adversely affect the overall frequency response of the network (peak-to-valley).



Loose connectors can cause loss of signal, the loss of ground integrity, and permit signal leakage. The two most common problems are loose and corroded connectors causing a loss of ground and/or loss of signal.

Signal suckout (change in the frequency response of the system) is generally caused by a loose connection and attenuates a specific frequency or frequency band.

Figure 1-27 Effect of Signal Suckout

Connectorizing Trunk and Distribution Cable The process for installing connectors on the trunk and distribution cables is dictated by the type of connector used. Different sizes of cable require different sized tools to prepare the cable for connectorization.

The following information will outline the basic installation procedures for the common connectors used in a CATV system. Consult the manufacturers of the equipment, cable and connectors for specific procedures and proper installation.



Pin Connector

To install a pin connector:

1 Install the connector to the device

2 Prepare the cable

3 Attach the connector to the cable

4 Install the body of the connector in the entry port of the equipment and tightened in accordance with the manufacturer’s specifications.

Pin Body MainNut

Integral Sleeve

Back Nut

Cable Preparation Tool

CENTERCONDUCTOR

LENGTH

CABLE

COREDEPTH

CORING TOOL



Main Nut Installation

Back Nut Main Nut

Feed-Thru Connector

To install a feed-thru connector:

1 Install the connector to the device

2 Prepare the cable


4 Install the body of the connector in the entry port of the equipment and tightened in accordance with the manufacturer’s specifications.

Body Integral Sleeve

Back Nut



Splice Connector

To install a feed-thru connector:

1 Prepare the cable


Back Nut Back NutMain Nut Main Nut

Body



Connectorizing Drop Cable The process for installing connectors on drop cable is dictated by the type of connector used. Currently, there are various types of connectors available and most require different installation procedures. These connectors range from the hex crimp connector to the compression connectors that have been developed in recent years.

The following information will outline the basic installation procedures for the common ”F” connectors used in a CATV system. The manufacturers of the equipment, cable, and connectors selected for use should be consulted for specific procedures and proper installation specifications.

The F connector is used to interface the drop cable to devices in the distribution equipment and all the connections made to the inside house wire.

In most CATV systems, the drop connector ranks the highest in subscriber-reported trouble calls. Improperly installed connectors will cause many forms of degradation to the signal, ranging from noisy pictures to multi-path ghosting and signal leakage. For this reason, it is imperative that the integrity of each connection and proper installation techniques be employed at the time of installation.

Cable Preparation

Prepare the drop cable as follows:

1 Remove 3/8” of the cable jacket from the end of the cable. Be sure not to cut the braid.

Figure 1-28 Drop Cable - Jacket Removed

2 Remove the jacket and fold back the braid.

Figure 1-29 Drop Cable - Braid Folded



3 Cut 1/4” of the foil and dielectric from the end of the cable. Remove the foil and dielectric material and expose the center conductor (be sure not to nick the center conductor).

Figure 1-30 Drop Cable - Dielectric Removed

Using a properly set prep tool will complete the cutting process in steps 1-3 simultaneously.

Figure 1-31 Drop Cable Preparation Tool

4 Install the connector onto the cable as illustrated below. There should be approximately 1/16” of the center conductor protruding from the end of the connector. On the inside of the connector, the dielectric will be flush with the center hole of the connector.

Figure 1-32 Connector Installed on Drop Cable

5 Using the proper size hex crimping tool, crimp the connector.

Figure 1-33 Crimped Connector



6 Install weather protection over the cable at this time and secure it accordingly. (If weather boots were used, they would be installed on the drop cable before the connector.)

7 The connector should be installed on the female F-connector or equipment port and tightened to the manufacturer’s recommended torque.

Splicing Practices Sound splicing and placement of all passive and active components is essential during the construction process for a new-build CATV or broadband communications system.

There are numerous factors that govern the splicing process of any communications system. Because of this, it takes a highly skilled and qualified craftsman to complete this work task. Improper splicing and poor workmanship will create extensive difficulties that will arise during system activation and impact the signals carried on the system.

The following recommendations should be adhered to for splicing aerial and underground plant:

1 Place equipment in accordance with the design. No deviations are permitted, unless approved by a qualified designer.

2 Cut cable to the exact length required for the connector used and the equipment configuration.

3 Do not exceed the minimum bending radius when forming or bending the coaxial cable.

4 Install cable connectors correctly and tighten them in accordance with the manufacturer’s specifications.

5 Bond and ground active equipment locations.

6 In aerial applications, attach all equipment connected with a housing-to-housing connector to the strand using equipment support brackets.

7 In aerial applications, place expansion loops properly and form them using an approved bending board or bending tool.

8 Clean the center conductor with a piece of Plexiglas or a center conductor cleaning tool. Do not use a knife to clean the center conductor.

9 In aerial applications, place amplifiers on the input side of the pole.

10 Place expansion loops on the input and output side of all active aerial equipment.



Underground Plant Splicing configurations are very different in underground plant facilities. There are many variables that have a direct impact on the final configuration for underground splicing.

Systems generally place active and passive equipment in upright pedestals, underground vaults, or other manholes that provide security and protection against the elements of harsh weather. Equipment should be mounted on a stake or bracket and protected from water migration.

Whenever possible, some excess coaxial cable should be left at all locations for future maintenance purposes.

Upright Pedestal Following is an illustration of a trunk amplifier, splitter and tap splice configuration. The splitter and tap are connected to the amplifier using housing-to- housing connectors.



Low Profile Trunk Amplifier Configuration Following is an illustration of a low profile pedestal with a trunk amplifier, directional coupler and tap splice configuration.

Upright Pedestal Tap Configuration Following is an illustration of a single tap mounted in an upright pedestal. This type of enclosure normally does not allow for excess cable to be placed; therefore the equipment should be mounted to the highest possible location.



Below Grade, Single Tap Configuration Following is an illustration of a tap placed below grade in a vault location. The tap is mounted to a mounting stake. One full wrap of excess cable is placed in the bottom of the vault for future maintenance. The connectors are protected from water migration by the use of shrink boot as the weatherguard.

Below Grade Trunk Amplifier Configuration Following is an illustration of a trunk amplifier installed below grade, a rare but, possible scenario. The amplifier is mounted to the top of the vault to prevent water migration.

Note that the amplifier is properly grounded and that the excess cable wraps are placed in the bottom of the vault.


Section 2 Fiber Optic Cable

Upon completion of this section, you will:

Understand the various type of fiber optic cable designs

Understand the basic fiber optic cable placement

Understand the various types of connectors and splicing techniques for use with fiber optic cable

Have a basic understanding of troubleshooting techniques and emergency restoration, such as fiber optic system components, test methods and equipment, and fault location and restoration

2-2 Fiber Optic Cable


Fiber Optic Cable Design Fiber optic cable design objectives are to protect the glass fiber from the outside environment. The fiber must be protected from forces such as impact, tensile, twist and compressive loads.

The fiber itself is degraded by moisture and if water were to get into a cable and freeze it could crush the fiber.

The most critical design parameter is temperature performance. The rate of expansion of glass is about 100 times less than the surrounding cable materials. Proper cable design is important to keep the many surfaces from imparting forces that would manifest as drastic increases in attenuation or, in an extreme case, fiber breakage.

Many different cable designs have been developed to overcome the environmental effects and forces on optical fiber. A few noted here are loose tube, central core, tight buffer, and slotted core.

Loose Tube Construction The characteristics of loose tube construction include:

• 1 to 12 fibers/tube.

• Gel filled tubes for protection.

• Tubes stranded around a dielectric or metallic strength member.

• Combination of loose tube & stranding pitch allows the cable to expand and contract up to 0.3% without imparting stress around the fiber.

Steel Central Member

No Armor

All Dielectric

..... .....

.....

..........

..........

.....

PE JacketAramid Yarn

FibersLoose Tube BufferSteel Central Member

..... .....

.....

..........

..........

.....

PE JacketAramid YarnFibersLoose Tube BufferDielectric Central Member

Dielectric Core/Armored

..... .....

.....

..........

..........

.....

PE Inner JacketAramid Yarn

FibersLoose Tube BufferDielectric Central Member

PE JacketSteel Armor

Figure 2-1 Loose Tube Construction

Fiber Optic Cable 2-3


Central-Core Tube Construction The characteristics of central-core tube construction include:

• Fibers encased in one large tube.

• Fibers separated into groups by ribbons or fibers bundled by colored ID threads.

• Ribbon designed for high fiber count -- up to 12 fibers/bundle.

• Core may be armored with added steel or dielectric rods for improved strength and temperature compensation.

Dielectric Core/Armored

All Dielectric Dielectric CoreDouble Armor

..........

PE JacketSteel ArmorRipcord

Wire Strength MemberCore Tube

12-Fiber Bundles

..........

PE JacketRipcord

Dielectric Member

Core Tube12-Fiber Bundles

..........

PE Inner JacketSteel ArmorRipcord

Wire Strength MemberCore Tube

12-Fiber Bundles

PE Jacket2nd Steel Armor

Figure 2-2 Central Core Tube Construction



Tight Buffer Construction The characteristics of tight-buffer construction include:

• Layer of plastic extruded directly onto the acrylate coated fiber.

• Tight structured fiber improves handling- fibers are larger and less sensitive to handling mishaps.

• Fibers are sensitive to external forces placed on the cable.

• Cables are comparatively large and difficult to design.

PVC JacketAramid YarnDielectric Strength Member

Tight Buffered Fiber(12)

Figure 2-3 Tight Butter



Slotted Core Construction The characteristics of slotted core construction include:

• Fibers placed in a longitudinal slotted cylindrical core mode of plastic and surrounded by armors or jacket layers.

• Difficult to handle.

• Expensive to manufacture.

Single Fiber(6 per slot)Slotted CoreStrength MemberWrapping TapePE Sheath

Figure 2-4 Slotted Core

Fiber Optic Cable Specifications Fiber optic cable specifications for telephone industry that are used for CATV and other industries include specifications written by GTE, SPRINT, REA, and Bellcore.

Bellcore TR-20 is the most comprehensive, referencing ASTM, or EIA-455 test procedures. This standard includes:

• Material qualification.

• Mechanical and environmental.

• Optical performance degradation limits.

• Lightning and rodent tests.



Overview • Joint review of the actual cable route conducted by both engineering and construction

teams.

• Obtain rights-of-way and permits from local, state and federal agencies.

• Pre-construction survey and preparation:

Identify problem areas.

Locate splice points.

Determine special requirements.

Utility locates.

“Make-ready.”

Tree trimming.

Pole hardware, anchors, downguys, strand placement.

Clear underground ducts.

Begin bores and install conduit in direct buried plant.

• Intermediate cable pulls and slack may be “figure-eighted.”

Methods of cable placement and construction techniques are similar to the installation of coaxial cable. Caution should be exercised to avoid exceeding the maximum pulling tension and bend radius. Swivels are needed to keep the cable from twisting.



Types of Cable Construction • Aerial Cable

Stationary reel (back-pull)

Moving reel (drive-off)

Cable strain reliefs provided by small 2” to 4” loops at all poles

Slack may be stored along the strand or protected in an enclosure

• Underground Cable

Ducts are cleared and mandrels passed through

Sub ducts installed and pull lines are provided by rodding or blowing into place

• Direct Buried Cable

Static plows used to trench the earth. Vibratory plows should not be used for fiber cable.

Split ducts are installed to protect cables from rocks or other sharp objects



Joining Optical Fibers It is often necessary to join optical fibers to extend the length of the fiber, to join a cable with a high fiber count to two or more cables with lower fiber counts or in the case of damage. Three methods are employed to join two fibers together:

• Fusion Splicing

• Mechanical Splicing

• Connectorization

Fusion Splicing The preferred method of splicing fiber optic cable is called fusion splicing. Prior to this operation, the fiber must be prepared for splicing. The steps for preparing the fiber are as follows:

1 The coating is removed from the glass ends using a specialized stripper.

2 The fiber ends are “cleaved” or scored to achieve a clean break.



3 The fiber ends are inserted into the splicer where they are precisely aligned -- either manually or automatically.

4 The splicer delivers a high temperature electric arc that fuses the two fiber ends.

5 The end result is an extremely low loss splice: 0.01 to 0.05 dB results are typical.

Mechanical Splicing Mechanical splices have been designed in several forms. These all share common attributes. They suffer losses in the order of 0.25 dB and greater, they require simple and relatively low-cost tooling to install, and can be easily installed in the field. Mechanical splices are suitable for temporary emergency restoration of a cut fiber, since the fiber can be connected more readily, but it is usual practice to follow up later with a permanent fusion splice. Mechanical splices can be unreliable for long term service.



Connectorization Another method of joining fiber is connectorization. This method employs specialized connectors that are usually employed at the node site, at the transmitter, or the headend. A bulkhead or mating adapter joins these connectors.



FC and SC connectors are the two most commonly used connectors.

Factory installed connectors may be purchased as pigtails or jumpers. A pigtail is a fiber with a connector at one end and bare fiber at the other. A jumper is a fiber with a connector at both ends. The pigtail is fusion spliced to the end of a bare fiber allowing the fiber to be connectorized. When there is a quantity of field fibers that must be connectorized, it is common to install all of the fiber connectors onto a single panel called a patch panel. A jumper is then used to connect the terminal equipment to the patch panel.



Flat Connector

• Worst Return Loss:> -14 dB

• Common MultimodeFiber Connector

Air Gap

Fibers do not touch each other.

Figure 2-5 Flat Connector

PC Connector

• Connectors are spring-loaded to keep fibers in tight contact with each other.

• Super PC end polish is rounded and improves return loss to > -45.

• Ultra PC end polish is an improvement of the Super PC, which is further polished to improve return loss to > -55.

• Return loss is severely degraded if connector is not mated to another connector or terminator.

• Good Return Loss:

> -30 dB

• Common Single-modeFiber Connector

Physical Contact

Fibers touch each other.

Figure 2-6 PC Connector



APC Connector

• End face of connector is polished to 8, 10, or 12 degrees, which causes reflections to be directed into the cladding, rather than back up the fiber.

• Preferred connector since return loss is not severely degraded if connector is not mated to another connector or terminated.

• Expensive to manufacture and not easily made in the field.

• Best Return Loss:Can be > -65 dB

• Preferred Single-modeFiber Connection

Angled & Radiused Endface

Figure 2-7 APC Connector



Return Loss If two fibers are separated by an air gap, optical energy will be reflected back toward the source of the light. This reflected energy is called return reflection or return loss. In a single mode interconnection with a flat end finish, the return loss amounts to -14 dB. If a 10-milliwatt laser is used to launch light into a fiber, and the fiber has a -14 dB reflection, then there will be .4 mw of light reflected back toward the laser, which can interfere with the operation of the laser. Optimum performance is achieved where reflection is kept to a minimum.

Early fiber optic connectors utilized ends that were flat finished and placed in close proximity, but did not touch each other. Improvements were later made by assuring fiber-to-fiber contact, thus eliminating the air gap between the fibers. The advantage of this design was that it improved the return loss to over 30 dB. This type of interface is called a physical contact or PC connector.

Further improvements to this connector were made by rounding its ends to better insure that the fibers touch.

Improvement of the end polishing technique has reduced return loss. Called the Super PC, this more efficient design produces a return loss of 45 dB or better. A further improved version is the Ultra PC connector, which has a return loss of 55 dB or better. It should be noted that the terms Super PC and Ultra PC are not industry standards and their return losses vary from vendor to vendor.



The most effective connector by far is the Angle Physical Contact Connector or APC. The uniqueness of the APC connector lies in the fact that the end of the fiber is polished at an angle of 8º, which causes any reflection to be directed 16 degrees off the axis of the fiber. This causes the reflected energy to enter the cladding or exit the fiber, rather than reflecting it back toward the laser source. The benefit in this is that it produces an improved return loss over all other varieties of connectors in a single mode interconnection with an angle polished (APC) connector In fact, the return loss amounts to 55 dB or better. As an example, if a 10-milliwatt laser is used to launch light into a fiber, and the fiber junction has a 55 dB reflection, there will be 0.00003 mw of light reflected back toward the laser. This amount of light is too low to interfere with the operation of the laser.

Air Gap

Flat FinishFlat Finish

PC Finish Round Finish

PhysicalContact

APC Finish8, 10, 12º

AngledPhysicalContact

Figure 2-8 Return Loss Examples



Connector Termination Patch panels normally utilize connectors at the headend to allow flexibility in system design and to allow easy access for connecting and disconnecting fibers. However, a phenomenon occurs when connectors are disconnected and reconnected. When non-APC connectors are taken apart, a severe reflection results, as previously described. If a laser transmitter is feeding more than one receiver, this can cause degradation of signals at the other receivers. It is then necessary to use optical terminators on any unused ports if PC or Super PC connectors are used in the panel. However, if APC connectors are used, terminators are unnecessary. The unmated APC connector acts as its own terminator, since it has excellent return loss when the connectors are not mated. The APC connector is preferred for use in patch panels.

Causes of Light Loss As fibers are joined together, there are several ways in which the loss can be introduced into the fiber system.

Lateral Misalignment

The core glass of each fiber end must be precisely aligned with each other. If the fiber axes are not aligned some or all of the light from one fiber may be lost into the cladding of the other fiber.

Figure 2-9 Lateral Misalignment Example

Angular Offset

If the two fibers are joined at an angle other than perpendicular, some of the light from one fiber may be lost into the cladding of the other fiber.

Figure 2-10 Angular Offset Example



Core Deformation

If the cores of each fiber end are not uniformly round or of different diameters, light may not flow through the splice efficiently.

Figure 2-11 Joint Loss Mechanisms, Core Deformation

Contamination/End Separation

Any contamination such as dust or dirt that might be present between the two fibers can scatter or block the path of the light and increase the loss of light through the fiber.

Figure 2-12 Joint Loss Mechanisms, Contamination and End Separation

Core Size/Shape Differences

Core size/shape differences cannot be controlled by the operator or splicing process. They are a function of the quality of the fiber being spliced.

Figure 2-13 Joint Loss Mechanism, Core Size/Shape Differences



Fiber Optic System Components

HEADEND

LaserTransmitter

PatchPanel

SpliceEnclosures

Splice(s) FieldSplice(s)

PLANT FIBER

Node

JumperCable

PigtailSplittersCouplersJumper Cables

Connectors

Figure 2-14 Components of a Fiber Optic System



Pre-Installation Field Testing • Visually inspect cable and reels for damage.

• Verify optical length, attenuation, anomalies, and continuity.

Test Methods and Equipment

Optical Loss Measurement or Attenuation by Substitution Method

• Reference value established to account for connector assemblies.

• Total Power: PT = PO - PI (dB)

• Fiber Loss/km = PT (dB) / Fiber Length = dB/km

• Compare loss against manufacturer’s specifications.

• Repeat measurements for all fiber bundles.

TX

~ ~

Reference Cable

Actual Fiber Under Test

P1 = ____ dBm

PO = ____ dBm

~ 5m ~ 5m

StabilizedLight Source

or Laser

OpticalReceiver/

Power Meter

RX

TX RX

Figure 2-15 EIA Standard, 455-171



Optical Time Domain Reflectometer (OTDR)

1310nm1550nm

OpticalCoupler

ElectricalTiming &Control

Variable PulseWidth Generator

Detection

Laser

Display

SignalAcquisition

and Processing

~ ~

OpticalPulse

Return/ReflectedPulse

ReflectedPulse

IncidentPulse

Front PanelConnector

Figure 2-16 Optical Time Domain Reflectometer Example

• Used to locate non-catastrophic failures.

• Measures distance, fiber loss, return loss and link return loss.

• As pulse width increases, more power is coupled into the fiber to measure longer fiber spans at the expense of resolution.

Pulse Duration (ns) Pulse Length (m)

3 1 20 4 100 20 1000 204



• Proper fiber index of refraction value setting needed for accurate distance measurements.

Type of Glass Refractive Index

Depressed Cladding @ 1310 nm 1.466 Depressed Cladding @ 1550 nm 1.467 Match Cladding @ 1310/1550 nm 1.470

0 10 20 30 40 50 km

AVG (256)

648A

5DB/DIV

1KM/DIV

PULSE (M)

25

1.4690

INDEX

1300 NM

Dead Zone

Reflective Spliceor Connector

Non-ReflectiveSplice or

Bend in Cable

End Connector

Noise Floor

Figure 2-17 Typical OTDR Display



Fault Location and Restoration

Too Low

Start

CalculateCable

Attenuation

Test Connectorat RX

with OTDR

DiagnoseCable with

OTDR at TX

Test Connectorat RX

with OTDR

MeasureReceived

Power

DiagnoseRX and/orElectronics

Acceptable

MeasureTransmitter

Output PowerDiagnose

TX

CalculateCable

AttenuationStop

OK

Too High

Acceptable

DiagnoseCable with

OTDR at RX

TakeCorrective

ActionFaultIdentified

Fault Identified

FaultIdentified

Fault Identified

Figure 2-18 Flow Chart of Emergency Restoration



Emergency Restoration of Fiber Cable System

• Key requirements:

Rapid temporary restoration.

Graceful transition to permanent repair.

• Recommendations:

Develop a plan for emergency restoration.

Plan for material, equipment and labor for emergency restoration.

Prioritize circuits for re-splicing.

Develop a plan for permanent repair.

• Restoration materials and equipment:

Develop a list of required materials and equipment.

Strategically locate required materials and equipment throughout system.

Purchase an Emergency Restoration Kit.

Emergency Cable Restoration – Alternatives

Cable Cut with Retrievable Slack

1 Locate damage point with OTDR.

2 Retrieve slack.

3 Splice fibers with temporary mechanical splices.

4 Verify end to end continuity utilizing OTDR or light source and power meter.

5 Make transition to permanent repair as soon as possible.

Cable Cut with No Slack

1 Locate damage point with OTDR.

2 Install new cable.

3 Splice in new piece of cable with temporary mechanical splices.

4 Verify end to end continuity utilizing OTDR as soon as possible.

5 Make transition to permanent repair as soon as possible.

Note: Down time can be further reduced by the use of an emergency restoration kit.



Typical Emergency Restoration Tool Kit

1 Restoration splice trays.

2 Two predrilled splice closures.

3 Approximately 150 feet fiber cable.

4 Temporary mechanical splice parts (twice fiber counts of cable).

5 Two complete tool kits consisting of:

• Alcohol packs

• Gel-off packs

• 88 vinyl tape

• Miller stripping tool

• Book of numbers

• Kim wipes

• Diagonal side cutting pliers

• Pliers

• Wrenches

• Cable sheath knife

• Ty-raps

• Snips

• Precision cleaving tool


•


Section 3 Fiber Optic Transmitter and Node Maintenance

After completing this section, you will have an understanding of:

Advantages of Fiber Optics

Basic Light Theory

Fiber Manufacturing

Network Components

Fiber/Coax Comparisons

AM Fiber Path Engineering

Laser Transmitter Technologies

Receiver Technologies

AM Fiber Network Performance

3-2 Fiber Optic Transmitter and Node Maintenance


Advantages of Fiber Optics

Transmission

• Attenuation – very low loss.

• Bandwidth – extremely large information handling capacity.

• Very secure medium of transmission.

Physical

• Small size.

• Light weight.

• Flexibility.

• Very large capacity/diameter ratio.

• No cable pressurization is required.

• No performance degradation due to moisture, oxidation, corrosion, etc.

Non-Conductive

• Fiber is a dielectric.

• Fiber is immune to EMI.

• Fiber is relatively immune to radiation.

• No electrical hazards to personnel.

• No problems with ground loops.

Note: The fibers themselves are non-conductors but the overall cable may be conductive.

Economical

• Capacity versus installed cost ratio very high.

• No requirements for frequency permit from FCC.

Fiber Optic Transmitter and Node Maintenance 3-3


Basic Light Theory • Like television and radio signals, light is electromagnetic energy that travels in waves.

• The distinction between different waves lies in their frequency and/or wavelength.

• Frequency: Number of sine-wave cycles per second, normally expressed as hertz (Hz).

• Wavelength: The distance between the same points on two consecutive waves.

• Frequency and wavelength are inversely related – higher the frequency, the shorter the wavelength.

Note: 60 Hz power has a wavelength of 3,100 miles. TV channel 2 has a frequency of 55.25 MHz and a wavelength of about 18 feet. Deep red light has a frequency of 429 THz and a wavelength of only 700 nm or about 28 billionth of an inch (1 nm = 1 billionth of a meter).

• Television and FM and AM radio waves have frequencies well below those of light.

• Light has a higher frequency and shorter wavelength.

• A higher-frequency carrier has greater information-carrying capacity.

• Infrared light is used because glass fibers carry infrared light more efficiently than visible light.

• Light travels slower than 186,000 miles per second (300,000 km per sec) in materials such as glass. Typical speed in glass is 125,000 miles per second (200,000 km per sec).

• Light traveling from one material to another changes speed, resulting in a change in its direction of travel; this property is called refraction.

• The amount that a ray of light is refracted depends on the index of refraction of the two materials.

• Light going from a higher index to a lower one refracts away from the normal.

• The angle of incidence can be increased past the critical angle, causing the light to be completely reflected back into the first material; therefore, not entering the second material.

• By controlling the refractive indices of the two materials, light can be made to continue traveling through the first material by total internal reflection. This is the fundamental principle of optical waveguides.

Fundamental Principle Total internal reflection in optical fiber results from the difference between the refractive index of the core material and that of the cladding material.

• Core Index = ncore

• Cladding Index = ncladding

• Fiber will propagate energy

• If ncore > ncladding



Core n1

n2

Cladding

n > n1 2

Partially refracted ray

Partially reflected ray

Totally reflected ray

Cladding

Total reflectionin this range

Light entering thiscone is totallyreflected

Partial reflectionin this range

CriticalAngle

Figure 3-1 Principle of Total Internal Reflection for Optical Fibers

CladdingCore

1 µsec 1 µsec

Figure 3-2 Multimode

Cladding

Core

1 µsec 1 µsec

Mode Field

Figure 3-3 Single-Mode



COATING

CLADDING (n2)

CORE (n1) 50 125 250

NOTE: ALLDIMENSIONS AREIN MICROMETERS

MULTI-MODE FIBER (TYPICAL)

COATING

CLADDING (n2)

CORE (n1) 10 125 250

NOTE: ALLDIMENSIONS AREIN MICROMETERS

n = INDEX OF REFRACTION

SINGLE MODE FIBER (TYPICAL)

n1 > n2 GIVES TOTAL TOTAL REFRACTION

Figure 3-4 Relative Fiber Size

Wavelength of Infrared Light, Compared to Frequency AM fiber optic systems utilize Infrared light as the carrier for information and data. Reference the drawing to see where infrared light falls in the spectrum of electromagnetic radiation. This light may be used at the wavelength of 1310 or 1550 nanometers. A nanometer can be expressed as one billionth of a meter (1/1,000,000,000 meter) or 1310 nanometer light can be expressed as 229,000 Gigahertz or 229 Million-Megahertz or 229 Tetrahertz. 1550 nanometer infrared light can be expressed as a frequency of 193,548 Gigahertz or 193.6 Million Megahertz or 193.6 Tetrahertz.

To convert wavelength to frequency:

(meters)light of Wavelengthcond)(meters/selight of Speed(Hz)Frequency =

So, for light with a wavelength of 1310 nm:

THz229Hz10229

meters101310ondmeters/sec103Frequency

12

9-

8

=×=

××=



(1 kHz.) 10

(1 THz.) 10

(1 MHz.) 10

(1 GHz.) 10

0

3

12

9

6

10

10

10

21

18

15

Frequency in Hz.

Subsonic

AM Radio

Sound

Shortwave Radio

TV & FM Radio

Visible LightUltraviolet Light

Radar-Microwave

Infrared Light

X - rays

Gamma Rays

Cosmic Rays

Visi

ble

Spec

trum

400 nm Ultraviolet

490 nm Blue455 nm Violet

580 nm Yellow550 nm Green

750 nm Red

620 nm Orange

850 nm800 nm Infrared

1,300 nm.

1,550 nm.

Figure 3-5 The Electromagnetic Spectrum

In the visible-light spectrum, color can be described in terms of its wavelength. For example, the wavelength of red light is longer than that of blue light.

Typical wavelengths used in fiber optics are 850, 1310, and 1550 nanometers (none of which are visible to the human eye). A nanometer is one billionth of a meter.



Figure 3-6 Spectral Loss of a Fiber Showing the Windows of Normal Operation



Optical Fiber Primary Parameters

Attenuation

The following table lists the factors that affect attenuation in fiber cable.

Effect Cause

Absorption Impurities Scattering Variances in the structure of the fiber Waveguide structural Micro/macro bending

Bandwidth

The following table lists the factors that affect the bandwidth or information carrying capacity of fiber cable.

Effect Cause Notes

Modal Dispersion

Each mode travels at a different velocity

Applies to multimode fibers only

Depends upon accuracy of index profile shapes independent of source width

Chromatic Dispersion

Each wavelength travels at a different velocity

Applies to both multimode and single mode fibers

Depends primarily on fiber materials

Increases linearly with increasing source width



Fiber Manufacturing

Figure 3-7 Outside Vapor Deposition

Figure 3-8 Inside Vapor Deposition



Figure 3-9 Preform Preparation

Figure 3-10 Fiber Drawing Tower



SignalProcessingEquipment Signal

Combining

OpticalTransmitters

OpticalSplitter

FiberOpticalNode

RFoutputReceiver

Processor

CombGenerator

Amplification &Splitting

HEADEND

Signal Collection- Satellites- Off Air Antenna- Microwave

Figure 3-11 Optical Network Components



Fiber/Coax Comparisons

Coax The following table lists the attenuation of various size coaxial cables at several frequencies.

Attenuation (dB/100') at

Cable Type

Center Conductor Size (in.) 50 MHz 350 MHz 550 MHz 750 MHz

865 MHz

860 0.203 .30 .83 1.06 1.24 1.33 750 0.167 .35 .97 1.24 1.48 1.61 540 0.124 .44 1.23 1.56 1.85 2.00 500 0.109 .52 1.43 1.82 2.16 2.34 F6 0.040 1.53 3.85 4.90 5.65 6.10



The following graph compares the cable length resulting in 10 dB of loss for various cable types at 50 and 865 MHz.

3333

2857

2273

1923

654752

621500

427

164

0

500

1000

1500

2000

2500

3000

3500

860 750 540 500 F6Cable Type

Dis

tanc

e (f

eet)

50 MHz

865 MHz

Figure 3-12 Cable Run Length vs. Cable Type for 10 dB Cable Loss



Fiber The following table lists the attenuation of fiber at several wavelengths.

Attenuation (dB/km, dB/3,281 ft) at Wavelength (nm)

Fiber Optic Cable Type 850 1310 1550 Single Mode 2.5 0.35 0.21 Multimode 4.0 0.60 0.45

Note: 1 km equals 3281 feet or 0.621 miles, and 1 mile equals 1.609 km.

The following graph compares the fiber length resulting in 10 dB of loss for various cable types at wavelengths of 850, 1310, and 1550 nm.

156,230

13,123

72,907

32,808

93,738

4,9210

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

1550 1310 850

Wavelength (nm)

Dis

tanc

e (f

eet)

Single mode

Multimode

Figure 3-13 Fiber Run Length vs. Wavelength for 10 dB Optical Cable Loss



Fiber vs. Coax for 10 dB Attenuation The following chart shows that 10 dB attenuation represents 156,230 feet of fiber at 1550 nm, 93,738 feet of fiber at 1310 nm, and only 752 feet of 860 cable at 865 MHz.

752

156,230

93,738

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

1550 nm 1310 nm Q R 860

Dis

tanc

e (fe

et)

Figure 3-14 Fiber vs. Coax, Cable Run Distance for 10 dB Loss



Fiber vs. Coax System Comparison The following figure compares a 93,700-foot run using .860 cable against the same length run using fiber.

1 2 3 4 5 6 7 8....................39 40(Many actives to fail, much degraded performance)

Coax

(No actives to fail, much better performance, lower cost)

Fiber

Headend

TXHeadend Fiber

Node

– 10 dB Optical Path Loss @ 1310 nm– 10 dB ÷ 0.35 dB/km = 28.6 km– 28.6 km x 3,281 ft/km ~ 93,700 feet

– QR 860 Cable (1.24 dB/100 ft @ 750 MHz)– Typical Amp Spacing 29 dB (2,339 ft)– 93,700 ft ÷ 2,339 ft/amp = 40 amps– 93,700 ft x 1.24 dB/100 ft = 1,162 dB total cable

Figure 3-15 93,700 feet Optical Fiber Plant vs. Coaxial Plant



AM Fiber Path Engineering

Optical Couplers

• Optical couplers are key devices for optical networks. Important applications include:

Optical splitting.

Optical attenuation.

Measurement of transmitted optical power (monitoring).

Measurement of optical reflections (OTDRs).

• Basic concept is narrow spacing between adjacent fiber cores allows coupling between the cores because the electromagnetic fields extend beyond the cores.

• Different techniques exist for the production of couplers:

Fused biconical taper.

Polishing of cladding.

Integrated waveguides.

• Various coupling ratios are accomplished by:

Stretching the fiber in the fused region to reduce or increase the core-coupling ratio.

Variable lateral spacing between the cores.

• For all truncated fibers, the fiber end is terminated within the coupler housing.

• Coupler ratios can be manufactured in 1% increments. Typical increments are 5% values.

Figure 3-16 Coupler Types



• Couplers are usually fusion spliced into the network or connectorized for patch panel use.

• Loss distribution generally specified in percent of coupled light.

• To define the theoretical split ratio of a coupler with two or more equal output legs:

( )

( )

( )

( )ndB

dB

dB

log10couplernx1 aFor 9

8log10coupler8x1 aFor 6

4log10coupler 4 x 1 aFor 3

2log 10 coupler 2 x 1 afor ratioSplit

=======

• Theoretical Loss in dB = 10 x Log (coupler %/100).

For example, using 75% coupler = 10 x Log (75/100) = -1.25 dB.

• A 1x2 coupler typically will have a splitting ratio of 50%. Loss calculations can be approximated with the following formula:

50/50 Coupler 10 x Log(0.5) = -3.01dB

70/30 Coupler For the 70% LEG = 10 x Log(0.7) = -1.55dB

For the 30% LEG = 10 x Log (0.3) = -5.22dB

• Excess loss = (stated loss in dB) - (nominal loss in dB).

• Excess loss is typically 0.5 to 0.8 dB and can be different for each leg.

• Typically chosen to keep total optical path loss on all legs relatively equal.

• Couplers can be cascaded to obtain higher receiver-to-transmitter ratios.



Output legs of an optical directional coupler are called the through and tap legs. They are differentiated by their respective power losses. The through leg has less loss than the tap leg. To illustrate, a DC75 coupler has losses at the tap leg of 6.3 dB and 1.5 dB at the through leg. The through leg transmits approximately 75% of the input power, hence the designation DC75.

The following are the typical insertion losses of the optical couplers, at 1310 and 1550 nm (excess loss included).

Typical Loss (dB)

Model Split (%) Tap Through Difference (Tap-Through)

DC50 50/50 3.2 3.2 0.0 DC55 45/55 3.7 2.8 0.9 DC60 40/60 4.2 2.4 1.8 DC65 35/65 4.7 2.0 2.7 DC70 30/70 5.5 1.6 3.9 DC75 25/75 6.3 1.5 4.8 DC80 20/80 7.3 1.2 6.1 DC85 15/85 8.4 1.0 7.4 DC90 10/90 10.5 0.7 9.8 DC95 5/95 14.0 0.4 13.6



Path Loss Calculations

• Path loss is sum of all losses in the optical path.

• Loss budget is defined as maximum allowable loss to obtain a given performance level.

• Typically allow an extra 3% to 5% of optical fiber for variances due to risers, sag, etc.

Calculations

1 Calculate loss of optical fiber (0.35 dB typical at 1310 nm).

2 Add 5% excess fiber for variances.

3 Calculate loss of fusion splices (Average 1 splice per 3.0 km @0.05 dB/splice)

4 Calculate loss of mechanical splices or connectors (Typical 1 per Tx, 1 per Rx @0.50 dB/connector pair).

5 Include 3 dB (actual loss will depend on individual system configuration) for headend fiber management.

6 Add in specified typical or maximum optical coupler loss.

Example

• Convert strand mileage to kilometers: 10.5 miles x 1.61 x 1.05 (5%) = 17.75 km

• Find fiber loss in dB: 17.75 km x 0.35 dB/km = 6.2 dB

• Approximate number of fusion splices: 17.75 km ÷ 3.0 km/splice ~ 6 splices

• Fusion splice loss: 6 x 0.05 dB = 0.3 dB

• Mechanical splice loss (connector pairs): 3 x 0.5 dB = 1.5 dB

• Coupler loss = 0 (none in this example)

• Total path loss: 6.2 + 0.3 + 1.5 + 3.0 = 11.0 dB

Figure 3-17 Path Loss Calculations Example



Typical Path Splitting Using an Optical Coupler

Figure 3-18 Typical Path Spiitting using an Optical Coupler

Path 1

The loss for path 1 is:

• Total Fiber = 6.4 mi x 1.61 km/mi x 1.05 (5%) = 10.82 km

• Fiber loss = 10.82 km x 0.35 dB/km = 3.79 dB

• Total splices = 10.82 km x 1 splice/3 km ~ 4 splices

• Splice loss = 4 x 0.05 dB/splice = 0.2 dB

• Connector loss = 4 connectors x 0.5 dB/connector pair = 2.0 dB

• Fiber management = 3 dB

• Coupler loss = 3.7 dB

• TOTAL PATH 1 LOSS = 3.79 + 0.2 + 2.0 + 3 + 3.7 = 12.7 dB



Path 2

Calculate the loss for path 2.

• Total Fiber = __________________________________________________________

• Fiber loss = ___________________________________________________________

• Total splices = _________________________________________________________

• Splice loss = ___________________________________________________________

• Connector loss = ________________________________________________________

• Fiber management = ____________________________________________________

• Coupler loss = _________________________________________________________

• TOTAL PATH 2 LOSS = ________________________________________________

Transmitter Output Power

Determine optical transmitter output power needed.

Path 1 = 12.7 dB

• Path 2 = _______ dB

• Select closest available _____ dBm



Laser Transmitter Technologies

Laser Subassembly

Cooler

Figure 3-19 Components of Laser Subassembly

Thermoelectric Cooler

Maintains the laser diode temperature via a closed loop controller.

Monitor Photodiode Measures optical output power. Used in a closed loop circuit to maintain constant power.

Lens Collects, collimates and focuses the laser light Optical Isolator Prevents reflections from connectors, splices and downstream user

equipment from entering the laser cavity. Laser Diode Chip Semiconductor “chip” that emits laser light.



Semiconductor Lasers Semiconductor lasers are usually made of InGaP (Indium Gallium Arsenide Phosphide). Electrically, they behave like a silicon P-N diode.

Typically, the active region dimensions are a few tenths of a micron thick, 1 to 2 microns wide, and several hundred microns long.

The following table lists the characteristics of Fabry-Perot (FP) and Distributed Feedback (DFB) lasers.

Fabry-Perot (FP) Distributed Feedback (DFB)

Structure

Metal ContactLaser Output

Active Layer

Metal Contact

n+

n

pp+

Grating

Metal ContactLaser Output

Metal Contact

n+

Active Layer

Emission Spectrum

Wavelength (nm)

Primary Light

Out

put P

ower

(Arb

itrar

y U

nits

)

1300 1310

1-5 dBbelowcarrier

1320

Secondary Mode LightTertiary Mode Light

Wavelength (nm)

Primary Light

Out

put P

ower

(Arb

itrar

y U

nits

)

1295 1305

35-40 dBbelowcarrier

1315

Secondary Mode LightTertiary Mode Light

Characteristics No cavity grating. Cavity grating.

Multiple modes emitted. Single mode operation eliminates Mode Partition Noise.

Highly sensitive to optical reflections Highly sensitive to optical reflections.

Fiber dispersion introduces mode partition noise in multimode fibers.



Fabry-Perot (FP) Distributed Feedback (DFB)

The following table lists the similarities and differences of FP and DFB lasers.

Similarities Differences

Output power is comparable: 10+ mW. DFB single mode operation eliminates Mode Partition Noise.

Output wavelengths of 1310 nm or 1550 nm. DFB used mostly as a forward path transmitter.

Frequency response is equal and extends into microwave range.

FP used mostly as a return path transmitter.

Linearity is equal in high quality versions of each type.

DFB has a lower Relative Intensity Noise (RIN).

Drive current is comparable: 50 to 80mÄ. DFB is more expensive to manufacture.

Both are sensitive to optical reflections.



Modulation Techniques

Direct

• Direct intensity modulation:

• RF current added directly to Laser dc bias current.

• Low RF drive current.

• Used with both DFB and FP lasers.

• Introduces Chirp (spreading of the optical bandwidth).

• Very high optical efficiency.

• Uncomplicated transmitter design.

• Most commonly used.

Voltage Source

RF inModulatedlaser light

RFCouplingCapacitor

LaserDiode

RFChoke

OutputFiber

Figure 3-20 Direct Modulation



Figure 3-21 DFB Laser in Linear Operation

Lasers modulated by high RF drive levels will distort or clip the original signal.

Figure 3-22 DFB Laser Clipping



With the more realistic input shown in the next figure, the voltage peaks that occur when many of the carriers are in phase during their amplitude maximums causes the Laser to be driven below the laseing threshold. This results in the absences of any signal, and therefore causes severe distortion and major BER problems with digital signals.

Clipping of analog signals will be displayed as white text horizontal flashes on a TV screen while the bit-error-rate of a digital signal will increase causing loss of data.

Figure 3-23 Laser Clipping, Complex Input



External

• External intensity modulation:

• Low chirp

• Used with YAG lasers or 1550 nm DFBs

• High RF drive power

• Extremely complicated transmitter

• Expensive

Waveguidefor

optical signalModulatorelectrodes

Optical(input)fiber

Optical(output)

fiberCATV signalsource

loadresistor

Figure 3-24 External Modulation



Figure 3-25 Externally Modulated Signal in Linear Operation

Figure 3-26 Externally Modulated Signal in Clipping



AM Optical Transmitters The following paragraphs describe the operation of a typical DFB transmitter

Figure 3-27 Typical DFB Transmitter

• Primary RF Input for Channel’s / Signal’s that are to be delivered to all subscribers.

• Narrow cast RF Input for Channel’s / Signal’s that are to be delivered to subscribers fed from specific nodes.

• Two gain stages amplify signal to the required drive level for the Laser.

• Electronic Pin attenuator to control this drive level via an automatic gain control function.

• Slope / flatness board, adjusted at factory for best Optical Link amplitude vs. frequency response.

• Directional coupler samples the RF total power level to provide proper drive level to the Laser via the Pin attenuator and provides signal cutoff if the signal goes above Laser damage point.

• Directional coupler following 2nd. Gain stage provides a calibrated RF TP to the front panel. Level will be a specific value for a specific channel loading.

• Laser module contains a predistortion (pre-emphasis) circuit that minimizes CSO, CTB, etc.

• A thermo-electric cooler (TEC) provides cooling or heating to keep the laser diode at 25 degrees C.



• Optical power TP provides scaled dc voltage that is equivalent to the laser optical power.

• Microprocessor & EEPROM

• Stores specific laser module parameters

• Monitors and controls dc and RF drive levels and optical output.

• Controls front panel test points, displays, etc.

Optical Power Test Point

Most manufacturers provide a scaled dc TP that is equivalent to the laser optical power.

Typical Motorola headend forward lasers are scaled such that 1 Volt equals 5 mW.

Example: dc TP reads 2.5 volts.

Using the formula

PmW = Etp x 5

PmW = 2.5 x 5

PmW = 12.5 mW of Optical Power

0 5 10 15 20 25

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Test

poi

nt v

olta

ge (

Vdc)

Optical power (mW)

P =E x5mW tp

Figure 3-28 Optical Power Test Point



Converting Between dBm and mW

Optical power is typically expressed in mW or dBm. To convert, use above chart or the formula below.

Example: Optical Power = 12.5 mW.

dBm = 10 x Log (PmW.)

= 10 x Log (12.5)

= 10 x 1.097

= 10.97 dBm

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

1 2 3 4 5 6 7 8 9 10 15 20 300

Milliwatts

dBm

dBm=10xlog(P )mW

Figure 3-29 Converting Between dBm and mW



RF Test Point Level vs. Channel Loading

The data in the following figure is for a typical Motorola Omnistar optical transmitter in the “PRESET” mode. (May vary for some models, refer to specifications)

A typical input of 77 NTSC analog channels between 54 and 550 MHz., and Digital channels between 550 and 750 MHz. will give +17 dBmV at the module RF TP.

This level will change based on 10 x log(ratio channels). Typical link C/N will change at this same rate.

Figure 3-30 RF Test Point Level vs. Channel Loading



Receiver Technologies

Diode Detectors

Power Supply

RF outModulatedlaser light

Photodetector

RF Choke

AC Coupling Capacitor

OpticalInputFiber

Figure 3-31 Diode Detector

Photodetector Converts laser photons to current. Voltage Source A reverse bias source that sets a linear operating point for the

photodetector. RF Choke Inhibits the signal current from entering the power supply. AC Coupling Capacitor Couples the signal current to the launch amplifier.

Pin Diodes

• Wideband

• Excellent responsivity

• Temperature stable

• Used in AM systems



AM Optical Receivers • AM receivers use PIN photodiodes.

• Typically have poorer peak-valley response than a trunk amplifier.

• Basically same receivers used for DFB, FP and YAG transmitters.

• Sensitivity nearly the same for 1310/1550 nm.

• High optical receive power (+1 dBm) can introduce distortion (CSO, CTB).

• Low optical receive power (-4 dBm and Less) can degrade link performance (CNR).

• Launch amplifiers are needed to provide useful output levels.

Photodetector Module

Input Pad

RFAmplifier

DCAmplifier

RFAmplifier

DC test point

SlopeControl

ThermalPad

RFAmplifier

DirectionalCoupler

Diplex Filter

H

LPINAttenuator

RFOUT

Automatic GainControl Board

~

PowerSupply

Photodetector

OpticalInputFiber

Figure 3-32 Typical AM Receiver Block Diagram

Photodetector Converts incident laser light to signal current. Input Pad Sets RF input level to following amplifier. RF Amplifier Low noise high gain amplifier. Automatic Gain Control Monitors a designated pilot frequency and adjusts a PIN

attenuator to maintain constant output level. Slope Control Compensates for forward cable loss. Thermal Pad Compensates for Hybrid changes over temperature. Directional Coupler Monitor port for ACB. Diplex Filter Separates forward and return path signals.



Note from the following figure that 1 dB optical power change results in 2 dB RF level change

12

14

16

18

20

22

24

26

28

30

-8 -7 -6 -5 -4 -3 -2 -1 0 1Received Optical Power (dBm)

Rec

eive

r Mod

ule

Out

put (

dBm

V)

Figure 3-33 Receiver Module Output Power vs. Received Optical Power



TCU

MAN

MANAUTO

(ADUoptional) Output

Retamp

BTN-S-1F(plugs intoequalizerfacility)

BTN-S-1R(plugs intoequalizerfacility)Ret out

JXP

JXP

JXP

JXP

JXP

FWD out

FWD 3

FWD 4

FWD 1

JXP

TP-20 dB

TP-20 dB

TP-20 dB

TP-20 dB

TP-20 dB

TP-20 dB

RET IN20 dB

OUT

Port 3

Port 4

Port 1

IN

Preamp

MDR

VarilosserFWD in

JXP

JXP-6A

Driver

H

L

H

L

H

L

H

L

H

L

JXP

JXP

Ret in

Ret 3

JXP

JXP

Ret 4

Ret 1

Statusmonitor

Flatnessboard Output

Output

OutputReturncombinerTo return laser(s)

via return options board

From optical receiver(s)via forward options board

Figure 3-34 BTN-75SH-SX, Block Diagram

SG2-FJBforwardjumperboard

Data lines

SplitterSplitter

Splitter TP

TP

TP

TP

TP

JXP

JXP

JXP

Receiver B

Receiver C

Receiver A

SG2-FBSforwardbandsplit

MDRAttenuatorSG2-FRB

POTADU JXP

+ 18.5Amplifier

JXP EQ

Amplifier

SG2 LID SG2 RF chassis

+24 VDC+5 VDCACV

JXPL

H

Diplexfilter

TP

L

H

Diplexfilter

TP

L

H

Diplexfilter

TP

L

H

Diplexfilter

JXPIngress

JXP

JXP

TP

TP

Optical transmitter

Optical transmitter

JXP

JXP

SG2-ABJforwardA/Bjumper

Upconverter

SG2-BCU/*

JXP

JXP EQ

Amplifier

Ingress

JXP EQ

Amplifier

Ingress

Status monitor

Switch

TP

-0.5

SG2-RPM/S

SG2-RPLPF

LP filter

LP filter

-3.5

-2.0

-2.0 -1.5

-1.0

-0.5-2.0

-0.5-0.5

JXP EQ

Amplifier

-1.0

-1.0

+18.5

-8.0

-2.0@ Min atten

+15 Lo+21.5 Hi

Power supply

TP

TP

Power supply

Ingress

LP filter

SG2-RPM/C

SG2-RPLPF

-3.5 -3.5-3.5+8.5

-2.0

Ingress control

HPfilter

-1.5

-1.0

All forward and return path test points (TP) are -20dB

Figure 3-35 SG2000 Block Diagram



AM Fiber Network Performance

Major Contribution to Noise and Distortion in AM System

Receiver

Processor

CombGenerator

SignalProcessing

SignalCombining

OpticalSplitter Fiber

Laser ChirpLaser RINLaser LinearityTransmitter Amplifier Performance

OpticalNode

RFoutput

Photodetector Linearity ResponsivityQuantum NoiseThermal NoiseReceived Optical PowerReceiver Amp Performance

Raleigh ScatteringChromatic DispersionPolarization Mode DispersionBrillouin Scattering

NonlinearComponents

HEADEND

PostAmplification

Figure 3-36 AM System Performance Criteria-Major Contributions to Noise and Distortion

• Overall link C/N performance affected by:

Laser relative intensity noise

Receiver quantum and thermal noise

Receiver photodetector responsivity

Received optical power

• Overall link CTB, CSO determined by:

Transmitter amplifier performance

Laser linearity

Photodetector linearity

Receiver amplifier performance

• Headend post amplification noise and non-linearities (C/N, CSO, CTB)

• Laser or modulator noise and non-linearities (C/N, CSO, CTB)

• Laser optical frequency stability “chirp” -- spreading of optical bandwidth within laser cavity.



• Fiber and coupler non-linearities:

Chromatic Dispersion- temporal spreading of multiple optical wavelengths.

Polarization modal dispersion- temporal spreading of polarizations.

Stimulated Brillouin scattering- energy transfer with feedback.

Raleigh scattering- due to imperfections in the fiber.

• Receiver and Photodiode noise and non-linearities (C/N, CSO, CTB).

Optimizing System Noise and Distortion Performance • C/N performance can be traded for CTB/CSO performance.

• 1 dB decrease in carrier levels decreases the C/N by 1 dB and improves CTB by 2 dB and CSO between 1 and 2 dB.

• 1 dB increase in carrier levels improves the C/N by 1 dB and degrades the CTB by 2 dB and CSO between 1 and 2 dB.

• Adjustment ranges are limited to +2 dB to -4 dB relative to C/N performance (i.e. -4dB CTB/CSO).

• Receiver RF output level is affected by these changes and by the changes in the modulation index.

Headend Contribution in AM Fiber System Performance • CTB and XMOD generated in headend adds with fiber performance at 20 Log rate.

• CSO adds at some rate between 10 and 20 Log.

• C/N performance degrades at 10 Log rate.

• Must maintain 10 dB better headend C/N for 1/2 dB impact to fiber (i.e., 60 dB C/N + 50 dB C/N = 49.6 dB C/N).

• Must maintain 24 dB CTB/CSO/XMOD for 1/2 dB impact to fiber (i.e., 89 dB + 65 dB = 64.6 dB).



Combining AM Fiber and RF Plant Performance • C/N adds on a power basis

• CTB and XMOD add on a voltage basis

• CSO adds somewhere between power and voltage basis

• Combining formulas are same ones used to combine trunk amps, bridgers, and line extenders:

+−=

+−=

+−=

−−

−−

−−

20CTB

20CTB

Total

15CSO

15CSO

Total

10N/C

10N/C

Total

RFFiber

RFFiber

RFFiber

1010log20CTB

1010log15CSO

1010log10N/C

Cascading Fiber Links • Fiber links cascade just like amplifiers.

+−=

+−=

+−=

−−

−−

−−

20CTB

20CTB

Total

15CSO

15CSO

Total

10N/C

10N/C

Total

2Fiber1Fiber

2Fiber1Fiber

2Fiber1Fiber

1010log20CTB

1010log15CSO

1010log10N/C

• Independent of laser type and laser modulation techniques.



CNR/CTB/CSO Addition

Addition of Two CTB Values - Example

dB47.59

1010log20CTB

dB69CTBdB63CTB

2069

2063

Total

2

1

=

+−=

==

−−

Or from chart on page 52 of Broadband Reference Guide RD-20

dB 59.473.53-63 Therefore;dB 3.53levelhigher toadded dB

db 6levels between Difference

===

The same result can be approximated from the following graph.

0.10

1.00

10.00

0 5 10 15 20 25 30 35 40

Difference between signals (dB)

dB a

dded

to h

ighe

r lev

el

10 log

15 log

20 log

Figure 3-37 CNR/CTB/CSO Addition



Performance Testing of CNR, CTB, and CSO • When performance testing, several channels must be measured and averaged over several

bands (low, mid, and high) due to variances in the slope of the distortions.

• CTB degrades in mid-band

• CSO degrades in upper and lower band

RelativePower

Frequency MHz

CNRCNR

CTBCTB

CSOCSO

55 110 220 450 550 750

Low Mid High

Figure 3-38 Performance Testing of CNR, CTB, and CSO



Impact of Noise on AM Fiber Optic Links • Noise poses fundamental operating limits with AM Optical links

• Every component, active and passive, adds noise to the AM system

• Cost of the AM system is determined by its noise performance

• Depth of modulation versus link noise determines C/N performance

C/N Limits Due to Laser RIN • Laser optical noise (RIN):

RIN is the ratio of random fluctuations of optical power to average optical power. Laser RIN limits the maximum C/N that a system can achieve. RIN deteriorates whenever reflections exist in the optical path. Optical isolators and fusion splicing minimize reflection-induced RIN.

• Laser RIN constant over path length

0 5 10 15 20 25 30 35

70

65

60

55

50

45

RIN = -160 dB/Hz

RIN = -155 dB/Hz

RIN = -150 dB/Hz

Path length in Km

C/N

Rat

io in

dB

YAG

DFB

FP

Figure 3-39 C/N Limits due to Laser RIN



C/N Limits due to Shot Noise

Detector Responsivity

• Ratio of detected photocurrent to incident optical power

• Measured in amps per watt

• Responsivity is limited to 1.046 A/W at 1310 nm, 1.248 A/W at 1550 nm

• Typical responsivity is .80 to .95 A/W

Shot Noise

• Shot noise due to discrete nature of electrons

• Shot noise limits best C/N that can be achieved at a given optical input power

• C/N output will drop 1 dB for each 1 dB drop in received optical power

Path length in Km

C/N

Rat

io in

dB

0 5 10 15 20 25 30 35

68

63

58

53

R = 1.046 A/W R = 0.950 A/W

R = 0.900 A/W

R = 0.850 A/W

Figure 3-40 C/N Limits Due to Shot Noise



Detector Amplifier Noise

• Gain stage required after photodiode for useful levels

• Gain stage adds noise to incoming signal

• Converts input current to output voltage; i.e., transimpedance amplifier

• Transimpedance value (measured in Ohms) impacts receiver's noise performance

0 5 10 15 20 25 30 35

80

75

70

65

60

55

Path length in Km

NF = 7.5 dB

NF = 6 dB

NF = 3 dB

NF = 4.5 dB

Figure 3-41 C/N Limits Due to AMP Noise



Limits to C/N • C/N is limited by the RIN of a laser.

• When increasing the optical path length, hence decreasing the received power, shot noise dominates. The C/N degrades 1 dB for every 1 dB drop in received power.

• Beyond the shot noise limit, the C/N degrades 2 dB for every 1-dB drop in received power due to the receiver noise.

Increasing Optical Path Length (dB)

C/N

(dB

) Receiver NoiseLimited

2:1

RIN Limited Shot Noise Limited 1:1

Figure 3-42 Limits to C/N

Link C/N Performance Summary • All of the preceding contributors combine to establish the final link C/N.

• The practical net link C/N that can be achieved for a typical AM optical link alone is about 51 to 53dB



Exercise

Optical Transmitter Optical Receiver

RF Input:+15 dBmV flat (with

77 NTSC chs 50-550 MHz

and digital signals550-750 MHz)

Optical Output:+4 dBm (2.5 mW)

Fiber Cable Length:approx. 11 kmAttenuation:approx. 4 dB

Optical Input:

_____ dBm

RF Output:

______ dBmV

RF T.P:

_____ dBmV

BTN-M NodeLaunch

Amplifier

Operational Gain: 20 dB

Node RF Output:

_____ dBmV

Reserve Gain:

_____ dB


Section 4 Coax Plant Maintenance

After completing this section, you will have an understanding of:

RF amplifier configuration

• Mechanical issues

• Basic amplifier components

• Unity gain concept

RF amplifier bench testing

• Use of the bench sweep

• Types of bench sweep equipment

Sweep and balance/maintenance

• Powering up the network

• Meter balance

• System sweep equipment

• What the sweep tells us

• Sweep problems and the effects

• Return (upstream) set-up/troubleshooting

4-2 Coax Plant Maintenance


RF Amplifier Configuration

Mechanical Issues

Housing Integrity

Use of proper torque in closing equipment housings provides RF seal to minimize egress and ingress (signal leakage) and minimizes the possibility of water entry into housings.

See the following figures for typical Motorola equipment torque and bolt tightening sequence. All manufacturers provide this type of information for their equipment.

Torque in thesequence shownto 5-6 ft-lbs

Figure 4-1 BLE-75

Coax Plant Maintenance 4-3


Torque in thesequence shownto 10-12 ft-lbs

Figure 4-2 MB-HSG

Torque in thesequence shown to10-12 ft-lbs.

Figure 4-3 BTN/BTD



The following table lists the maximum torque for Motorola equipment.

Equipment Model Number Max. Torque (ft-lbs)

SJ-series housing STH-7 8 X-series housing XHSG 15 SX-series housing SXHG 15 MB-series housing MB-HSG 12 JLX, JLC, JLE housings JLX-HSG 5 BTD-series housing BTA-SXHG 12 BLE-series housing BLE-HSG 6 Aperture plugs All 5 Passives and taps SSP, FFT 3

Module Integrity

All module hold down screws must be tight for proper electrical operation and for proper heat dissipation to the housing and then to the outside world.

If the unit uses heat sink compound, care must be exercised to ensure that the surfaces are kept clean of any dirt, grit, etc., as this would defeat the purpose of the compound. To keep them clean, spare modules that are stored in a service vehicle should be placed in plastic bags.

Newer equipment has many hybrid integrated circuits that dissipate a large amount of heat/power, therefore proper heat sinking is a must!

Module Cover Integrity

The cover is an integral part of the amplifier’s electrical circuit. All of the cover screws must be in place and tight. If not feedback may occur from the output of the amplifier to the input. This will cause amplitude versus frequency variations or oscillation, which can damage the amplifier and/or cause distortion in the TV signals being delivered by the system.



A loose cover may couple some of the signal from the output of the amplifier to the input. Since the gain of the amplifier is greater than one, the circuit may oscillate.

Less than 35 dB of loss(coupling path from loose cover)

35 dB.

of Gain

Figure 4-4 Oscillation Diagram

An amplifier can be set-up and aligned with the cover totally removed. Then when the cover is replaced and properly tightened, the response will be virtually unchanged.



Basic Amplifier Components All amplifiers have basic components as shown in the following block diagram of a typical line extender amplifier. This line extender is equipped for two-way operation with the forward signal path and return signal path separated via diplex filters.

The following paragraphs describe the function of each of the major components.

ManualAutomatic

(Option)

LevelControl

Level

EQ-*

PAD

Preamp

PAD

BODE

InterstageEqualizer

PostAmplifier

16

TP20 dB

TP20 dB

input

H

L

DiplexFilter output

H

L

DiplexFilterReturn amp kit

(Option)

TP20 dB

JXP-*A

SEE-*

JXP-*A

TP20 dB

(Option)

LCB-5 orauto. fuse

24V

PowerSupplyPower

stop

Figure 4-5 Line Extender Amplifier Block Diagram



Diplex Filters

The diplex filters separate the forward and return signal spectrums and directs them to the correct path in the amplifier.

ManualAutomatic Level

Control

Level

EQ-*

PAD

Preamp

PAD

BODE

InterstageEqualizer

PostAmplifier

16

TP20 dB

TP20 dB

input

H

L

DiplexFilter output

H

L


(Option)

TP20 dB

JXP-*A

SEE-*

JXP-*A

TP20 dB

(Option)

LCB-5 orauto. fuse

24V

PowerSupplyPower

stop

Figure 4-6 Diplex Filter Diagram



The following figure depicts the frequency response of a typical diplex filter.

H(Hi-pass)

L(Lo-pass)

5 MHz 40 MHz

Typical insertion loss inthe 5 - 40 MHzspectrum is 40 - 50 dB.

54 MHz 750 MHz

Typical insertion loss inthe 54 - 750 MHzspectrum is > 1 dB.

5 MHz 40 MHzTypical insertion loss inthe 5 - 40 MHzspectrum is > 1 dB.

54 MHz 750 MHzTypical insertion loss inthe 54 - 750 MHzspectrum is 40 - 50 dB.

Common

Figure 4-7 Diplex Filter Frequency Response Diagram



Input and Output Test Points

Input and output test points are typically at -20 or -30 dB. Most amplifiers now have directional coupler test points. This type of TP will have an accuracy of ~ ±1 dB. Resistive or transformer type test points, have an accuracy of ±2 dB at best.

Amplifiers have separate test points for forward input and output, return input and output, or some combination of these.

ManualAutomatic

(Option)

LevelControl

Level

EQ-*

PAD

Preamp

PAD

BODE

InterstageEqualizer

PostAmplifier

16

TP20 dB

TP20 dB

input

H

L

DiplexFilter output

H

L


(Option)

TP20 dB

JXP-*A

SEE-*

JXP-*A

TP20 dB

(Option)

LCB-5 orauto. fuse

24V

PowerSupplyPower

stop

Figure 4-8 Input and Output Test Points



Equalizer and Pad

The equalizer compensates for the input level tilt caused by the variation in attenuation of cable with frequency. Refer to the following figures for the detail of an equalizer’s function.

The pad at the amplifier input is used to establish the proper operating input to the amplifier based on the system design.

ManualAutomatic

(Option)

LevelControl

Level

EQ-*

PAD

Preamp

PAD

BODE

InterstageEqualizer

PostAmplifier

16

TP20 dB

TP20 dB

input

H

L

DiplexFilter output

H

L


(Option)

TP20 dB

JXP-*A

SEE-*

JXP-*A

TP20 dB

(Option)

LCB-5 orauto. fuse

24V

PowerSupplyPower

stop

Figure 4-9 Cable/Equalizer Function Diagram

Equalizers for Motorola equipment are available in 2 dB steps from 0 to 30 dB for frequency limits from 350 through 550 MHz. For 750 MHz and above, equalizers are in 1 dB steps from 2 to 22 dB. A 0 and 1 dB equalizer is available for all frequencies. Pads are available in 0.5 dB steps from 0 to 26 dB.



Note that the equalizer creates the exact opposite response as the cable except for the 1 dB insertion loss at the high frequency.

The net result of this is a flat response across all frequencies at a total attenuation that is 1 dB greater than the cable loss at the high frequency.

Figure 4-10 Cable/Equalizer Function



Pre-amp Stage and Interstage Pad

Most of today’s amplifiers have 2 or 3 stages of gain. These gain stages are manufactured by a number of third party vendors and all cable equipment manufacturers use these vendors. These hybrid integrated gain stages typically have individual gains of between 16 and 20 dB dependent upon their application

The input or pre-amp stage will be designed for best noise figure and a compromise on output level capability. The interstage pad that follows, if the amplifier has one, is used for certain special operating conditions or temperature compensation.

ManualAutomatic

(Option)

LevelControl

Level

EQ-*

PAD

Preamp

PAD

BODE

InterstageEqualizer

PostAmplifier

16

TP20 dB

TP20 dB

input

H

L

DiplexFilter output

H

L


(Option)

TP20 dB

JXP-*A

SEE-*

JXP-*A

TP20 dB

(Option)

LCB-5 orauto. fuse

24V

PowerSupplyPower

stop

Figure 4-11 Pre-amp Stage and Interstage Pad



Bode Equalizer

The Bode equalizer that follows the pre-amp is a special slope/gain control circuit that is driven manually, thermally, or via an automatic level controlling circuit. The Bode is designed to vary in attenuation the opposite to the change in attenuation of cable versus temperature.

ManualAutomatic

(Option)

LevelControl

Level

EQ-*

PAD

Preamp

PAD

BODE

InterstageEqualizer

PostAmplifier

16

TP20 dB

TP20 dB

input

H

L

DiplexFilter output

H

L


(Option)

TP20 dB

JXP-*A

SEE-*

JXP-*A

TP20 dB

(Option)

LCB-5 orauto. fuse

24V

PowerSupplyPower

stop

Figure 4-12 Bode Equalizer



The following figure shows the variation in attenuation versus frequency versus temperature for a span of cable that has 25 dB of loss at 750 MHz. The temperature variation is ±50º F. Note that the variation at high frequencies is greater than at lower frequencies.

Figure 4-13 25 dB of Cable vs. Temperature Change



The following figure shows the level variations that the amplifier would experiences for the same span of cable, assuming it was equalized at the nominal temperature.

Note that the level variations at 750 MHz are almost 3 dB across the total temperature swing. This, of course, would multiply as the signals continue through additional amplifiers if it is not compensated for.

Figure 4-14 Cable/Equalizer Function vs. Temperature



The action of the Bode when the cable attenuation increases with an increase in temperature is to decrease its insertion loss in the opposite direction from the cable with the net result of no change in levels.

0 50 150 250 350 450 550 650-5

-4-3

-2

-1

0

1

2

3

4

5

Frequency - MHz

Ref

eren

ce (d

B) Bode Hot

Cable Attenuation Hot

750

Figure 4-15 Bode Board Equalizer, Temperature Increase



The action of the Bode when the cable attenuation decreases with a decrease in temperature is to increase its insertion loss in the opposite direction from the cable with the net result of no change in levels.

0 50 150 250 350 450 550 650-5

-4-3

-2

-1

0

1

2

3

4

5

Frequency - MHz

750

Bode Cold

Cable Attenuation Cold

Ref

eren

ce (d

B)

Figure 4-16 Bode Board Equalizer, Temperature Decrease



Interstage Equalizer

The interstage equalizer typically establishes most of the output operational level tilt that is common in today’s amplifiers.

A typical interstage equalizer may provide equalization for about 13.5 dB of cable at 750 MHz and create 10 dB of interstage tilt between 50 and 750 MHz. Different amplifiers may have different amounts of interstage equalization/tilt.

ManualAutomatic

(Option)

LevelControl

Level

EQ-*

PAD

Preamp

PAD

BODE

InterstageEqualizer

PostAmplifier

16

TP20 dB

TP20 dB

input

H

L

DiplexFilter output

H

L


(Option)

TP20 dB

JXP-*A

SEE-*

JXP-*A

TP20 dB

(Option)

LCB-5 orauto. fuse

24V

PowerSupplyPower

stop

Figure 4-17 Interstage Equalizer Diagram

Tilted output levels provide an improvement in the amplifier distortion performance and allow for flatter levels to be available at cable customers’ television sets.

In some cases, flatness controls are incorporated in the interstage equalizer or on a separate board (shown in following figure) also located between the pre-amp and output stage(s).

Figure 4-18 Flatness Board



The typical effects of controls incorporated into a flatness control circuit are shown in the following figure. Some coupling between controls is normal. Additionally, care should be taken when adjusting the narrow frequency controls that effect the low frequency end.

These adjustments should not be changed unless a quality sweep signal is being observed. Do not adjust them using a signal level meter!

Figure 4-19 Effect of Flatness Controls



Post Amplifier

The output or post-amplifier stage is designed for best output level capability and a compromise on noise figure. As stated earlier, most of today’s amplifiers have 2 or 3 stages of gain. These gain stages are manufactured by a number of third party vendors and all cable equipment manufacturers use these vendors. These hybrid integrated gain stages typically have individual gains between 16 and 20 dB dependent upon their application.

ManualAutomatic

(Option)

LevelControl

Level

EQ-*

PAD

Preamp

PAD

BODE

InterstageEqualizer

PostAmplifier

16

TP20 dB

TP20 dB

input

H

L

DiplexFilter output

H

L


(Option)

TP20 dB

JXP-*A

SEE-*

JXP-*A

TP20 dB

(Option)

LCB-5 orauto. fuse

24V

PowerSupplyPower

stop

Figure 4-20 Post Amplifier



Level Correction

Various types of level correction are typically provided as options. These methods include those described below.

1 Open-loop, thermal control, which adjusts amplifier gain/slope by sensing housing temperature and correcting for fixed cable loss values. This method is less accurate than AGC and is not recommended in areas of the network where amplifiers are installed in a different thermal environment than the cable.

2 Closed-loop, AGC/ALC control, which samples the level of a single or group of channels at the output of an RF amplifier detects them and corrects the preset level by control of interstage electronic attenuator (bode) to reduce or increase gain/slope as needed. The channel used for this type of control must be a standard NTSC analog channel, not scrambled, or digital.

3 Thermal pads, which add or remove flat attenuation to compensate for gain changes in the hybrid gain stages over temperature.

ManualAutomatic

(Option)

LevelControl

Level

EQ-*

PAD

Preamp

PAD

BODE

InterstageEqualizer

PostAmplifier

16

TP20 dB

TP20 dB

input

H

L

DiplexFilter output

H

L


(Option)

TP20 dB

JXP-*A

SEE-*

JXP-*A

TP20 dB

(Option)

LCB-5 orauto. fuse

24V

PowerSupplyPower

stop

Figure 4-21 Level Correction Diagram



Return Amplifier Kit

An optional Return Amp Kit is typically available to be placed in the amplifier in order to activate the return signal path. It is comprised of a Hybrid integrated gain stage and accompanying pads and equalizer.

The basic function of these components is similar to those in the forward path except for their frequency range that today is typically 5 to 40 MHz.

You will note that the equalizer and a pad follow the Gain stage in the return path versus being in front of the Gain stages in the forward path. The reason that this is necessary will be covered later in this course.

ManualAutomatic

(Option)

LevelControl

Level

EQ-*

PAD

Preamp

PAD

BODE

InterstageEqualizer

PostAmplifier

16

TP20 dB

TP20 dB

input

H

L

DiplexFilter output

H

L


(Option)

TP20 dB

JXP-*A

SEE-*

JXP-*A

TP20 dB

(Option)

LCB-5 orauto. fuse

24V

PowerSupplyPower

stop

Figure 4-22 Return Amp Kit



Power

ManualAutomatic Level

Control

Level

EQ-*

PAD

Preamp

PAD

BODE

InterstageEqualizer

PostAmplifier

16

TP20 dB

TP20 dB

input

H

L

DiplexFilter output

H

L


(Option)

TP20 dB

JXP-*A

SEE-*

JXP-*A

TP20 dB

(Option)

LCB-5 orauto. fuse

24V

PowerSupplyPower

stop

Figure 4-23 Power Diagram



Mini-Bridger

This is a type of amplifier that is common in systems today, differences are:

• Two separate full level output stages/ports, able to drive 2 distribution cables. In some cases one of these outputs is configured to operate at a lower level and drive a short cascade of Trunk amplifiers.

• An optional plug in Split/DC-10 that allows one of these outputs to drive 2 cables at reduced level. Therefore a total of 3 outputs.

• A total of 3 Gain stages are required to make up for the additional loss of the 2-way splitter, the loss of the JXP-Therm, and the fact that the amplifier has a greater operational gain specification.

• Powering for this amplifier is similar to the basic amplifier (fused on the input and optional fuse or circuit breaker on each of the possible distribution outputs).

Figure 4-24 Mini-Bridger Amplifier Block Diagram (MB-750D-H)



A newer version of the Mini-Bridger type of amplifier.

• Similar to MB-750 D-H, with various improvements.

• Accommodates Status Monitor.

• Directional coupler TPs for forward input and return output.

• JXP facility for both return input legs.

___ dBmV required

Use See equalizerand JXP pad for

flattest response andcorrect level

H

L

Return amp

H

L

H

L

Pre-amp MDR

MBD

ADU/TDU

10TP -20 dB-0.5 dB loss

Use 25 dBgain

hybrid

RA - kit

JXP

Status Moniterout and-20 dB

return in TP

-3.5 dB loss

JXP

JXPSee-0.5 dB loss

MBD-DC

MBD-SPLT

TP-20 dB

Feederoutput port 3

Feederoutput port 4

Diplex FilterJXP

Drive levelMan

Drivecontrolselectjumper

Drive unit

MainGain

Diplex Filter-0.5 dB loss

Inport port 1-0.5 dB loss

TP-20 dB

EQ

JXP

Thermal comp

Interstageamp

JXP

Feederdoubler

JXP

Expressfeederdoubler

Diplex Filter-0.5 dB loss

TP-20 dB

Express feederoutput port 2

-0.5 dB loss

Statusmoniter in

JXP

Figure 4-25 MB-75SH* Amplifier Diagram

A third type of amplifier that is common in systems today. The differences are:

• Four separate full level output stages/ports, able to drive 4 distribution cables. In some cases one of these outputs is configured to operate at a lower level and drive a short cascade of Trunk amplifiers. This amplifier is ideal for use in dense metropolitan areas with high homes passed per mile.

• Also has a total of 3 Gain stages for the same basic reasons as the Mini-Bridger amp.



See the fusing diagram for powering options typical of this type of amplifier.

Output

Output

Flatnessboard

RET.AMPL

Equalizerfacility

Equalizerfacility

ReturninRET OUT

JXP

RA-Kit

JXP

JXP

JXP

JXP

FWD OUT

FWD 3

FWD 4

FWD 1

JXP-THERM

TP-20 dB

TP-20 dB

TP-20 dB

TP-20 dB

TP-20 dB

TP-20 dB

TP-20 dB

OUT

Port 3

Port 4

Port 1

INH

L

Preamp BODE

ADU

MDR-750/*

JXP

FWD IN

JXP

MID

-16

-16 H

L

H

L

H

L

H

L

JXP

JXP

RET IN

RET 3

JXP

JXP

RET 4

RET 1

Statusmonitor

Output

Output

-16

Figure 4-26 BTD Amplifier Block Diagram



F3 F4 Port 3Port 1

F5 Port 4

F6 Out

F7Port 2

F2

DCPowerSupply

F1In

Surge Suppressoror

Electronic Crowbar

Figure 4-27 BTD Fusing Diagram

FUSE FUNCTION RATING

F1 Passes AC to or from IN port 10A

F2 DC power supply fuse (always required). BCB-5 (Circuit Breaker) optional at this location. Also used to disconnect DC power from amplifier module.

5A

F3 Passes AC to or from Port 1 10A



F6 Passes AC to or from OUT Port 10A

F7 Always required except when power from AC input (Port 2) is to be blocked at this location.

10A



Unity Gain Concept The following group of figures graphically describes the Unity Gain Concept, first with Flat operating levels through a Basic Amplifier to show the simplest concept. Tilted operating levels are next presented in the Basic Amplifier to demonstrate that Flat or Tilted works the same. The C/N problem of this approach is pointed out. Finally a Real Amplifier is shown with Interstage equalization and Tilted operating levels to demonstrate how this approach corrects the C/N issue, while still retaining the advantage of the Tilted output Operating levels.

Unity Gain concept: The amplifier compensates for the loss or attenuation of the cable and passive equipment that precedes it.

Output30 dBmV

Output30 dBmV

CoaxLoss 20 dB

Input 10 dBmV

20 dBGain

Figure 4-28 Basic Amplifier Unity Gain



This is a graphical representation of the Operating Levels and the Gains and Losses of a Unity Gain section of a system.

The bottom graph shows the Gain or Loss of each part of the system. The upper graph shows the operating level at that point in the system after taking into account the impact of the preceding part of the system.

Since all of the parts can not be shown in one figure, they are continued in following figures.

This first figure shows the levels starting Flat @45 dBmV at the output of an Amplifier and progressing through a two-way split and a length of cable to the input of a second Amplifier. It then shows the impact of a proper Equalizer and is continued on the next figure.

10

20

30

40

50

-30

-20

-10

0

10

20

30

50MHz

750MHz 50

MHz

Am

p. O

utpu

tLe

vel

45 dBmv flat

750MHz

+41dBmvflat

+20dBmv

Mhz

50MHz

+36 dBmv

Amplifier

Loss

/Gai

n in

dB

Leve

l in

dBm

v

50MHz

750MHz

-4dB flat

Loss

of 2

-way

Sp

lit Loss

of C

able

Loss

of E

Q

750MHz

50MHz

-21 dB50

MHz

750MHz

-1 dB

Leve

l Fol

low

ing

Cab

le

Leve

l Fol

low

ing

2-W

ay S

pit

1130 ft.

QR-540 CableEQ-* PAD

2 Way

Split

Preceding

Amp.

-5dB

-17 dB

750

Figure 4-29 Basic Amplifier, Flat Output Levels



This figure continues at the Amplifier input and shows how the Equalizer compensates for the slope of the cable, creating flat levels that are then attenuated by the Pad to provide the proper input to the Amplifier Gain Block.

The gain of the Gain Block brings the levels back up to the Flat 45 dBmV operating level at the output of the Amplifier. This complete process is called the Unity Gain Concept and is the basis for most Cable System operation.

E.Q-* PAD* GainBl’k

-30

-20

-10

0

10

20

30

10

20

30

40

50

50MHz

750MHz

+20 dBmv

+19 dBmvflat

50MHz

750MHz

50MHz

50MHz

750MHz

750MHz

+45 dBmvflat

Cable

50MHz

750MHz

29 dBflat

50MHz

750MHz

-17dB

-1dB 50MHz

750MHz

-3dBflat

Leve

l Fol

low

ing

Cab

le

Loss

of

EQ

Leve

l Fol

low

ing

EQ.

Leve

l Fol

low

ing

Leve

l Fol

low

ing

Gai

n Bl’k

(Uni

ty G

ain)

Amplifier

Loss

/Gai

n in

dB

Leve

l in

dBm

v +36dBmv

Loss

of

Pad

+16 dBmvflat

29 dB

Gai

n of

Bl’k

Figure 4-30 Basic Amplifier, Flat Output Levels Continued



This and the next figure depict the same process as the first two. The only difference is that the Amplifier operating output levels have 10 dB of tilt between 50 MHz and 750 MHz with the level at 750 MHz equal to 45 dBmV.

You will note that the input to the amplifier now only has 6 dB of tilt versus the 16 dB of tilt that it had in the Flat case.

Figure 4-31 Basic Amplifier, Tilted Output Levels



As we follow the levels through the Amplifier, note that following the Equalizer there is 10 dB of tilt between 50 MHz and 750 MHz and that at the final output the levels are back to 45 dBmV at 750 MHz with 10 dB of tilt for unity gain.

The actual signal level going into the Gain Block at 750 MHz is the same as it was for the Flat case, but at 50 MHz the signal level going into the Gain Block is 10 dB lower.

This would cause the C/N contribution of the Amplifier to be 10 dB worse at 50 MHz versus at 750 MHz, which is not good.

Figure 4-32 Basic Amplifier, Tilted Output Levels Continued



We will now present the Tilted output Levels through a Real Amplifier that includes a Pre-amplifier gain stage, an Interstage Equalizer, and an output gain stage.

Figure 4-33 Real Amplifier, Tilted Output Levels



As we progress through the Real Amplifier it will be noted that the Equalizer that is at the input of the amplifier only compensates for 6 dB of cable slope which causes the Tilted levels to be Flat at the input of the Pre-amplifier gain stage. Additionally the gain of this stage is only 17dB.

Since the Pre-amplifier is the primary contributor to establishing C/N the resultant C/N will be the same across all frequencies.

Figure 4-34 Real Amplifier, Tilted Output Levels Continued



• Between the two gain stages is a Bode equalizer that changes its insertion loss the opposite of what cable changes with temperature. The Bode is shown at the mid-point of its operating range where it is nominally flat.

• Next we have the Interstage Equalizer that introduces an additional 10 dB of slope between 50 MHz and 750 MHz. This along with the equalizer at the input of the amplifier totally compensates for the cable that precedes the amplifier.

• The original 10 dB of operational tilt is now restored and the output Stage brings the levels back to 45 dBmV with 10 dB of tilt operating level for our Unity Gain.

• Since the input levels to the output stage are high the impact on C/N caused by tilted levels at this point is minimal.

• The additional advantage gained by having tilted output levels is an improvement in the amplifier distortion performance and the tilted levels allow for flatter levels to be provided to cable customers’ television sets.

17 dBInterstage

EQ.18 dB

Bode

Preamp

Output

Stage

Loss

/Gai

n in

dB

Leve

l in

dBm

v

10

20

30

40

50

50MHz

750MHz

+33 dBmvflat

+28 dBmvflat

50MHz

750MHz

50MHz

50MHz

750MHz

750MHz

+45 dBmv

Leve

l Fol

low

ing

Prea

mp

Leve

l in

dBm

v

+27 dBmv

Leve

l Fol

low

ing

Bod

e

Leve

l Fol

low

ing

Inte

rsta

ge

Leve

l Fol

low

ing

Out

put s

tage

(Uni

ty G

ain)

+35 dBmv

+17 dBmv

-30

-20

-10

0

10

20

30 50MHz

750MHz

+18 dBflat

750MHz

-1dB

Loss

/Gai

n in

dB

Loss

of

Inte

rsta

ge E

Q

-6 dB

-4 dB

Loss

of

Bod

e

-11dB50

MHz

Out

put S

tage

AMPLIFIER

-1 dB

-9dB-5 dB-5 dB

Gai

n of

Figure 4-35 Real Amplifier, Tilted Output Levels Continued



The previous figures dealt with the Unity Gain Concept assuming that the Amplifiers involved were identical and had the same output operating level. In some cases the output levels of the amplifiers may be different, such as an Optical node followed by Mini-Bridgers followed by a number of Line Extender amplifiers. Under these conditions the different amplifiers may have different output operating levels.

If this is the case the only difference is that the Gain of the next amplifiers may need to be adjusted up or down to compensate for the difference in levels. The concept is still logically the same.



RF Amplifier Bench Testing

Use of the Bench Sweep Bench Testing of amplifiers is recommended prior to their installation in the field. This will allow for the best system Amplitude versus Frequency response.

Care must be exercised when setting up the powering options within the amplifier in order to prevent damaging the test equipment. It is best to test the amplifier with a simulated length of cable and the passives that will precede it in the location it is to be placed in the field. In this way the amplifier can be pre-aligned with the proper equalizer, pads, and reserve gain required.

Figure 4-36 Test Equipment Connections for Bench Sweeping



Types of Bench Sweep Equipment There are two basic types of Bench Sweep test equipment available:

• Standard analog CW sweep as depicted above

• Newer Network Analyzer type sweep equipment

Figure 4-37 Typical Test Equipment and Connections



Sweep and Balance/Maintenance

Powering up the Network

• It is recommended that the system be powered up for at least 1 to 2 hours prior to aligning the amplifiers.

• Each amplifier should be pre-configured with the appropriate fuses/circuit breakers and the power option functions set per system design. The amplifiers then should be installed in their locations to allow for warm-up to operating temperature.

Standby Power Supply

• Most system outages are power related.

• In order to ensure the continued delivery of video and telephony services to the homeowner, standby power supplies with backup batteries are essential in case of a power outage. To match with telco’s 99.99% reliability (58 minutes/year), must have longer standby life.

• Periodic power supply/battery maintenance is essential.

Figure 4-38 Power Supply Example



Voltage Drop (Coaxial Cable)

Furnishing power to a number of amplifiers from a common source, the AC power supply, results in a voltage drop across the DC resistance of the coaxial cable consistent with Ohm’s Law. Various methods of calculating the voltage drop exist, and the appropriate method depends on the desired end result. The system designer needs to know where to locate the power supplies, and he is likely to perform calculations from the last active device to the likely proposed location. RF cable sizes may have to increase after detailed power design. A technician, on the other hand, may be interested in the correct voltage at a point in the system and he will calculate the drop starting from the power supply to the point of interest.

Voltage drop is calculated using Ohm’s law:

RIE ×= Where:

(Ohms) eresistenacR(Amperes) urrentcI(Volts) voltageE

===

Note: Detail of these calculations were covered in the Design Basics Course.



Figure 4-39 Fiber Backbone

Figure 4-40 Tree and Branch



Meter Balance

• Input Pad: Typically this pad value establishes the proper design input to the amplifier

• Equalizer: Start value may be selected by several methods.

Use value from system design.

Formula: output level - Input level - Passive loss = Cable

Cable - Interstage Equalizer = Fwd. Eq. Value

(All values above are in dBmV/dB at the system maximum frequency.)

Cable Tilt method described in Design Basics course (pages 6-44).

• It is important that the amplifier has proper Gain Reserve in order to be able to compensate for temperature variations.

System Sweep Equipment The following outlines the actual meter balance of a typical amplifier including an example.

Temperature Required Gain Reserve (dB)

ºF ºC 350 MHz 450 MHz 550 MHz 750 MHz

-20 -29 4.0 5.5 6.0 7.0

10 -12 3.5 4.5 5.0 6.0

40 4 3.0 3.5 4.0 5.0

70 21 2.5 2.5 3.0 4.0

95 35 1.5 2.0 2.0 3.0

120 49 1.0 1.5 1.5 2.0

BLE-75 SH/JH Set Up Procedure

Manual Set-Up

1 Verify and record AC/DC voltages. AC = 38 or 55 Vac rms or higher measured with a true rms voltmeter. DC = 24 V. ±0.4V. Verify amplifier powering is per the design.

Example: AC = 62.4 V.ac as measured on a non-true RMS meter.

AC = 55.5 V ac RMS after correcting for meter error by subtracting 11%(6.86 V).

DC = 23.9 V

Set powering for stop.



2 Input Test Point

a Measure and record RF input levels, determine input pad value to achieve design RF input level @750 MHz, and place in the Input pad JXP position.

Note: This should be done with an equalizer in place, see next step.)

Example: Measured level = 23 dBmV

Design level = 18 dBmV

Pad = 5 dB.

b Determine the forward equalizer value and place in forward equalizer location. (Refer to the below formula or start with EQ. value from design print.)

Example: Output level of +47dBmv. minus Input level of +23dBmv. minus 8.5dB. of passive loss = 15.5dB. of cable.

15.5dB of cable minus interstage eq. of 10dB. = 5.5dB. eq.

Use an EQ-750-6

3 Output Test Point

a Place the Auto/Man suitcase jumper in the Man./Top position (refer to page 7 in BLE manual) and adjust the manual level control to maximum (fully clockwise).

b Note the RF output level at 750 MHz, determine ambient temperature reserve gain number (page 11 or 13 in BLE manual or RD-17 page 65) and reduce the manual gain control by that amount.

Example: Measured output at 750 MHz. at full gain = +53 dBmV.

Reserve Gain @ 750 MHz.for 70 deg. = 4 dB

Output Level at 750 MHz. with gain reduced = +49 dBmV

c Determine the pad value required to achieve design output level @750 MHz and place in the midstage JXP position (should not be > 3 to 4 dB).

Example: Design output level = +47 dBmV

Pad value = 2 dB

FWD OUT T.P. level = +47 dBmV

d Verify the output operational tilt by measuring the level at Ch-2 versus the level @750 MHz (should be 10 dB), adjust forward equalizer value to achieve proper tilt, the manual control may be adjusted no more than ±0.5 dB if needed.

Example: FWD OUT = 37 / 47 dBmV

e If there is less than 5 dB of aerial cable @750 MHz and the ADU option is not to be used, the set-up is complete.



4 Thermal Drive Set-Up/Output Test Point (fter performing manual setup)

a Install the TDU board in the location noted. Move the Auto/Man suitcase jumper to the Auto/Bottom position. (Refer to pages 7,10, and 17 in BLE manual.)

b Place the suitcase jumper on the TDU board as outlined below. L or 10 dB for 5 to 15 dB of aerial cable @750 MHz. M or 20 dB for 15 to 25 dB of aerial cable @750 MHz. H or 30 dB for 25 to 35 dB of aerial cable @750 MHz.

c Check the RF output level @750 MHz and adjust the TDU board level control to match the level set in the Manual procedure above (design output level).

Example: FWD OUT T.P. level = 37/47 dBmV

5 Automatic Drive Setup/ Output Test Point (after performing manual setup.)

a Install the ADU board in the location noted. Move the Auto/Man suitcase jumper to the Auto/Bottom position. (Refer to pages 7 and 10 in BLE Manual.)

b Check the RF output level @750 MHz and adjust the ADU Auto Level control to match the level set in the Manual procedure above (design output level).

Example: FWD OUT T.P.level = 37 / 47 dBmV This completes the field alignment of the amplifier. Appropriate notations of all operating levels, pad values, powering conditions should be made at this time.

• OUTPUT LEVEL @750 MHz - INPUT level @750 MHz - PASSIVE loss = CABLE

• CABLE - INTERSTAGE EQUALIZER (10 dB) = FWD. EQUALIZER VALUE Note: These formulas assume that all amplifiers have the same operational output tilt.

• LDR-750/10: Interstage Equalizer/Response Board compensates for ~ 10 dB of cable @750 MHz that provides 7 dB of operational tilt, and also has response controls to adjust amplifier flatness.

• BCS-75-*, Cable Simulators should be used when < 10 dB of cable exists. (Refer to page15 and 16 of the BLE manual for more detail concerning the use of cable simulators.) Also the insertion loss of a BCS must be considered when selecting the input JXP value. A BCS insertion loss = Its value +1 dB.



Forward and Low System Sweep Original sweep was continuous analog and required that the programming signal be removed from the system. Not popular with subscribers.

High Level and Low Level analog were next. These could be used on a live system, with minimal interference to subscribers, but caused problems to data and have limited resolution.

Today’s system sweep equipment resolves most of these problems with certain limitations.

Today’s Sweep Methods

Both of these systems use momentary insertion of signals at various frequencies that are programmable by the user. This minimizes impact on both video and data carriers on the system.

They both continuously monitor the transmitted levels at the headend and forward correction data to the companion sweep receiver, via a data carrier. They can display signal amplitude data along with frequency response display.

In most cases the sweep receiver can also function as a signal level meter and a limited spectrum analyzer.

Wavetek

Calan

50 MHzTelemetry

Data

= measurementpoint

GB = Guard Band

Figure 4-41 Sweep Methods



Most systems today operate with tilted levels on the amplifier output.

750 MHz47dBmV

50MHz37dBmV

350 MHz41.3dBmV

200MHz39.1dBmV

550MHz44.1dBmV

Figure 4-42 10 dB Tilt 50 to 750 MHz Ideal Frequency Response

50MHz

750MHzDesired Response(Never Happens)

Actual Response

Figure 4-43 10 dB Tilt 50 to 750 MHz Typical Frequency Response



Normalized

This system sweep display has had this tilt Normalized out to show a Flat response.

Normalizing the response allows for better observation of the system peak-to-valley response.

The following four figures depict a number of common system response problems.

50 150 250 350 450 550 650 750-10

-8-6

-4

-2

0

2

4

6

8

10

Frequency - MHz

Reference(dB)

Figure 4-44 Typical System Normalized Response



High End

Possible causes:

• Water or moisture in cable/fittings.

• Incorrect or miss aligned equalizer, 550 MHz Eq in 750 MHz System.

• Amplifier match or flatness adjustments incorrect.

50 150 250 350 450 550 650 750-10

-8-6

-4

-2

0

2

4

6

8

10

Frequency - MHz

Ref

eren

ce (d

B)

Figure 4-45 High-End Rolloff



Low End

Possible causes:

• Loose center conductor screw, capacitive connection.

• Amplifier flatness adjustments incorrect.

• Cracked shield.

• Corrosion.

50 150 250 350 450 550 650 750-10

-8-6

-4

-2

0

2

4

6

8

10

Frequency - MHz

Ref

eren

ce d

B

Figure 4-46 Low End Rolloff



Ringing

Possible causes:

• Discontinuity caused by damaged cable or bad passive device X feet away from amplifier.

• Unterminated cable.

• If amplifier TP is not a directional coupler type this effect could be normal if there is a passive device less than several hundred feet from the amplifier.

50 150 250 350 450 550 650 750-10

-8-6

-4

-2

0

2

4

6

8

10

Frequency - MHz

Reference(dB)

Figure 4-47 Response Ringing



Possible system problems caused by improper alignment if a system sweep is not utilized.

May Not PassCTB & XMOD

May NotPass C/N

50MHz

550MHz

Figure 4-48 Improper Alignment

Figure 4-49 Forward Headend Sweep Injection Points Diagram



Any failure or problem in the forward system only effects those subscribers that are fed downstream from the problem location.

Headendor

Node(Municipal Water

Plant)

Figure 4-50 Typical Tree and Branch Forward System

Any ingress or other extraneous signal problem in the return system effects all subscribers no matter where they are located or where the problem occurs.

Headendor

Node(Municipal Sewer

Plant)

Figure 4-51 Typical Tree and Branch Return System



In the forward direction the pad and equalizer are located on the input side of the gain stages, but in the return direction the pad and equalizer are located following the output side of the gain stage. The following figures will explain why this occurs.

Figure 4-52 Typical Line Extender Block Diagram



As shown in the following figure, for the forward system, the different cable and passive losses in each leg are compensated by the pad and equalizer at the input of each amplifier.

= 2026 17 =

17

OP Gain = 35 dBIn = 10 dBmV Out = 45 dBmVAll Amplifiers

Total passive loss = 2.2 + 1.3 + 1.3 + 1.5 = 6.3 dBTotal cable loss = 1850 ft @ 1.5 dB per 100 ft = 27.7 dB

Total Loss @ 750 MHz = 34.0 dB

Leg #1



Leg #2

28 dB Equalizer1 dB pad

To Set up Leg #1

-12

10

=


To Set up Leg #2

Figure 4-53 Forward Gain and Losses @750 MHz



For the return system, however, the different cable and passive losses cannot be compensated for by the single set of pad and equalizer at the input of the return amplifier. The following figure shows that Leg #1 requires a 6 dB equalizer and a 16 dB pad, while Leg #2 requires a 2 dB equalizer and a 3 dB pad.

= 2026 17

17

10

OP Gain = 25 dBOut = 45 dBmV In = 20 dBmVAll Amplifiers



Leg #1



Leg #2


To Set up Leg #1


To Set up Leg #2

-12

Which Valuesdo We Use?

Figure 4-54 Return Gain and Losses @40 MHz, Pad and Input Equalizer



For this reason the padding and equalization are done at the output of the preceding amplifiers as shown in the following figure.

2026 17

17

OP Gain = 25 dBOut = 45 dBmV In = 20 dBmVAll Amplifiers



Leg #1



Leg #2

-12=

10

=


To Set up Leg #2


To Set up Leg #1

Figure 4-55 Return Gain and Losses @40 MHz, Pad and Eq. on Output



Return system set-up must be accomplished via some method of knowing the signal levels at the headend. This can be as simple as the technician at the headend with the radio link or a TV camera link as shown in the following figure.

Today most of the system sweep equipment manufacturers have options that allow for return level or sweep measurements that are transmitted to the field via a data carrier.

System

HL

HL

SweepTransmitter

CarrierGenerator

TV

LH

Modulator

Splitter

SpectrumAnalyzer

SweepReceiver

Headend

Camera

Figure 4-56 Return System Test Setup


Section 5 System/FCC Proof of Performance

Upon completion of this section you will have an understanding of network components and proof procedures.

5-2 System/FCC Proof of Performance


Network Components

CombGenerator

(50-550 MHz)Optical

Transmitter

Combiner

Combiner

Pad

Pad

Fiber

OpticalSplitter

OpticalNode

CoaxPlant

Headend(550-750 MHz)

Figure 5-1 Network Components

Optical Nodeor Coax Plant

Output TP

Tunable BandpassFilters

Receiver ModuleRF Output

Preamp(when needed)

RF SpectrumAnalyzer

Printer

VariableAttenuator

Figure 5-2 Testing System Components

System/FCC Proof of Performance 5-3


Figure 5-3 Major Components Location Diagram



FCC Customer Tap Proof of Performance

Total Test Points

• Proof of performance test points chosen shall be balanced to represent all geographic areas served by the cable system.

• At least one-third of the test points shall be representative of subscriber terminals most distant from the system input and from each microwave receiver (if microwave transmissions are employed), in terms of cable length.

Number of Physically Separate Plants (HE + AML Hubs)1 2 3 4 5 6 7 8 9 10 11 12 13

1-999 No Testing Required1,000 - 12,500 6 6 6 6 6 6 7 8 9 10 11 12 13

12,501 - 25,000 7 7 7 7 7 7 7 8 9 10 11 12 1325,001 - 37,500 8 8 8 8 8 8 8 8 9 10 11 12 1337,501 - 50,000 9 9 9 9 9 9 9 9 9 10 11 12 1350,001 - 62,500 10 10 10 10 10 10 10 10 10 10 11 12 1362,501 - 75,000 11 11 11 11 11 11 11 11 11 11 11 12 1375,001 - 87,500 12 12 12 12 12 12 12 12 12 12 12 12 1387,501 - 100,000 13 13 13 13 13 13 13 13 13 13 13 13 13

Total SubscribersServed

Figure 5-4 Total Test Points Required

Channels to Test The FCC requires testing of a minimum of four channels plus one additional channel for every 100 MHz (or fraction thereof) of cable distribution system upper frequency limit.

The following table lists the number of channels to be tested according to the upper frequency limit.

Upper Limit Channels to Test Upper Limit Channels to Test

101 – 216 MHz 5 500 – 600 MHz 9

216 – 300 MHz 6 600 – 700 MHz 10

300 – 400 MHz 7 700 – 800 MHz 11

400 – 500 MHz 8 800 – 900 MHz 12



Alternatively, the minimum number of channels to be tested can be determined from the number of activated channels on the system.

Number of Activated Channels Channels to Test

≤ 22 5

23 - 35 6

36 - 53 7

54 - 68 8

69 - 85 9

Aural Carrier Frequency The frequency of the aural carrier must be 4.5 MHz ±5 kHz above the frequency of the visual carrier at the output of the modulating or processing equipment of a CATV system and at the subscriber terminal.

Figure 5-5 Audio Carrier Frequency Criteria



Visual Carrier Signal Level Minimum visual signal level at the subscriber’s television is 0 dBmV. Additionally, the visual signal level as measured at the end of a 30 meter (100 foot) cable drop that is connected to a subscriber tap shall not be less than +3 dBmV.

100 Ft.

3 dBmV

Ø dBmV

Bond to PowerNeutral

Figure 5-6 Visual Signal Level Criteria



Visual signal level on each channel measured at the end of a 30 meter (100 foot) drop cable shall not vary more than 8 decibels within any 6 month interval which must include four tests performed in six-hour increments during a 24-hour period in July or August and during a 24-hour period in January or February.

Maintain 3 decibels (dB) of the visual signal level of any visual carrier within a 6 MHz nominal frequency separation.

Figure 5-7 24-Hour Stability Test

Maintain overall peak-to-valley of 10 dB of measured signal visual level on a CATV system of up to 300 MHz of cable distribution system upper frequency limit, with a 1 dB increase for each additional 100 MHz of cable distribution system upper frequency limit. Requirements detailed in the following table.

Maximum Peak-to-Valley (dB)

System Frequency Limit (MHz)

Maximum Peak-to-Valley (dB)

System Frequency Limit (MHz)

11 400 14 700 12 500 15 800 13 600 16 900

Figure 5-8 24-Hour Stability Test



Aural Carrier Signal Level The RMS voltage of the aural signal shall be maintained between 10 and 17 decibels below the associated visual signal level. This requirement must be met both at the subscriber terminal and at the output of the modulating and processing equipment (generally the headend).

For subscriber terminals that demodulate and remodulate the signal (for example, baseband converters), the RMS voltage of the aural signal must be maintained between 6.5 and 17 dB below the associated visual signal at the subscriber terminal

Figure 5-9 24-Hour Stability Test: Video/Audio Carrier Levels



In-Band Channel Response The amplitude characteristic shall be within a range of ±2 decibels from 0.75 MHz to 5.0 MHz above the lower boundary frequency of the cable television channel, referenced to the average of the highest and lowest amplitudes within these frequency boundaries.

• Prior to December 30, 1999, the amplitude characteristic may be measured after a subscriber tap and before a converter that is provided and maintained by the cable operator.

• As of December 30, 1999, the amplitude characteristic shall be measured at the output of the subscriber terminal (if operator provides set-top).

Figure 5-10 In-Channel Frequency Response Measurement Area for a Cable TV Channel



Carrier-to-Noise The ratio of RF visual signal level to system noise shall be as follows:

• From June 30, 1993 to June 30 1995, not less than 40 decibels.

• As of June 30, 1995, not less than 43 decibels.

The carrier-to-noise ratio (C/N) is a measure of the noise contribution of the distribution plant, independent of the noise present on the signal as received. It is defined as the ratio between sync peak power and RMS noise, measured in a 4 MHz bandwidth.

Figure 5-11 Carrier-to-Noise Requirements



Coherent Disturbances The ratio of visual level to the RMS amplitude of any coherent disturbances such as intermodulation products, second and third order distortions or discrete-frequency interfering signals not operating on proper offset assignments shall be as follows:

• The ratio of visual signal level to coherent disturbances shall not be less than 51 decibels for non-coherent channel CATV systems, when measured with modulated carriers and time averaged.

• The ratio of visual signal level to coherent disturbances which are frequency-coincident with the visual carrier shall not be less than 47 decibels for coherent channel cable systems, when measured with modulated carriers and time averaged.

Figure 5-12 Coherent Disturbance Diagram

Low Frequency Disturbances The peak-to-peak variation in visual signal level caused by undesired low frequency disturbances (hum or repetitive transients) generated within the system, or by inadequate low frequency response, shall not exceed 3% of the visual signal level.

Measurements made on a single channel using a single unmodulated carrier may be used to demonstrate compliance with this parameter at each test location.



Terminal Isolation Shall be not less than 18 decibels. In lieu of periodic testing, the cable operator may use specifications provided by the manufacturer for the terminal isolation equipment to meet this standard.

Shall be sufficient to prevent reflections caused by open-circuited or short-circuited terminals from producing visible picture impairments at any other subscriber terminal.

The following table lists the tap-to-tap isolation of the FFT-K series taps.

Tap to Tap Isolation 5-30 MHz 20 dB 30-750 MHz 25 dB 750-1000 MHz 20 dB



FCC Customer Tap Proof of Performance Following is the matrix of tests required by the FCC.

TEST RULE SPEC CHANNELS WHERE? WHEN EQUIPMENT NEEDED

AURAL CARRIER FREQUENCY

76.605(a)(2) +4.5MHz

±5kHz

7 for most systems

Headend & Subscriber Terminal

2x a year, max 7 months apart

Tuned Frequency Counter, Spectrum

Analyzer, or Cheetah

MIN VISUAL SIGNAL LEVEL

76.605(a)(3) 0dBmV or 3dBmV TAP

All Terminal or Tap Plus 30m Drop


SLM + Cable or Cheetah

VISUAL SIGNAL CHANGE

76.605(a)(4) Within 8dB within 6 months


2x a year, max 7 months apart +4 tests, 6 hours

apart in 24 hours


ADJACENT VISUAL SIGNAL

76.605(a)(4)(i) Within 3dB over 24 hrs.



apart in 24 hours


ANY VISUAL SIGNAL IN

BANDWIDTH

76.605(a)(4)(ii) Within 10dB for 300MHz +1dB

for each additional 100MHz



apart in 24 hours


AURAL CARRIER LEVEL

76.605(a)(5) -10 to -17 dB from VIS carrier

All Headend & Subscriber Terminal


apart in 24 hours


IN BAND CHANNEL RESPONSE

76.605(1)(6) ±2dB from 0.75 to 5.0MHz in

Channel

7 for most systems

After Tap & Before Set-top *


Demod, WVFRM Mon, Analyzer or

Cheetah

C/N RATIO 76.605(a)(7) 43dB 7 for most systems

Subscriber Terminal


SLM, Analyzer or Cheetah

COHERENT DIST (CTB,CSO,XMOD)

76.605(a)(8) 51dB (47dB) for HRC

7 for most systems

Subscriber Terminal


Spectrum Analyzer

TERMINAL ISOLATION

76.605(a)(9) 18dB All Subscriber Tap 2x a year, max 7 months apart

SLM, Spectrum Analyzer or Mfg.

Specs

LF DISTURBANCES

(HUM)

76.605(1)(10) 3% of VIS Level One, using a single

unmodulated carrier

Subscriber Terminal


SLM, Analyzer, Sweep or Cheetah

CHROMA DELAY 76.605(a)(11)(i) 170 nsec 7 for most systems

Headend Triennial Demod, WVFRM Mon, Vectorscope

DIFF GAIN 76.605(a)(11)(ii) ±20% 7 for most systems


DIFF PHASE 76.605(a)(11)(iii) ±10º 7 for most systems


SIGNAL LEAKAGE

76.605(a)(12) a) 15, b) 20 microvolts/meter

a) < 54MHz & >216MHz

b) 54-216MHz

3m from & 3m directly below

system components

Once a year, max 12 months apart

Field strength meter and horizontal dipole



Notes: 1. *Prior to December 30, 1999. As of December 30, 1999, at subscriber’s terminal.

2. Number of channels tested depends on 76.601(c)(2) which specifies tests be made on a minimum of 4 channels plus one additional channel for every 100MHz of upper frequency limit. For example, 216MHz systems - 5 channels: 35 channel systems with 36 as the high carrier - 6 channels; 300 to 400MHz - 7 channels.

3. Measurements to be performed at subscriber terminals may be done at any convenient test point in the system provided that data is included to relate the measured performance of the system as would be viewed from a nearby subscriber terminal.

4. Proofs should be made at six widely separated points for systems with > 1000 and < 12,501 subs: one additional point for each 12,500 subs or fraction; points balanced throughout all geographic areas with at least 1/3 representative of extremities. System taps can be used if data recorded relates to what viewer would see.



Testing Methods

Headend System RF Measurements The following test equipment is required to perform these tests:

• HP8591 or HP8593 Spectrum Analyzer, or equivalent

• Wavetek Tunable Bandpass Filters, or equivalent

• C6PA post amplifier, or equivalent

The RF level measurements will record the RF video and audio carrier levels of each C6M modulator channel in the headend at the headend system output test point. Also the carrier-to-noise (C/N) will be measured at this test point.

Note: If the channel employs sync suppression type scrambling equipment, it will be necessary to place the scrambler in the standby mode during these measurements. After both RF measurements have been obtained, the scrambler may be placed back into normal active service.

Video Carrier Level and Video/Audio (V/A) Separation

A Hewlett Packard HP8591 spectrum analyzer, or equivalent, is required for these tests.

The output signal level of the headend will depend on the number of channels on the system and the combining configuration. The following diagram shows a sixty-channel headend.

SpectrumAnalyzer

HeadendTest Point

12 Signal Sources @+ 60 dBmV

12 Channels + 60

12 Channels + 60

12 Channels + 60

12 Channels + 60

HC-8-12

HC12-20

HC12-20

HC12-20

HC12-20

HC12-20dB

Loss+40 dBmV

+40

+40

+40

+40

60 Channels+28 dBmV

Figure 5-13 Headend System RF Levels



To perform these tests:

1 Connect the spectrum analyzer input to the headend system output test point.

2 Set the analyzer as follows:

• Video Bandwidth: 300 kHz min.

• Log Scale: 10 dB/div.

• Span: 6 MHz min.

• Resolution: 300 kHz

• Units: dBmV

3 Tune the spectrum analyzer to the appropriate video carrier frequency to be measured. Fine-tune the analyzer so that both the channel’s video and audio carrier are displayed.

4 Adjust analyzer input attenuation so that the video carrier is at calibrated reference level on the display. Most cases, this reference line will be the top graticule of the display.

5 Measure and record both video carrier level and video/audio separation on the appropriate data sheet.

6 Repeat steps 3 through 6 for the remaining channels in the headend.

Video Carrier-to-Noise (C/N) Ratio

The following test equipment is required to perform these tests:

• Hewlett Packard HP8591

• Wavetek Tunable Bandpass Filters or equivalent

• C6PA or equivalent post amplifier

The procedure for the video carrier-to-noise ratio measurement is described in the NCTA RECOMMENDED PRACTICES FOR MEASUREMENTS ON CABLE TELEVISION SYSTEMS, 2nd Edition, 1993. We will be using the test procedure utilizing the Spectrum Analyzer. A copy of the procedure is attached for your convenience.

VISUAL, AURAL CARRIER LEVEL: 24 HOUR VARIATION

Definition

Visual carrier level in a cable television system is the RMS voltage of a channel’s visual (picture) carrier, considered as a sinewave, at the peak of the modulation envelope, measured across a termination impedance that matches the internal impedance of the cable system. Aural carrier level in a cable television system is the RMS voltage of a channel’s aural (sound) carrier measured across termination impedance that matches the internal impedance of the cable system, generally expressed with reference to the channel’s associated visual carrier level.



FCC §76.601 (c) (3)

The operator of each cable television system shall conduct semi-annual proof-of-performance tests of that system, to determine the extent to which the system complies with the technical standards set forth in section 76.605 (a) (4) as follows. The visual signal level on each channel shall be measured and recorded, along with the date and time of the measurement, once every six hours (at intervals of not less than five hours or no more then seven hours after the previous measurement), to include the warmest and the coldest times, during a 24-hour period in January or February and in July or August. FCC §76.605 (a) (3)

The visual signal level, across a terminating impedance which correctly matches the internal impedance of the cable system as viewed from the subscriber terminal, shall not be less than 1 mV across an internal impedance of 75 ohms (0 dBmV). Additionally, as measured at the end of a 30 meter (100 foot) cable drop that is connected to the subscriber’s tap, it shall not be less than 1.41 mV across and internal impedance of 75 ohms (+3 dBmV). (At other impedance values, the minimum visual signal level, as viewed from the subscriber terminal, shall be the square root of 0.0133(Z) mV and, as measured at the end of a 30 meter (100 foot) cable drop that is connected to the subscriber tap, shall be 2 times the square root of 0.00662(Z) mV, where Z is the appropriate impedance value.) FCC §76.605 (a) (4)

The visual signal level on each channel, as measured at the end of a 30 meter cable drop that is connected to the subscriber tap, shall not vary more than 8 decibels within any six-month interval which must include four tests performed in six-hour increments during a 24-hour period in July or august and during a 24-hour period in January or February, and shall be maintained within:

i) 3 decibels (dB) of the visual signal level of any visual carrier within a 6 MHz nominal frequency separation;

ii) 10 dB of the visual signal level on any other channel on a cable television system of up to 300 MHz of cable distribution system upper frequency limit, with a 1 dB increase for each additional 100 MHz of cable distribution system upper frequency limit (e.g., 11 dB for a system at 301-400 MHz; 12 dB for a system at 401-500 MHz, etc.); and

iii) A maximum level such that signal degradation due to overload in the subscriber’s receiver or terminal does not occur.

FCC §76.605 (a) (5)

The RMS voltage of the aural signal shall be maintained between 10 and 17 decibels below the associated visual signal level. This requirement must be met both at the subscriber terminal and at the output of the modulating and processing equipment (generally the headend). For subscriber terminals using equipment that modulates and remodulates the signal (e.g., baseband set-tops), the RMS voltage of the aural signal shall be maintained between 6.5 and 17 decibels below the associated visual signal level at the subscriber terminal.



Test Procedure

The following equipment is required:

• A signal level meter (SLM) or Spectrum Analyzer. The SLM should have been calibrated immediately prior to this test. If a spectrum analyzer is used, it should have been recently calibrated.

• A precision step attenuator, which may be a part of a quality SLM.

• A subscriber drop cable (30 meters).

• CATV set-top typical of units used in test point region.

Following is a block diagram showing proper set-up of the equipment used in this procedure.

Set-Top

Signal Level Meter(SLM) or

Spectrum Analyzer

30 Meter Drop

Subscriber Tap

Feeder Cable

Figure 5-14 Test Equipment Set-Up Block Diagram, Visual Carrier Level

Prior to performing this measurement, be sure that all signal processor standby carriers are set to precisely the same levels as the visual carriers they replace. Record the make, model number, and most recent date of calibration of each unit of test equipment used for the test.

1 Fine-tune the SLM to the visual carrier to be measured and, if applicable, adjust the SLM compensator as shown on its calibration chart.

2 Insert or remove attenuation from the SLM precision attenuator until the SLM reads within its linear region on the dB scale. Re-fine-tune the SLM if necessary to find the peak amplitude of the channel’s visual carrier.

3 Record the measured compensated visual carrier level. Record the air temperature, time and date of the measurement.



4 Now fine tune the SLM to the channel’s associated aural carrier and remove attenuation from the precision attenuator until the SLM again reads within its linear region on the dB scale. Fine-tune again if necessary to find the peak amplitude of the channel’s aural carrier.

5 Compute and record the aural carrier level with respect to its associated visual carrier level.

Test Methodology

At the headend, perform the procedure for all active cable system channels at the headend. (76.605 (a) (5))

At each test point, repeat the procedure for all active cable system channels for each of the system test points at the end of the 30m drop. (76.605 (a) (3))

At each test point, repeat the procedure for each of the systems test points at the output of the set-top terminating the end of the 30-meter cable drop. The SLM should be tuned to the visual and aural carriers produced by the set-top, and testing performed by tuning the set-top to each active cable system channel. (76.605 (a) (3)), (See note 4)

At each test point, re-measure and record all visual carriers (only) at the end of the 30m cable drop using step 3 through step 5 at six hour intervals three more times (total of four tests in 24 hours). It is only necessary to measure aural carrier levels during the first of the four test intervals. (76.605 (a) (4), (76.601 (c) (3))

Notes, Hints and Precautions

There are acceptable alternate measurement procedures for this data. Signal level meters with multi-channel capability and built-in chart recorders or automated spectrum analyzers with data output ports for computer interface may automate the signal level measuring process. When using these devices, follow the manufacturer’s recommended procedures and insure that the equipment has been properly and recently calibrated.

If the alternate method, using a spectrum analyzer, is used and the spectrum analyzer has 75-ohm input impedance, it may be used directly. An appropriate 75-ohm to 50 ohm minimum loss pad would have to be used with a 50-ohm impedance spectrum analyzer; be careful to take into consideration the loss of the pad. For accurate measurement, an IF bandwidth of 200 kHz or greater is required. Video filters are not to be used; use the widest video bandwidth.

In general, CATV scrambling systems for premium programming distort the visual and aural level measurements. It is recommended that scrambling be turned off during the measurement of a channel’s visual and aural carrier levels. If this is inappropriate, adjust measurements by applying an established and documented correction factor (difference between peak visual level scrambled versus not scrambled) to the SLM data, or use a spectrum analyzer instead of a SLM. When using a spectrum analyzer, measure the visual carrier level at the peaks of any non-suppressed synchronization pulses or apply an established and documented correction factor (difference between peak visual level scrambled versus not-scrambled) to the data.



Baseband set-tops remodulate any tuned channel and generally produce fixed levels for visual and aural carriers independent of tuned channel levels. If a baseband set-top or an RF set-top incorporating volume control or AGC is used, it is acceptable to record the video carrier level and the aural carrier level on one channel, and note any variation (if any) caused by changing channels. It should be verified that the FCC visual and aural level regulations are met allowing for the set-top contribution.

If it is established and documented that the set-top (of any type) has at least 0 dBmV of output visual carrier level and the proper aural carrier level with an input signal of the same characteristics, field set-top tests for signal level measurements do not have to be performed. Similarly, all types of non-volume control set-tops should be characterized for their affect on relative aural carrier level and it confirmed that the cumulative effect of set-tops and plant variation meets the FCC requirement.

Subscriber visual level is normally expressed in decibels referenced to 1 mV RMS across 75 ohms and is measured at the coaxial cable terminal intended for connection to the subscriber television equipment. Subscriber aural carrier level is normally measured at the same point, under the same condition as the associated visual carrier. It is also expressed in decibels referenced to 1 mV across 75 ohms or, as in Section 76.605 (a) (5), measured with respect to the associated visual carrier.

Twenty-four hour variation of a particular visual carrier is the decibel difference between the maximum level excursion and the minimum level excursion of the carrier over any 24-hour period.

AURAL CARRIER CENTER FREQUENCY

Definition

The aural center frequency measurement is the difference in frequency of the aural carrier and the associated visual carrier.

FCC §76.605 (a) (2)

The aural center frequency of the aural carrier must be 4.5 MHz ±5 kHz above the frequency of the visual carrier at the output of the modulating or processing equipment of a cable television system, and at the subscriber terminal.

Procedure

The following test equipment is required:

• A frequency counter covering the direct frequency range to be measured.

• A demodulator with 4.5 MHz audio subcarrier output.

• A channel selector (if a set-top is used, it must be an RF non-volume-control type.)



Connect the equipment as shown in the following diagram.

Set-Top(if required) Demodulator Frequency

CounterSystem

or Source

Figure 5-15 Test Equipment Set-Up Block Diagram, Aural Carrier Frequency Separation

Read the frequency of the 4.5 MHz subcarrier output of the demodulator directly on the frequency counter.

Best results are obtained with no modulation on the sound carrier. If this is not possible, choose a long gate time on the frequency counter. Discussion

The FCC rules require that the aural center frequency measurements be conducted at the headend or last point of modulation and at the set-top output at each test point. However, as mentioned above, the aural frequency delivered to the subscriber can be carefully characterized by measuring at the last point of modulation and at the output of a baseband set-top. Recommended practice is to measure an adequate sample of set-tops. Keep in mind that if several types of set-tops are in place in the system, this process must be done to sufficiently characterize all types of set-tops in use.

The FCC has eliminated standards for video carrier frequency tolerance outside of the FAA frequency bands (108.0 to 137.0 MHz and 225.0 to 400.0 MHz). Within those ranges the video carrier frequency must be properly offset per FCC rule 76.612.

VIDEO SWEEP OF MODULATORS OR PROCESSORS Measuring In-Band Response Using a Video Sweep Signal and Spectrum Analyzer

The procedure measures the response variations from modulator or processor input to either headend output or, as with the above procedures, to the field test points. It requires interruption of normal video programming. It differs from multiburst testing in that it provides a continuous response curve and allows the use of a spectrum analyzer rather than a waveform monitor for display. Procedure


• Spectrum analyzer with peak-hold capability. A non-peak-hold unit may be used if the display is photographed with a time exposure to record the trace maximum.

• Video Sweep Signal Generator. This unit generates a conventional video signal in which the visual information is replaced by a frequency varying monotonically from 0.5 MHz to 5.0 MHz or higher. This signal may be at the line rate or frame rate.

• Test Modulator (for processor channels only).



Connect the equipment as shown in the following diagram. Use the test modulator only when measuring processors. Unless the modulator has been previously characterized for response, it should first be measured using this procedure (and a back-to-back connection between modulator and demodulator) and its measured variation subtracted from the data obtained when measuring the processor under test.

TestModulator

(forProcessorTestingOnly) Spectrum

Analyzer

VideoSweepSignal

Generator

Modulatoror

ProcessorUnder Test

inRFout HE

TestPoint

Figure 5-16 Test Equipment Set-Up Bock Diagram Video Sweep of Modulators/Processors



Perform the tests as follows:

1 Tune the spectrum analyzer to the channel under test. The spectrum analyzer should be in log mode. Set the sweep rate so the trace can “paint” the response when the peak hold mode is enabled as shown in the figure on the following page.

2 If frequency markers are available on the analyzer, set them to 0.5 MHz below the visual carrier and 3.75 MHz above the visual carrier (F1 and F2 in the figure).

3 The variation, in dB peak-peak, is half the difference between the maximum and minimum amplitudes (L1 and L2 in the figure) between the frequency markers, excluding the visual carrier itself. In the example shown, half the peak-to-peak variation is approximately 2.75 dB that exceeds the allowable variation.

4 Photograph or print the display, if desired, for a permanent record.

Frequency Rangeof Interest

1 MHz/div

Pk-Pk

F1 F2

L1

L2

10 d

B/d

iv VisualCarrier

AuralCarrier

Figure 5-17 Typical Video Sweep Response



VISUAL CARRIER-TO-NOISE RATIO

Definition

Visual carrier-to-noise ratio is the power in a sinusoidal signal whose peak is equal to the peak of a visual carrier, divided by the associated noise power in a 4 MHz bandwidth. This ratio is expressed in dB. Procedure


• A spectrum analyzer with IF resolution bandwidth of 300 kHz (100 kHz is acceptable if basic sensitivity is ample for the measurement).

• A 75 ohm variable RF attenuator.

• A tunable bandpass filter or bandpass filters for the channels to be measured. Minimum of 6 MHz noise power bandwidth is required.

• A broadband low noise preamplifier: 10 dB noise figure maximum with 20 to 30 dB gain (not required if the spectrum analyzer has an optional plug-in pre-amplifier).

• A thermometer for -40° F to 140° F.

Set up the equipment as in the following diagram.

SpectrumAnalyzer

ModulatorUnder Test

Combiner

Headend System

HeadendTest Point

To CableSystem

Var.Attn.

VariableBand-Pass

Filter

C6PA

Figure 5-18 Visual Carrier-to-Noise Diagram



Perform the tests as follows:

1 Record the ambient temperature reading at time of measurement.

2 Check that the spectrum analyzer is properly calibrated in accordance with manufacturer’s instruction. Most critical is the accuracy of the log scale tracking.

3 Adjust spectrum analyzer as follows:

Resolution IF Bandwidth: 300 kHz

Video Bandwidth: 100 Hz desired (no greater than 300 Hz)

Log Scale: 10 dB/div.

Frequency Span: 1.0 MHz/div.

Scan Time: Automatic

4 Set spectrum analyzer input attenuator (with no input signal) so that noise floor is at least 70 dB below the top graticule line.

5 Tune spectrum analyzer to the carrier to be measured.

6 Adjust spectrum analyzer as follows (assumes a modulated carrier):

Resolution IF Bandwidth: 300 kHz or 1 MHz

Video Bandwidth: 300 kHz or 1 MHz

Log Scale: 2 dB/div.

Frequency Span: 1 MHz/div.

Scan Time: Automatic

7 Tune bandpass filter to peak the video carrier; fine tune spectrum analyzer for peak level.

8 Adjust variable RF attenuator to set peak video carrier at the top graticule line on the spectrum analyzer. This is the reference.

9 Readjust spectrum analyzer settings as in step 3.

10 Carefully adjust tuning so that video carrier peak is on centerline of the display.

11 Shut off carrier under test at the headend or, at least, remove modulation (do not choose a carrier used for AGC or ASC for this test).

12 Determine a point at a frequency above the video carrier where the energy on the spectrum analyzer is at a minimum level. This will normally be 2 to 3 MHz above the video carrier. Bandpass filter must be retuned to cover the 2 to 3 MHz shift. Record the number of dB between this point and the top line reference.



13 Add to the number determined in step 12 the correction factor given by the spectrum analyzer manufacturer for the IF bandwidth used, (e.g. for a 300 kHz bandwidth the correction factor is 11.2 dB, for a 4 MHz noise bandwidth equivalent, and 2.0 dB for detector linearity). Total correction typically is 13.2 dB.

For example, A reading of the noise level that is 56 dB below the reference would give the following:

-56 dB + 13.2 dB = -42.8 dB noise level or a carrier-to-noise ratio of 42.8 dB

Note: This measurement has a practical limit of 55 dB C/N or lower. Alternative Procedure

An alternative test can be made using a Signal Level Meter. The following equipment is required:

1 A signal level meter (SLM) with known noise correction factor (includes correction for bandwidth and detector noise response).

2 A bandpass filter (BPF) for the set-top output channel or, if a set-top is not used, a BPF for each channel to be tested.

3 An RF set-top with a good noise figure, and sufficient signal handling capability to avoid distortions, which might effect measurement accuracy. It must have channel-tuning capability equal or greater than the system to be tested.

4 A low noise preamplifier for low level test points.

5 A 75 ohm variable RF attenuator (ATT) from 0 to 60 dB with increments of 1 dB.

6 A thermometer measuring -40° F to 140° F.

Note: Carrier-to-noise is difficult to measure accurately on an active system because of intermodulation distortions generated by the many system signals and because of the sideband components of channels adjacent to the one under test. When possible, the above test should be made when all signals, except the gain and slope control pilots, are turned off. Since this is almost impossible, it is suggested that carrier-to-noise measurements are recorded both with and without full active signal so that future tests of the operating system can detect changes in the readings related to those taken at proof of performance with full signal loading.



Discussion

Carrier-to-noise ratio is expressed in dB. The carrier, or the signal, in the above definition, is the desired energy level to which undesired random Gaussian noise is compared, when the random noise has a bandwidth equal to or greater than the desired signal. This noise is also called white noise which, by definition, is independent of frequency within the band of interest. It is the energy inherent in all matter and varies in amplitude with the thermal agitation of the material. In cable television systems, the principal sources of this form of noise are active devices, i.e., headend processors, preamplifiers, and cable repeater amplifiers. The power ratio of the desired carrier to the random noise is an important design parameter that becomes a figure of merit to determine system performance. Noise manifests itself as picture impairment to the eye and ranges from “not visible” at ratios greater than 50 dB to “significantly objectionable” at ratios less than 30 dB. For ratios in the range of 36 to 46 dB, picture impairment from noise becomes a function of viewer interest in program content, the viewer’s critique, and TV receiver characteristics.

Measurement of carrier-to-noise ratio is one of several basic measurements performed on a cable television system. It is useful as a system maintenance tool. For cable television purposes, noise and carrier measurements are referenced to 75-ohm impedance and 4 MHz bandwidth.

For channels with video modulation, it is necessary to remove modulation from modulators, or to revert to standby carrier operation for RF-to-RF headend processors. There are certain times when channels simply cannot and should not be removed from operation for any period during the 24-hour broadcast day. However, broadcast channels can usually be measured during commercial breaks. Measurements made with standby carriers or measurements made when a specific channel is disconnected from the headend do not represent total “noise-to-picture.” Total noise is a combination of off-the-air inputs, preamplifier and processor contributions. Performance Objective

A cable television system should measure ratios of 43 dB or better, excluding noise contribution from source material. Rebuilds of older systems should have as an important goal achieving significantly better carrier-to-noise performance, at least 48 dB, to allow for future HDTV channels.



Coherent vs. Non-Coherent In an HRC (Harmonically Related Carriers) system the video carriers are spaced at 6 MHz multiples. The HRC is a coherent headend.

In a coherent headend, the carrier frequencies are tied together by a comb generator. A comb generator produces an extended series of harmonics of 6 MHz (1x6, 2x6, 3x6, ... 15x6, 16x6, ... 50x6, 51x6, ... etc.). These frequencies are very exact multiples of 6 MHz. The output of the comb generator goes to every modulator or signal processor. Each modulator or signal processor is tuned to accept from the comb generator only the frequency of its own particular output signal. The modulator or signal processor then uses that signal from the comb generator as a reference frequency: it locks its output video carrier at that frequency.

Freq.Compar-

ator

VideoIF

ModulatorAudio

IFModulator

OutputFreq.

Converter

LocalOscillator

ModulatedTV Signal(52.75 - 58.75 MHz Channel VideoCarrier at 54 MHz)Base Band

Audio

Base BandVideo

Band PassFilter

54 MHz

CombGenerator

f = ∑ 6nout(MHz)

n=1

kFreq.

Compar-ator

VideoIF

ModulatorAudio

IFModulator

OutputFreq.

Converter

LocalOscillator


Audio

Base BandVideo

Band PassFilter

60 MHz

Freq.Compar-

ator

VideoIF

ModulatorAudio

IFModulator

OutputFreq.

Converter

LocalOscillator


Audio

Base BandVideo

Band PassFilter

402 MHz

Figure 5-19 Coherent vs. Non-Coherent Diagram



In a STD (Standard-Channel Plan) the video carriers (except channels 5 and 6) are spaced at 6 MHz intervals starting at 55.25 MHz. This frequency allotment scheme is considered a non-coherent system.

The following diagram shows where CSO and CTB beats fall within a given channel in a non-coherent system.

Figure 5-20 Coherent Disturbances



Second Order Distortions: Carrier-to-Second Order Beat Ratio Definition

This is the ratio, expressed in decibels, of the peak level of the RF signal to peak level of the interference. The amplitude distortion of the desired signals is caused by the second order curvature of the non-linear transfer characteristic in system equipment. Procedure


• A RF signal generator. More than one might be required.

• A spectrum analyzer.

• A bandpass filter for each frequency band to be measured or a tunable bandpass filter covering the frequency band.

Following is a block diagram showing the proper test equipment set-up.

SpectrumAnalyzer

AMP

BPF

Figure 5-21 Carrier-to-Second Order Beat Ration, Test Equipment Set-Up Block Diagram


1 Remove all carriers from the system, except pilot carriers.

2 If equipment allows it, adjust the channel 2 and channel 13 headend equipment to standby mode to generate a CW carrier at the nominal operating levels. If the equipment does not have this feature, then substitute signals from external generators must be coupled into the system at 55.25 MHz and 211.25 MHz, at the normal operating level of these channels.

3 At the headend, insert a CW signal generator into the system and tune to 156.00 MHz. Adjust to the proper level relative to the Channel 2 and Channel 13 carriers.



4 Adjust the reference carrier frequency so that it is slightly offset from 156 MHz to allow the different beat between Channel 2 and Channel 13 (which occurs at 156 MHz) to fall within ±100 kHz of the 156 MHz carrier. This frequency is chosen because it establishes the condition wherein a second order beat will fall close to Channel 2, the Channel 13 and the 156 MHz carriers. Second order distortion can then be measured at three frequencies across the system passband.

5 At the desired measurement location (usually at the end of the longest trunk) connect the bandpass filter and spectrum analyzer to the CATV system and tune the spectrum analyzer to the channel under test. The use of the bandpass filter prevents the possibility of generating a false distortion in the spectrum analyzer. A variable frequency bandpass filter should also be tuned to peak the carrier. The spectrum analyzer should be set as follows:

IF Bandwidth: 30 kHz

Video Bandwidth: 10 Hz

Scan Width: 50 kHz/div.

Vertical: 10 dB/div.

Scan Time: 0.2 sec/div.

6 Adjust the gain of the spectrum analyzer to bring the peak of the carrier to the top graticule line. This is the reference level. Notify the headend to remove this reference carrier from the system. Measure the level of the second order beat product remaining on the instrument.

7 Carrier-to-second order beat is the difference in the two levels expressed in dB.

8 Repeat steps 5 through 7 for each of the specific carriers to be measured. Performance Objectives

Good engineering practice results in 60 dB carrier-to-second order beat for any beats that fall within a standard television channel.

Discussion

Passive elements can contribute to this distortion in a cable television system; however, the distortion most often is due to the active equipment. Without a very narrow band instrument such as a spectrum analyzer or a wave analyzer, it is difficult to measure this phenomenon on an active cable system in the presence of typical system carrier-to-noise ratios. In this procedure, carrier to second order beat ratio is measured using a beat generated by intermodulation of two CW carriers.

For systems carrying signals at frequencies higher than 300 MHz the same basic procedure can be used. Use two carriers, one at the low end of the system passband and one at the high end. Mathematically subtract the low frequency from the high frequency carrier. The result is the frequency to which the signal generator representing the inserted carrier should be tuned. For example, in a 450 MHz system with highest video carrier at 450 MHz, insert a carrier at 390 MHz.



Composite Second Order Distortion: CW Carriers: Standard Frequency Plan (not HRC) Definition

Composite second order distortion is the amplitude distortion of a desired signal caused by the second order curvature of the non-linear transfer characteristic occurring in CATV system equipment. It is the ratio, expressed in dB, of the peak level of the desired RF signal to the peak of the cluster of distortion components falling in the side band of the desired signal at ±0.75 MHz and ±1.25 MHz around the carrier.

Note: In practice, only the distortion that falls above the frequency of a desired television carrier at +0.75 MHz and at +1.25 MHz is measured.

Procedure


• A spectrum analyzer with 30 kHz IF bandwidth capability.

• A variable 75 ohm attenuator.

• A bandpass filter for each channel where distortion is to be tested or a tunable bandpass filter. The filter should have a bandwidth of between 1 MHz and 6 MHz.

The following figure is a block diagram showing the proper test equipment set-up.

SpectrumAnalyzer

AMP

BPFATT

Figure 5-22 Composite Second Order Distortion, CW Carriers Test Equipment Set-up Diagram




1 Switch all headend signals to the CW mode and adjust to the same levels as established for a headend operating under modulation conditions. A headend signal simulator can be substituted. In either case, signals should be within 10 kHz of assigned frequency.

2 Adjust the spectrum analyzer as follows:


Video Bandwidth 300 Hz or less

Scan Width: 500 kHz/div.

Vertical: 10 dB/div.

Scan Time: 0.2 sec/div. (Auto)

Note: When using a spectrum analyzer with minimum video filtering capabilities of greater then 10 Hz, the composite second order distortion display may be noisy and should be read at the middle of the trace.

3 Connect the spectrum analyzer to the system as shown in the diagram on the previous page. Where possible, have 6 to 10 dB in the input attenuator.

4 Tune the spectrum analyzer and center the carrier of the channel to be measured on the screen. Tune the bandpass filter for peak reading.

5 Adjust the variable attenuator in conjunction with level controls on the analyzer to establish a full screen “0” dB reference for the peak level of the RF signal being tested.

6 Tune spectrum analyzer to move the carrier peak 2.5 divisions to the left. The main distortion area should be centered. Change the scan width to 50 kHz per division. Repeak the bandpass filter.

7 Have the channel under test disconnected from the system at the headend. The remaining display on the spectrum analyzer indicates the level of the composite second order distortion in the region of the carrier (caused by all channels except channels 5 and 6). The number of dB below the “0” dB reference established in Step 5 (above), that the peak of the composite intersects on the screen, is the measurement of signal-to-composite second order distortion.

Performance Objectives

Good engineering practice dictates that the ratio of carrier to distortion should be at least 53 dB for non-coherent systems.



Discussion

In a multi-channel distribution system, using the standard frequency plan, where most of the carriers are spaced at a constant frequency interval, there is a moderate buildup of second order distortion components as channels are added to the system. Most of them cluster at ±1.25 MHz around the carriers. The number of these components is greatest at channels nearest the high end of this continuous spectrum. The composite second order beats caused by channels 5 and 6 appear at ±0.75 MHz around certain carriers.

The test is difficult to conduct on an active system. It requires relatively expensive test equipment and it requires interruption of normal programming to change headend signals to CW. This is required in order to observe distortion in the presence of system noise. While second order distortion components also fall 0.75 MHz and 1.25 MHz below a desired carrier, those falling above the carrier are more subjectively offensive. The I.F. response of a standard television set filters out the lower sideband distortion components.

The worst case for composite second order distortion is in an IRC (Incrementally Related Carrier) system. Channels 5 and 6 are shifted to frequencies of 79.25 MHz and 85.25 MHz respectively and all channels are frequency locked to a comb of frequencies defined by (6n +1.25) MHz where n can be any positive integer number from n = 9 and higher as the state of the CATV System art allows. The same test procedure can be used. The second order composite distortion falls at ±1.25 MHz around all carriers and, since it is frequency locked, appears as two distortion components at each channel. As before, the beat ±1.25 above the desired carrier is most critical. The power in this frequency locked beat (which is the sum of the power in all the beat components falling into this channel) results in a subjectively more offensive distortion and therefore becomes the limiting distortion in IRC systems. It reduces the subjective benefit of phase locked systems. It is recommended that, for systems planned to be over fifty channels, HRC (Harmonically Related Carriers) be used when the benefit of phase lock is required. With the HRC frequency plan, all composite second order beats fall on carrier frequencies and can not be distinguished from third order composite beats.

For IRC systems, a figure for carrier-to-composite second order distortion ratio of 53 dB is appropriate.

Figure 5-23 Second order beats produced by carriers at 50 and 200 MHz



Third Order Distortion: CW Carriers Definition

Composite third order distortion is the amplitude distortion of desired signals caused by third order curvature of non-linear transfer characteristics in system equipment. It is the ratio, expressed in dB, of the peak level of the RF signal to the peak of the average level of the cluster of distortion components centered on the carrier.

Procedure

The following test equipment is required for this procedure.



• A bandpass filter for each channel to be tested or a tunable bandpass filter. The filter should have a bandwidth of between 1 MHz and 6 MHz.

The following figure is a block diagram showing the proper test equipment set-up for this procedure.

SpectrumAnalyzer

BPFATT

AMP

Figure 5-24 Third Order Distortion, CW Carriers Test Equipment Set-Up Block Diagram




1 Switch all headend signals to the CW mode and adjust to the same levels as established for a headend operating under modular conditions. A headend signal simulator can be substituted. In either case, signals should be within 10 kHz of assigned frequency. All signals required to meet FCC rules must be within ±5kHz of assigned frequency.

2 Adjust the spectrum analyzer as follows:


Video Bandwidth: 300 Hz or less

Scan Width: 50 kHz per div.

Vertical: 10 dB per div.

Scan Time: 0.2 sec. per div. (Auto)

Note: When using a spectrum analyzer with minimum video filtering capabilities of greater than 10 Hz, the composite third order distortion display will be noisy and should be read at the middle of the trace.

3 Connect the spectrum analyzer to the system as shown in the previous diagram. Where possible, have 6 to 10 dB in the input attenuator.

4 Tune the spectrum analyzer and center the channel to be measured on the screen.

5 Adjust the variable attenuator in conjunction with level controls on the analyzer to establish a full screen “0” dB reference for the peak level of the RF signal being tested.

6 Have the channel under test disconnected from the system at the headend. The remaining display on the spectrum analyzer indicates the level of the composite third order distortion in the region of the carrier. The number of dB below the “0” dB reference established in Step 5 (above), that the peak of the composite intersects on the screen, is the measurement of signal-to-composite-third order distortion.

Performance Objectives

Good engineering practice dictates that the ratio of carrier level to distortion level should be at least 53 dB for non-coherent systems. For systems tested by this non-coherent method but operated in the coherent mode, an operating level that results in a 47 dB carrier-to-distortion ratio is acceptable as good engineering practice.



Discussion

In a multi-channel distribution system where all, or most, carriers are spaced at a constant frequency interval, there is a rapid buildup of third order distortion components as channels are added to the system. They cluster at the carrier frequencies. The number of these components is greatest at channels nearest the middle of this continuous incremental spectrum.

This test is one of the most meaningful in determining the quality of a cable television system because it most closely relates to overall subjective performance of a system. It is particularly true of systems that carry more than twelve channels. However, the test is difficult to conduct on an active system. It requires relatively expensive test equipment and it requires interruption of normal programming to change headend signal to CW. This is required in order to observe distortion in the presence of system noise.

Figure 5-25 Second and Some Third Order Beats Resulting from Carriers at 50, 200, and 300 MHz



Third Order Distortion: Modulated Carriers Definition

Composite third order distortion is the amplitude distortion of desired signals caused by third order curvature of non-linear transfer characteristics in system equipment. It is the ratio, expressed in dB, of the peak level of the RF signal to the peak of the average level of the cluster of distortion components centered on the carrier. Procedure

The following test equipment is required for this procedure.




The following figure is a block diagram showing the proper test equipment set-up for this procedure.

SpectrumAnalyzer

BPFATT

AMP

Figure 5-26 Third Order Distortion, Modulated Carriers Test Equipment Set-Up Block Diagram




1 Increase the level of all channels at the headend by 3 dB, except the pilot carriers.

2 Connect the spectrum analyzer to the system as shown in the block diagram. Where possible, have 6 to 10 dB in the input attenuator.

3 Tune the spectrum analyzer such that the channel to be measured is centered on the screen. The spectrum analyzer should be adjusted as follows, when establishing the “0” dB reference level.

IF Bandwidth: 300 kHz or greater

Video Bandwidth: Maximum, video filter “off”

Scan Width: 0.5 MHz per division

Vertical: 10 dB per division

Scan Time: 5 ms per div. or slower

4 In conjunction with the level controls on the analyzer, adjust the variable attenuator to establish a full screen “0” dB reference for the peak level of the RF signal under test.

5 Have the channel under test disconnected from the system at the headend and adjust the spectrum analyzer as follows:


Video Bandwidth: 10 Hz*

Scan Width: 50 kHz per division

Vertical: 10 dB per division

Scan Time: 0.2 sec. per division

Note: *When using a spectrum analyzer with minimum video filtering capabilities of greater than 10 Hz, the composite third order distortion display will be noisy and should be read at the middle of the trace.

6 The display remaining on the spectrum analyzer will now indicate the level of the composite third order distortion in the region of the carrier. The number of decibels below the “0” dB reference established in Step 4 that the peak of the composite intersects on the screen, is the measurement of signal-to-composite third order distortion.

Performance Objective

Good engineering practice dictates that the ratio of carrier level to distortion level should be at least 59 dB for non-coherent systems when the signals are raised 3 dB above normal level. For systems tested by this non-coherent method but operated in the coherent mode, an operating level that results in a 53 dB carrier-to-distortion ratio is acceptable as good engineering practice.



Discussion

The discussion of the rapid buildup of third order distortion products also applies to this measurement discussion. This test is a compromise method, compared to one in which headend signals are switched to the CW mode. It does have the advantage of creating minimum impact on subscriber viewing but it requires relatively expensive test equipment. It also requires increasing headend levels in order to observe the distortion on the instrument in the presence of system noise. The test gives a quantitative measurement, although with system levels raised, distortion will most likely be apparent on the television screen.



Third Order Distortion: Cross Modulation Definition

Cross Modulation is a distortion, resulting from the non-linearity of a system, which causes a carrier in the system to be modulated by the various desired signals carried on the other channels in the same system. Cross Modulation is specifically defined as the ratio of the peak-to-peak amplitude of the modulation on the test carrier (caused by the signals on other carriers) to the peak level of the test carrier. Procedure

The following test equipment is required to measure Cross Modulation.

• A signal source that has the capability to generate the number of carriers required for the rated capacity of the amplifier or system under test. Further, it must be able to have individual level control for each carrier. Each carrier must be available unmodulated or synchronously modulated with a 100% square wave at a 15734 ± 20 Hz. The outputs of these individual carriers should be combined into a single output for test. The levels of the combined carriers should be adjustable from equal level to a +12dB of tilt. It is imperative that carriers, when in the CW mode, are free of residual modulation. Sidebands should be at least 100 dB down.



The following figure shows the test equipment arrangement for this measurement.

SignalSource

VariableAttenuator

SystemUnderTest

FixedAtt. BPF Receiver Tuned

Voltmeter

SignalLevelMeter

Some receivers incorporatethe signal level meter andthe tunable low frequencyvoltmeter.

Figure 5-27 Third Order Distortion, Cross Modulation Test Equipment Set-Up Block Diagram




1 Place 30dB attenuation in the variable attenuator.

2 Place all carriers stipulated for use in this test to the CW position.

3 Adjust each carrier to the proper level at the output of the unit or system under test corresponding to its minimum specified operating level. These level settings should achieve the prescribed operating tilt.

4 Tune the receiver and the band pass filter to the channel under test.

5 Turn all carriers except the particular channel under test to the 15734 Hertz 100% square wave modulated position. Check to be sure that the peak level of each modulated carrier has the same peak as in the CW mode.

6 Measure the cross modulation performance with the calibrated receiver. (A suggested modulation procedure is described in the Discussion section of this procedure.)

7 Remove attenuation from the attenuator and record the cross modulation measurement at the new output level. Change of the attenuator setting in 2 dB steps is reasonable until the measurement is nearing the equipment specification; then a reduction in attenuation should be in 1dB steps. A well-behaved unit will indicate a worsening of distortion of 2 dB for every 1 dB increase in output level.

8 Select other carriers across the frequency spectrum of the unit or system under test. Switch this carrier to the CW mode and have all offending carriers synchronously modulated at the 15734 Hz rate. Repeat steps 4 through 7.

Performance Objective

When this measurement procedure is used to check individual amplifiers, they should be tested at the operating level and tilt specified by the equipment manufacturer and to the cross modulation performance stated in that specification.

The carrier-to-cross-modulation measurement of a fully operating system, at the point of highest distortion, should not be worse than 53 dB.



Discussion

By definition, Cross Modulation distortion is the measure of the amplitude modulation transferred onto a channel by other carriers in the system. Therefore, to measure it, an envelope detector is required. A spectrum analyzer measures the energy in the carrier wave and the side bands of a channel under test independently of the phase relationship of these side bands to the carrier. It has been proven that simultaneous cross modulation measurements of an amplifier with an envelope detector and with a spectrum analyzer may differ greatly. The resultant cross modulation distortion varies from amplitude modulation to phase modulation as a function of frequency, with the result that the standard envelope detector indicates significantly more favorable results than those from a spectrum analyzer. Subjective tests of system performance under controlled conditions show that system performance more closely follows Composite Triple Beat distortion than it does Cross Modulation distortion. Because of this, Cross Modulation is considered to be of secondary importance to other distortion measurements.



Modulation Distortion at Power Frequencies Definition

Modulation distortion at power frequencies is the amplitude distortion of the desired signals caused by the modulation of these signals with components of the power source. It is the percentage of the level of the peak-to-peak interference compared to the peak level of the RF signal. It is also stated as the ratio, expressed in dB, of the peak level of the RF signal to the peak-to-peak level of the interference. Procedure

The following test equipment is required for this measurement.

• A RF signal generator for the television frequencies (or a headend modulator or processor capable of CW transmission).

• A signal level meter (SLM) with a video output terminal. (A spectrum analyzer can be used as an SLM.)

• A standard oscilloscope with at least 10 mV/division sensitivity.

• A low-pass filter with 1,000 Hz cut-off frequency. The low pass filter will remove system noise.

The following figure is a block diagram showing the proper set-up of the test equipment used in this procedure.

LPF SLMSCOPE

AMP

Figure 5-28 Modulation Distortion at Power Frequencies Test Equipment Set-Up Block Diagram




1 At the headend, insert a CW signal into the system at the level of the video carriers. Test should be conducted for several frequencies across the full bandpass of the system.

2 Adjust the oscilloscope setting as follows:

• Set input attenuator to 0.1 volts/division.

• Adjust horizontal sweep to 5 milliseconds/division.

• Switch input signal to dc coupling.

• Adjust centering of the trace to be five or six divisions below the top graticule line.

3 Connect a drop cable from the system test point to the input of the SLM.

4 Connect the video output of the SLM to the oscilloscope via a low pass filter.

5 Tune the SLM to the CW carrier under test.

6 Adjust the SLM gain to raise the oscilloscope trace by five divisions. This is the reference for the RF peak level.

7 Switch the oscilloscope input to ac coupled and center the trace.

8 Set the input attenuator to 10 mV/division.

9 For the above settings percentage = 2 x number of divisions of peak-to-peak modulation. Performance Objective

A good engineering practice is 3 percent or less hum distortion. If the modulation waveform is essentially a smooth sinusoidal, the subjective effects are minimal. If this waveform shows an abrupt transition or a spike, it will be more apparent on the television screen and a more conservative rating of 2 percent or less should be used. Discussion

This distortion can occur in any cable television system component, active or passive, through which source power passes. System hum modulation can be tested by utilizing any CW signal carried on the system.

Other low frequency variations should also be measured. These may include very low frequency ranges such as AGC hunting and system intermittence due to wind and other factors. Frequencies above 60 Hz might also be encountered due to switching power supplies and may occur between 1 to 20 kHz. Remove the low pass filter and adjust the oscilloscope horizontal sweep rate to search for this distortion.


Section 6 Spectrum Analyzer Basics

After completing this section, you will have an understanding of spectrum analyzer functions

6-2 Spectrum Analyzer Basics


Overview The RF spectrum analyzer is calibrated to measure the RMS value of a CW signal’s voltage level. It is a tuned voltmeter with variable bandwidths and a peak voltage detector.

SOFT Keys HARD Keys

Numeric Key Pad

Hewlett Packard 8591C

UNITs Keys

Figure 6-1 Hewlett Packard 8591C Spectrum Analyzer

Measuring modulated carrier levels requires proper utilization of the spectrum analyzer controls with the application of calibration and correction factors. Proper warm-up time before instrument calibration will yield more accurate results. Measurements are only as precise as the instrument’s accuracy and uncertainty specifications.

Cable TV system impedance is 75 Ohms; therefore measurement errors can be caused by a mismatch between the analyzer impedance and system impedance. Analyzers for cable TV measurements are manufactured with 75-Ohm input impedance.

Improper use of readily available 50-ohm cable adapters will not only cause impedance mismatch but may also break the input connector on the analyzer.

Spectrum Analyzer Basics 6-3


Analyzer Functions The following figure is a functional block diagram of a typical spectrum analyzer. The components shown in the diagram are discussed in the following paragraphs.

preampLO

mixer

H

V

RFinputsig.

sweepgenerator

RBW detectorVBWfilter

inputattn.

display

logamp

Figure 6-1 RF Spectrum Analyzer Basics



Input Attenuator The input attenuator protects the mixer from high input levels. With high input levels the mixer maybe driven into the non-linear range of operation. Signal compression and distortions can arise in the non-linear range.

As the attenuation value is increased by 10 dB, the noise floor of the spectrum analyzer increases by 10 dB. The reference level remains unchanged.

50 dB attenuation

40 dB

30 dB

20 dB

10 dB

0 dB 10 dB noise floor increase

Figure 6-2 Input Attenuator

Preamp The internal preamp increases the sensitivity of spectrum analyzer for measuring low-level signals. The gain and noise figure of this preamp must be accounted for when measuring for C/N, CSO and CTB. Care must be exercised when using the preamp, because it may add distortion to the measured signals.



Mixer The mixer is a device that combines the input signal with the local oscillator (LO). It produces the original signal, LO, and sum and difference frequencies at its output. If the input to the mixer is sufficiently high the output level will not track the input level. This is called gain compression and can effect the amplitude accuracy of the measurement as well as create distortions in the displayed signal.

If the input to a mixer in a spectrum analyzer is increased beyond its linear range, gain compression results. Increase the attenuation setting on the analyzer until the signal stops growing.

Attenuation = 50 dBAttenuation = 30 dB

Figure 6-3 Mixer Compression



Resolution bandwidth (RBW) or IF Filter The RBW filter sets the bandwidth of the signal to be detected. When measuring a CW signal, the shape of the signal is the actual shape of the filter.

A filter is composed of capacitive and inductive tuning elements that need to be fully charged for proper display of a signal. If a fast changing signal is filtered with a narrow bandwidth filter, the response time of the filter distorts the actual shape and level of this signal. HP spectrum analyzers use a Gaussian shaped filter that, because of its fast charge and discharge time, minimizes this effect.

When measuring a CW carrier, the amplitude remains unchanged while the RBW setting is changed. However, when measuring the amplitude of a carrier in a CATV system where NTSC analog modulated carriers exist, the RBW should be set at either 1 MHz or 300 kHz to reject adjacent carrier energy and to capture the true peak level of the modulated signal.

When measuring random noise, as the resolution bandwidth is increased the noise power is increased due to the wide spectral content of noise.

As the resolution bandwidth is increased on a modulated carrier, the amplitude will increase. When measuring NTSC modulated carriers, the minimum recommended setting for resolution bandwidth is 300 kHz to capture the information in the video signal. If the bandwidth is set too high (3 MHz) the adjacent channel power will influence the total measurement.

Ch 3 Video Carrier

Ch 2Aural Carrier

3 MHz. RBW

30 kHz RBW

300 kHz. RBW

Figure 6-4 Resolution Bandwidth with NTSC Modulated Carrier



Data Carriers Data carriers look and act like noise when viewed on a RF spectrum analyzer. The displayed amplitude of a data carrier changes as the resolution bandwidth is changed.

As the resolution bandwidth is changed the digital carrier-to-noise remains the same. However, analog carrier-to-noise (converted to a 4 MHz bandwidth) values do not, although visually it appears to change.

Figure 6-5 Resolution Bandwidth, Data Carrier

Log Amplifier The large dynamic range of the spectrum analyzer is accomplished through logarithmic amplification. High level input signals are compressed to accommodate the peak detector.



Video Detector The envelope detector converts an IF signal to a video signal. The detector is either a peak level detector, as when measuring a carrier, or a sample detector, as when measuring noise with the averaging function on. When performing noise measurements, the sample detector is chosen to average out the random amplitude fluctuations.

There are three types of video detection in most analyzers: peak detection for measuring CW or modulated carriers, sample detection for measuring noise, and negative peak (minimum hold) for other measurements.

A digital signal can have peak excursions that exceed the average power level by 6 to 10 dB or more. When setting a digital signal power level, it is desirable to know the peak power as well as the average power so as to avoid signal distortions created during clipping of an over-modulated laser transmitter. This could create inter-symbol interference in the demodulated data.

Digital Signal

PeakPower

Peak / AveragePower ratio (dB)

This Level measured usingMax. Hold FunctionThis Level measured

using Signal Averaging

Figure 6-6 Video Detector, Data Carrier



The examples in the following figure show the difference between the average and peak power for two types of digital carriers as measured on a RF spectrum analyzer. The 64 QAM signal shows a difference of 9.86 dB and the QPSK a difference of 7.51 dB.

The spectrum analyzer gives an approximation of the peak value of a signal, but is not fast enough to show all the peaks (the highest and fastest peaks will not be captured).

Example of peak vs. averagefor a 64 QAM signal

Example of peak vs. averagefor a QPSK signal

Max. (Peak) hold

Average

Max. (Peak) hold

Average

Figure 6-7 Examples of Peak vs. Average Power



Video Bandwidth (VBW) Filter The VBW filter is a low-pass filter that smoothes or averages the displayed noise. The VBW filter can have the effect of averaging out the modulation energy of a modulated carrier that would make the carrier level appear lower than it really is. To prevent this condition the filter must be set as large or larger than the RBW filter when making modulated carrier measurements.

As the value of the video bandwidth is changed on a modulated carrier, the amplitude will change. When measuring NTSC analog signals, the video filter should be set equal to or greater than the resolution bandwidth filter.

VBW = 300 kHz

VBW = 30 kHz

carriers offset for cla

Figure 6-8 Video Bandwidth, Modulated Carrier

Video Averaging HP digital spectrum analyzers provide the capability to perform digital averaging on a trace. This function is used to smooth noisy displays when measuring digital signal levels or when making the noise part of a C/N measurement.

Sweep Generator The sweep generator provides sweep control signals for the local oscillator and display. The sweep time is coupled with the RBW, VBW and SPAN settings in automatic mode. Therefore, when the RBW, VBW, or SPAN settings are changed, the sweep time of the generator changes to accurately sweep through the internal filters response time.



Trace Position A spectrum analyzer gives an accurate measurement only within its calibrated display range, which is typically 70 dB. In the left plot below, the measurement is made outside the calibrated range and marker #1 measures –20.13 dBmV. For the display on the right, the reference level was changed to bring the trace within the calibrated range for a more accurate reading of –22.00 dBmV. The indicated error is 1.87 dB. Even worse is the 11.15 dB error in the measured noise level at marker #2.

Figure 6-9 Trace Position



Marker Noise Function Some spectrum analyzers have a function called marker noise. This function will properly measure noise levels by averaging the noise power on consecutive points about the marker location. This function will also compute the proper power level by correcting for the internal log amp and detector effects. Lastly, the readout will be calculated in a resolution bandwidth of 1 Hz.

The left trace is measured without any corrections and the marker readout displays 9.90 dBmV in a 30 kHz bandwidth (RBW). When converting to a 1 Hz bandwidth without any analyzer corrections the new value is 9.90 - 10xlog(30 kHz/1 Hz) = -34.87 dBmV/1Hz.

The marker noise function corrects for analyzer log and detector effects to correctly display the noise power as –32.85 dBmV/1Hz for a difference of +2.02 dB, which is quite close to the expected 1.98 dB.

Figure 6-10 Marker Noise Function



Noise-to-Noise or Beat-to-Noise Correction When performing noise measurements, if the analyzer noise floor is within 10 dB of the system noise level, it will influence the amplitude reading. The reading will appear higher than normal.

Figure 6-11 Noise-to-Noise and Beat-to-Noise Levels

In the example above, if the system plus analyzer noise reading is –34.0 dBmV and the difference between the system noise and analyzer noise is 4 dB, the noise-to-noise correction factor from the following graph and table is 2.2 dB. Therefore the true amplitude level of the system noise is –34.0 dBmV –2.2 dB = – 36.2 dBmV.

The +1.25 MHz CSO beat has a level reading of – 27 dBmV. The difference between the beat and the system plus analyzer noise is 7 dB. The beat, beat–noise correction factor from the following graph and table is 0.97 dB. Therefore the true amplitude of the beat is –27.0 dBmV – 0.97 dB. = –28.0 dBmV.



Figure 6-12 Noise-to-Noise Correction



C/N Measurement The following figure shows how C/N measurement range and accuracy are effected by the carrier level input to the spectrum analyzer with and without the preamplifier.

Figure 6-13 C/N Measurement



CSO and CTB Measurement These charts show how the CSO and CTB measurement range and accuracy are effected by the carrier level input to the spectrum analyzer with and without a preselector.

Figure 6-14 CSO and CTB Measurement



Other Considerations Analyzer Gain Compression and Distortions - To verify that the internal mixer is not compressing signal amplitude, bring the signal peak just below the reference line and change the display to 1 dB/div. Now increase the attenuation setting of the instrument and note any amplitude changes. If the signal amplitude changes by 0.5 dB or more, the mixer is overloaded. Continue to increase the attenuation setting until the signal level does not change.

Note: If the carrier amplitude fluctuates more than 0.2 dB when viewed with an amplitude scale setting of 1 or 2 dB/div., use the max-hold function to capture any of the carriers peak swings.

To verify if distortions are created internally due to mixer overload, increase the input attenuation and view the change in their level. If they change less than the amount of attenuation step size, the mixer is overloaded.

Trace Position - When measuring signal amplitude levels, it is important to position the trace above the last graticule for accurate measurements. The display scale fidelity, which defines the accuracy of the measurement at any given trace position, is specified for all but the lower graticule location.

Measure Uncal - This indicator denotes the spectrum analyzer settings will not correctly display the signal’s amplitude and frequency values. This can happen when the RBW, VBW, span or sweep time has been altered from their auto-coupled mode of operation to a condition beyond their calibration range.

VBW/RBW Ratio - To ensure that the VBW filter is as large or larger than the RBW filter when making carrier level measurements, set this value to 1 or higher.

Switching Uncertainty -- Switching uncertainties effect amplitude accuracy and occur when the RBW and attenuation settings have been changed during a measurement.

Reference Level Accuracy and Display Scale Fidelity - Amplitude measurements not made at the reference level will have a known uncertainty. This uncertainty is listed in the manufactures data sheet as the reference level accuracy. The spectrum analyzer reference level is calibrated to a traceable standard; any other reference level used to make measurements will have an uncertainty associated with it.

Display Scale Fidelity - The uncertainty of an amplitude measurement when the signal to be measured is at a place other than the reference level; for instance, when using a marker on a signal peak other than at the reference level. Both uncertainties add to give a total uncertainty value that must be accounted for when making amplitude measurements.

Noise Level Corrections -- Noise is a signal with constantly varying amplitudes that has a Gaussian distribution. When noise passes through the bandwidth filters, log amp, envelope detector and an averaging circuit of a spectrum analyzer, the mean noise amplitude becomes skewed by a known amount.



In the linear display mode, after passing through the IF filter, the Gaussian envelope now takes on a Rayleigh distribution. And since the spectrum analyzer is a tuned peak detecting voltmeter, calibrated to indicate the rms value (-3 dB) of the peak of a sine wave, the mean value of the Rayleigh-distributed noise is scaled by the same amount. Scaling of the Rayleigh-distributed noise envelope produces an error that displays the noise level 1.05 dB below its true value.

When the Rayleigh-distributed noise is passed through the log amplifier, the higher noise levels are compressed more than the lower noise levels which when filtered skew the mean noise value by 1.45 dB below its true value.

Due to the shape of the RBW filter, the difference between the equivalent noise-power bandwidth of the near-Gaussian shaped filter and a true “brick wall” or rectangular filter, will have the effect of displaying the noise level 0.5 dB too high. The above corrections will combine to produce a displayed noise level that is approximately 2.0 dB too low.


Section 7 Signal Leakage

Upon completing this section, you will be have an understanding of:

Ingress

Egress

FCC rules, signal leakage limits, mandated engineering procedures, and practical considerations

7-2 Signal Leakage


What are Ingress and Egress?

Ingress Ingress is the introduction of unwanted signals into the cable plant. These unwanted signals emanate from the off-air users of the radio spectrum such as pagers, off-air broadcasters, FM radio stations and two-way radios. Ingress occurs when these signals mix with and sometimes override the desired signals. The result is unacceptable picture quality.

Egress Egress is the leakage of signal out from the confines of the CATV plant. This leakage is caused primarily by defective and/or improperly installed connectors; typically located in the drop portion of the plant. Egress of signal can also be caused by defective RF-shielding components, such as those found around the lid of an amplifier or passive device. Damage to the coaxial cable itself, such as a cracked shield, results in signal leakage out of the plant.

Ingress/Egress Effects Basically ingress/egress is a two-way street. If signals can leak out of the system, signals can also leak into the system.

Ingress

• Interferes with picture quality.

• “Ghost” images appear within CATV channels from over-the-air broadcasts.

• Short-wave broadcasts can disrupt signal transmissions in the return band.

• Noise build-up in the return band can over-power return receivers in the headend.

Egress

• Subject to FCC rules.

• Interferes with signals authorized to use the electromagnetic spectrum:

Ham Radio

Over-the-air Broadcast TV

FM Radio

Government Communications

FAA Voice Communications

• 75 Ohm termination loss resulting in waveform changes caused by reflections.

Signal Leakage 7-3


FCC Requirements

Rules You must have a copy of The Code of Federal Regulations, title 47-Telecommunication and part 76-Cable Television Service. A copy of Part 76 is included at the end of this section. Other FCC rules of interest are listed in the following table.

Part Service Description

15 Radio Frequency Devices 18 Industrial, Scientific, Medical 21 Domestic Public Fixed Services 69 Home Electronics 73 Broadcast 81 Maritime 83 Shipboard 87 Aviation 89 Safety Land Mobile 91 Industrial Land Mobile 93 Land Transportation 94 Private Operational Fixed 95 Personal Radio 97 Amateur Radio G Governmental

7-4 Signal Leakage


Signal Leakage Limits The following table lists the leakage limits specified by the FCC.

From (MHz) To (MHz) Leakage Level Measured at a Distance

-- 54 15 µV/m 100 feet/30 m

54 216 20 µV/m 10 feet/3 m

216 1000 15 µV/m 100 feet/30 m

108 137 10 µV/m 1,500 feet/452 m*

225 400 10 µV/m 1,500 feet/452 m*

Note: * At 1500 feet/450 m in air space above system (fly-over specification).

Mandated Engineering Procedures Signal Leakage measurements shall be taken with a calibrated field strength meter

and a horizontal dipole antenna (resonant half wave).

Signal Leakage 7-5


The antenna is positioned:

1 Three meters from the system component.

2 Three meters above ground level.

3 Directly below the system component (if possible).

4 At least 10 feet/3 meters from other conductors

10 ft/3 m

10 ft/3 m

The antenna, once positioned, is rotated about a vertical axis until the maximum meter reading is detected.

The maximum meter reading is expressed in the field strength RMS value of the synchronizing peak for each cable television channel for which signal leakage can be measure.

Video providers operating in the 108 to 137 and 225 to 400 MHz frequency bandwidths must:

1 Demonstrate compliance with a cumulative leakage index of less than 64.

2 Regularly monitor the physical plant by substantially covering the geographic area every three months.

3 Maintain a log of signal leakage indicating:

a All signal leakage exceeding 20µV/m at a distance of 3 meters in the aeronautical radio frequency bands.

b Date and location of each leakage source.

c Date leakage was repaired.

d Probable cause of leakage.

Prior to providing service to any subscriber in a new section of cable plant if operating in the 108-137 and 225-400 MHz bandwidths, the operator shall:

1 Demonstrate compliance with a cumulative leakage index of less than 64.

2 Ascertain that no individual leak in the new section of plant exceeds 20 µV/m at 3 meters in the aeronautical frequency bands.

7-6 Signal Leakage


Video providers shall not operate or provide service in the 108 to 137 and 225 to 400 MHz radio frequency bands until:

1 Notification to the FCC of all signals carried in the aeronautical radio frequency bands (FCC Form 325).

2 CLI (Cumulative Leakage Index) is demonstrated to be below 64.

3 Proper frequency offsets are maintained in the aeronautical radio frequency bands.

Calculating Cumulative Leakage Index The Cumulative Leakage Index (CLI) is calculated using the following equation:

( )V/m in leakage measuredEWhere

EEEE MileageMonitored

agePlant Milelog10CLI

,isThat

leak each of sum MileageMonitored

agePlant Milelog10CLI

n

2n

23

22

21

2

µ=

++++×=

×=

L

Note that the monitored mileage must be at least 75% of the plant mileage.

As an exercise, calculate the CLI for the following conditions. Plant Miles: 1000

Plant Miles Driven: 750

Leakage Recorded: 3 at 450 µV/m

30 at 150 µV/m

300 at 50 µV/m

What is the CLI? ___________

Alternatively, the CLI can be met by measuring the leakage in the airspace above the system by doing a fly over and recording the signal strength that proves that at an altitude of 450 meters (1500 feet), the field strength is not greater than 10 µV/m.

This measurement must be made once each calendar year. Detail of this alternative appears in section 76.611, (a), (2).

Signal Leakage 7-7


Frequency Offsets The transmission of carriers or other signal components capable of delivering peak power levels equal to or greater than 10-5 watts at any point in the system is prohibited:

1 Within 100 kHz of the frequency 121.5 MHz.



Converting Between dBmV and µV/m To convert dBmV to µV/m:

20dBmV

MHz

20dBmV

MHz

MHz

20dBmV

10freq21

101000freq021.0

Vfreq021.0m/V101000V

××=

×××=

µ××=µ×=µ

As an example, convert –41 dBmV measured at 100 MHz to µV/m:

m/V71.1800891.021001010021

10freq21m/V

2041

20dBmV

MHz

µ=×=

××=

××=µ−

To convert µV/m to dBmV

×

×= µ

MHz

m/VdBmV freq21

Elog20V

As an example, convert 18.71 µV/m measured at 100 MHz to dBmV

( )

dBmV

freqE

VMHz

mVdBmV

4105.220

00891.0log202100

71.18log20

1002171.18log20

21log20 /

−=−×=

×=

×=

××=

×

×= µ

7-8 Signal Leakage


The following table lists the leakage limits (in dBmV) for sub-channels T7 through T13 and EIA standard channels 2 through 158.

Distance Distance Distance DistanceChannel dBmV (meters) Channel dBmV (meters) Channel dBmV (meters) Channel dBmV (meters)

T7 -18.1 30 36 -49.8 30 77 -57.6 30 118 -60.5 30T8 -24.3 30 37 -50.0 30 78 -57.7 30 119 -60.6 30T9 -27.9 30 38 -50.2 30 79 -57.8 30 120 -60.6 30T10 -30.4 30 39 -50.3 30 80 -57.9 30 121 -60.7 30T11 -32.4 30 40 -50.5 30 81 -58.0 30 122 -60.8 30T12 -34.0 30 41 -50.7 30 82 -58.1 30 123 -60.8 30T13 -35.3 30 42 -50.8 30 83 -58.1 30 124 -60.9 30

2 -35.3 3 43 -51.0 30 84 -58.2 30 125 -61.0 303 -36.2 3 44 -51.1 30 85 -58.3 30 126 -61.0 304 -37.0 3 45 -51.3 30 86 -58.4 30 127 -61.1 305 -38.2 3 46 -51.4 30 87 -58.5 30 128 -61.2 306 -38.8 3 47 -51.6 30 88 -58.6 30 129 -61.2 307 -45.3 3 48 -51.7 30 89 -58.7 30 130 -61.3 308 -45.6 3 49 -51.9 30 90 -58.8 30 131 -61.4 309 -45.9 3 50 -52.0 30 91 -58.8 30 132 -61.4 30

10 -46.1 3 51 -52.1 30 92 -58.9 30 133 -61.5 3011 -46.4 3 52 -52.3 30 93 -59.0 30 134 -61.5 3012 -46.7 3 53 -52.4 30 94 -59.1 30 135 -61.6 3013 -46.9 3 54 -55.0 30 95 -39.6 3 136 -61.7 3014 -42.1 3 55 -55.2 30 96 -40.2 3 137 -61.7 3015 -42.5 3 56 -55.3 30 97 -40.7 3 138 -61.8 3016 -42.9 3 57 -55.4 30 98 -41.2 3 139 -61.8 3017 -43.3 3 58 -55.5 30 99 -41.7 3 140 -61.9 3018 -43.7 3 59 -55.7 30 100 -59.2 30 141 -62.0 3019 -44.0 3 60 -55.8 30 101 -59.3 30 142 -62.0 3020 -44.4 3 61 -55.9 30 102 -59.3 30 143 -62.1 3021 -44.7 3 62 -56.0 30 103 -59.4 30 144 -62.1 3022 -45.0 3 63 -56.1 30 104 -59.5 30 145 -62.2 3023 -49.7 30 64 -56.2 30 105 -59.6 30 146 -62.2 3024 -49.9 30 65 -56.4 30 106 -59.6 30 147 -62.3 3025 -47.6 30 66 -56.5 30 107 -59.7 30 148 -62.4 3026 -47.9 30 67 -56.6 30 108 -59.8 30 149 -62.4 3027 -48.1 30 68 -56.7 30 109 -59.9 30 150 -62.5 3028 -48.3 30 69 -56.8 30 110 -59.9 30 151 -62.5 3029 -48.5 30 70 -56.9 30 111 -60.0 30 152 -62.6 3030 -48.7 30 71 -57.0 30 112 -60.1 30 153 -62.6 3031 -48.9 30 72 -57.1 30 113 -60.2 30 154 -62.7 3032 -49.1 30 73 -57.2 30 114 -60.2 30 155 -62.7 3033 -49.3 30 74 -57.3 30 115 -60.3 30 156 -62.8 3034 -49.5 30 75 -57.4 30 116 -60.4 30 157 -62.8 3035 -49.6 30 76 -57.5 30 117 -60.4 30 158 -62.9 30

Signal Leakage 7-9


Good Engineering Practices The following suggestions will help to assure compliance with FCC requirements and to maintain a clean system.

1 All vehicles should have signal leakage detection equipment.

2 All maintenance employees should be required to file leakage reports daily.

3 All systems should have someone assigned to signal leakage repair.

4 Meeting or exceeding CLI requirements helps tremendously toward minimizing ingress into the return path. A good rule to follow is; if a leak can be detected, it should be fixed to ensure a clean return path.

Plant Maintenance, Proof of Performance and Signal Leakage Rev[1]. A

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