EMC TESTING: A LABORATORY’S PERSPECTIVE

PRESENTED TO THE ROCKFORD CHAPTER OF THE IEEE EMC SOCIETY - MARCH 26TH 2015 BY DEREK WALTON L F RESEARCH

WHAT IS THIS PRESENTATION

ABOUT?

Testing Rather than Design

The intent of Testing

Different Tests

Testing Pitfalls

Some debugging techniques

Laboratory Accreditation

TESTING RATHER THAN

DESIGN

Loads of info on EMC design - Much less on testing

Books

Magazines

Symposiums

Workshops

Training Courses

On-the-job Training

Standards - to a point

THE INTENT OF TESTING

To recreate, in a controlled manner, the environment in which something will operate

To verify a design performs as expected in that environment

To use engineering to overcome impractical test hurdles

To understand whats happening during a test to ensure correct EUT Evaluation

To use repeatable and reproducible methods regardless of test location

To make sure folks are ethical

“THE” EMI TEST AFTER ALL, EMI TESTS ARE ALL THE SAME RIGHT?

The four basic EMI testsConducted Emissions - The measuring of electrical noise leaving an EUT ( mostly ) on electrical conductors

Radiated Emissions - The measuring of electrical noise leaving an EUT by means ( mostly ) of radio waves

Conducted Susceptibility - The behavior of an EUT to electrical noise arriving ( mostly ) on electrical conductors

Radiated Susceptibility - The behavior of an EUT to electrical noise arriving ( mostly ) by means of radio waves

Measuring Conducted Emissions

Why do we measure conducted emissions?

Direct Connection to other devices

Low frequency indirect coupling

How do we measure conducted emissions?

Current monitor probe

Voltage probe

Line Impedance Stabilization Network ( LISN )

Absorbing Clamp

Special Case - Clicks

Conducted Emissions - Background

Are we measuring Voltage or Current?

Voltage? From where to where?

Current? Around what loop?

Conducted Emissions - BackgroundCont…

What should “X” be?

How long should “L” be?

Where do we measure?

Conducted Emissions - Current Monitor Probe

Essentially a transformer with burden resistor

Primary is the wire or bundle under test, secondary the measuring instrument ( MI )

Affords good isolation between EUT and MI

Easy to use clamp on device

Electro-statically shielded

Conversion factor usually called Transfer Impedance units dB Ohms

Same device measures any combination of wires

Conducted Emissions - Voltage Probe

Simple device similar to a current clamp or an oscilloscope probe

CVP has Isolation between EUT and MI, voltage probe has NO Isolation between EUT and MI

Electro-statically shielded

Voltage probe Ideally needs a ground connection close to measurement point - may require floating MI. CVP does not.

Conversion factor usually given in dB

Conducted Emissions - LISN

A lumped representation of impedance

Must have connection to a reference plane for measurement and safety

Sample port provides Non-Isolated signal for the MI

MI needs protection from coupled transients

Conducted Emissions - Absorbing Clamp

A variation on the current probe that tries to control impedance also

Used in place of low frequency Radiated emissions tests

Requires cable “raceway”

Typically requires multiple small sweeps

Conducted Emissions - Clicks

A transient emissions rather than continuous emissions test

Uses LISN

Special receiver or laborious manual test

Mainly used on appliances that switch regularly during operation

Conducted Emissions - Harmonic and Flicker

Most frequently required for Equipment greater than 50 Watts drawing power from the grid

Harmonics is a test to determine how much the EUT distorts the power source

Flicker is a test to determine how much the EUT disturbs the power source amplitude

Measuring Radiated Emissions

Why do we measure Radiated Emissions?

Protect over the Air signals

Minimize risk of causing undesired events

What are ambients?

Test Sites

Tools to measure Radiated Emissions?

Absorbing Clamps

E-Field Antennas

H-Field Antennas

Radiated Emissions - BackgroundMade up of Electric ( E ) and Magnetic ( H ) field components

The “nature” of a radio wave depends on how far you are from the source

Far away: Plane wave.

Ratio of E to H = 377 Ohms

Close in ( often called near-field ): It depends on the source being dI/dt or dV/dt

Ratio of E to H complex

Crossover point λ/2π ( ish )

Radiated Emissions - Ambients

What are Ambients

How can we stop them

Radiated Emissions - Test Sites

Free field Site

Ground Plane

Open Area Test Site ( OATS )

Shield Room

SAC

FAC

Turntable

Mast

Ground Plane

Reverb

Transmission Line Device

Radiated Emissions - Free Field Site

No attempt or inability to control the EMI properties of the area

Often used for in-situ testing

Advantage: Cost, Location

Disadvantage: Ambient Hell

Radiated Emissions - Ground Plane

A conducting plane that resides under the EUT and extends well beyond the area testing takes place

Provides a controlled reference plane consistent between locations

Mimics the local structure where an EUT might finally reside.

Provides an attachment point for measuring and impedance control networks

Radiated Emissions - Ground Plane

Not always the floor of the test area

Military, Aerospace, automotive tests specify a test bench with ground plane on it

Ground plane may be Copper, Aluminum, Brass, Steel, Carbon Fibre, or none

Ground plane “bonded” to chamber by low impedance connections

Physical layout of equipment on the ground plane tightly controlled by test standard

Radiated Emissions - Open Area Test SiteA ground plane constructed to meet a performance criteria typically over 2 frequency ranges:

30 MHz to 1 GHz - NSA

1 GHz to 40 GHz - SVSWR

Above 1 GHz Ground Plane covered by absorber

If covered by an RF transparent structure, NSA and SVSWR need to be volumetric

Needs to be “ Flat ”

Ideally should be Earthed

May require sheet metal in the sweet-spot, mesh may be OK elsewhere.

Edges best “ petaled “

No nearby obstructions that could reflect RF

NSA = +/- 4 dB from ideal

Radiated Emissions - CALTS

High precision ground plane specifically for antenna calibration

NSA = +/- 1 dB

Special Antennas

BIG!

FLAT!

$$$$$$

Radiated Emissions - Background - Shielding

An RF shield works by reflecting the RF - Think mirror and also by absorbing the RF - Think dissipation

Better conductivity/ Thicker material = better shield.

Shield compromised by openings, slots, seams, penetrations ( especially the EUT )

Outside signals reflected away or absorbed

Internal signals from EUT also reflected

Radiated Emissions - Semi-Anechoic Chamber (SAC)

Typically a metal box that has the walls and roof lined with absorber

Ferrite Tiles

Dissipating Cones

Still needs to meet NSA and SVSWR: must be volumetric

Radiated Emissions - Fully Anechoic Chamber (FAC)

Basically a SAC in which the floor is also covered with Absorber material

Used mainly for Radio device testing

Radiated Emissions - Turntable

SAC/FAC kills reflections, what about emissions outside of the direct antenna path?

Turntable rotates so all sides illuminate the antenna

FAC turntable may rotate in X, Y and Z planes.

Turntable must be flush with and bonded to the ground plane

Radiated Emissions - Antenna Mast

What about emissions that don't leave horizontally?

Receive antenna moves up and down a mast. Typically 1 to 4 metres

Antenna must tilt on mast to “boresight” on the EUT.

Radiated Emissions - Reverb Chamber

Conducting box with NO absorbing material

EUT resides in a “sweet-spot” in the chamber

Physical paddle rotates slowly to stir the field

EUT orientation irrelevant

Receive antenna orientation irrelevant

Significantly lower cost to own

Radiated Emissions - Transmission Line Devices

Strip-Line: A plate suspended above a ground plane

Tri-Plate: A plate suspended between two ground planes

TEM Cell: Essentially a co-ax cable with access to the area inside with one end terminated in a resistive load

GTEM: Essentially a co-ax cable with access to the area inside with one end terminated by field absorbing material as well as a restive load

Radiated Emissions - Measuring Tools

More than 1 test antenna needed

Electric fields

Reference ( Roberts ) Dipole

Biconical ( Basket ) Antenna

Log Periodic Dipole Antenna

Hybrid Bi-Log

Horn Antenna

Double Ridged Wave Guide ( DRWG ) Antenna

Monopole Rod

Magnetic Fields

Loop Antenna

Antenna Terminology

Radiated Emissions - Reference Dipole

Often Called the Roberts Dipole

Elements adjustable

Calculable factors for a given element length

Still the FCC reference Antenna

Radiated Emissions - Biconical AntennaSmall diameter elements replaced with “Egg beaters”

Fixed tuning

Newest designs calibrated by Network Analyzer

Preferred FCC test antenna

Radiated Emissions - Log Periodic Dipole Array

A series of different length dipole antennas sharing a common feed

Covers wide frequency range

With the development of NEC factors can be modeled

Preferred FCC test antenna

Radiated Emissions - Bi-Log Antenna

A combination of the biconical and log periodic antennas

Useful over a wide range of frequencies

Undesirable at close test distances

Factors need to be measured

Test Lab preferred antenna

Radiated Emissions - Background

Horn Antenna essentially open ended Waveguide

Used with a “launcher” to adapt horn to co-ax cable

Factors accurately calculable

High Gains possible

Radiated Emissions - DRWG Horn Antennas

Double Ridged Wave Guide Horn ( DRWG ) antenna uses ridges to greatly expand the useable frequency range of a horn antenna

Older DESEC Horn superseded by NEC derived model

Radiated Emissions - Monopole Rod Antenna

Monopole Rod Antenna is arguably not really a antenna

Under most circumstances the test set-up capacitively couples with the rod part of the antenna

Should the Counterpoise be grounded or not? Some antennas cannot support both test set-ups

New version has optical feeder cable

Radiated Emissions - Loop Antenna

Loop antennas are basically all the same, used for Magnetic field measuring

Most models are Electro-statically shielded

Best loops are balanced feeds

DO-160G still using a compass

Radiated Emissions - Absorbing Clamp

Not really an Antenna, but used to determine radiated emissions

Relatively repeatable measurements

Used mainly for Appliance testing

Radiated Emissions - Background

Antenna Factor

Antenna Gain

Directivity

Beam width

Side lobes

Balun Symmetry

Power

VSWR

Radiated Emissions - Background

Measuring Instrument

Receiver

Spectrum Analyser

Preselector

Peak Vs QPeak Vs Average

Cables

Filters

Preamplifiers

Attenuators

Radiated Emissions - Sanity Check

Sanity checks essential in EMC testing

Radiated Emissions best sanity check is a reference radiator

Arguably the best Ref. Rad. is a Spherical Source

Performing Conducted Susceptibility

Frequency Domain Tests

AF Conducted Susceptibility - Hz to MHz

RF Conducted Susceptibility - Hz to GHz

Time Domain Tests

Voltage Variations - Microseconds to Seconds

“Surges”- Milliseconds to Microseconds high energy

“Spikes” - Microseconds to Nanoseconds low energy

ESD

Conducted Susceptibility Background

Why Two Tests?

Conducted AF tests essentially are performed on power leads

The intent is to expose an EUT to the voltage ripple on the power source that is likely to be observed when in use.

Conducted RF tests essentially are performed on all cables

The intent is to SIMULATE the current that would be induced on a cable by an antenna too big/powerful to fit inside a shield room

Conducted Susceptibility AF Tests

Programmable Power Supply: technique supplies both EUT power and the test signal

Test signal may be one frequency OR multiple harmonics combined into a complex wave shape

Mechanism for limiting test signal independent of EUT power needed

Signal Source, Power Amplifier and Coupler: technique separately generates test signal and couples via a transformer or a capacitor

Signal Source normally an ARB

Coupling mechanism usually a transformer

EUT power supply needs protection from the test signal

Test signal difficult to discriminate on power leads due to EUT fluctuations

Conducted Susceptibility AF Tests

Conducted Susceptibility RF Tests

Coupling Decoupling Network: This device isolates the EUT and the support equipment and provides Controlled application of test signal in accordance with the theoretical model

EM Clamp: An injection transformer that contains the ability to add series impedance to the support equipment end of the cable.

Inductive Clamp ( often called a bulk current clamp ) : An RF current transformer

Conducted Susceptibility Coupling Decoupling Network

Designed to force cable impedance to be around 150 Ohms, intrudes into wiring

Intended to be used on all cables entering and leaving an EUT

Test level determined in an RF jig prior to test

Forward power is the control parameter, no limiting

Many different types

Conducted Susceptibility EM Clamp

Construction tightly controlled by CISPR

Intended to try to force EUT cable impedances to 150 Ohms with no intrusion

Test level determined in an RF jig prior to test

Forward power is the control parameter, must use limiting

Conducted Susceptibility Inductive Clamp

RF injection transformer with defined Insertion Loss

Has no ability to control cable impedances

Ideal for in-situ testing, or MIL/Aerospace/Automotive

Tests differ in use:

Forward Power

Net Power

Pre-determined limit

Real Time

Most test use limiting

Conducted Susceptibility Voltage Variations

Intent of these tests is to simulate changes in the power supply voltage and where appropriate frequency

Usually “slow” but have almost no limiting impedance

Often caused by load shedding, power transfer or application of power to a heavy load

Conducted Susceptibility Load Shedding

Typical tests:

Automotive Load Dump

Finite but very high energy change in voltage for hundreds of milliseconds

Aerospace Load Dump

A zero impedance source change in voltage for about a second

EUT must “ride” the event

Test signal derived by dedicated power supply or Surge generator.

Conducted Susceptibility Power Transfer

Fluctuations resulting in power dropping to Zero

Power Grid Load redistribution

Change over of power source

EMI

Cosmic Events

microsecond to multi-second interrupts

Multiple interrupts

Test signal derived by specialized power supply

Conducted Susceptibility Load application

Fluctuations in power that do not result in drop out

Typical causes application of heavy loads

Motors

Starters

RADARs

Ovens, Heaters

Test signal derived by dedicated power supply

Conducted Susceptibility Surges

A relatively short duration increase in power voltage, with significant energy content

A relatively short duration increase in signal voltage on lines that cross regions where changes in potential with respect to Earth is possible

Inter-plant communications

Telephone lines

Indirect Lightning

Test signal usually derived by specific simulator

Conducted Susceptibility Surges

Conducted Susceptibility Spikes

A relatively fast change in power supply voltage, usually the transient source impedance is reflected in the role played by the power cables impedance

Spikes couple on to test cable from adjacent cables and structure.

Not intended to cause damage, more intended to cause upset.

Sources: relay and solenoid coils, Switch contacts, motors, motor drives, indirect lightning

Simulated by switching inductive devices or by specific simulator.

Indirect Lightning

Multiple Burst Lightning

Conducted Susceptibility SpikesAll these spikes are couple onto Interconnect leads

Conducted Susceptibility Electrostatic Discharge

ESD simulates the injection of charge into, or onto an EUT.

Most ESD Simulators represent the human body model: there are a number of them! Most ESD events are only a few nanoseconds long

Aerospace ESD is an Air Discharge test: typically 15 kV

Commercial ESD requires Air and Contact tests, in addition, coupling plane tests are possibly required: typically 8 kV contact and 15 kV Air

Automotive ESD requires Air and also contact including directly onto EUT pins: typically up to 25 kV

Gaming machines may require up to 50 kV!

Conducted Susceptibility Electrostatic Discharge

Air Discharge Contact Discharge

Conducted Susceptibility Electrostatic DischargeHorizontal Coupling Plane Vertical Coupling Plane

Performing Radiated Susceptibility

Frequency Domain Tests

Electric Field Susceptibility - Hz to GHz

Magnetic Field Susceptibility - DC to MHz

Test Philosophies

Time Domain Tests

EMP - Microseconds

Magnetic Fields - typically damped Sine-waves

ESD

Radiated Susceptibility Background

The generation of and Electric, Magnetic or Plane Wave Field

Illegal to transmit without license

Low frequency E-Fields ( typically under 100 MHz ) difficult to generate inside a chamber = $$$$$$

High frequency E-Fields ( typically 100 MHz to low GHz ) relatively easy but still = $$$

Very high frequency E-Fields ( typically a few GHz and up ) need specialized equipment = $$$$$$$$

Magnetic fields relatively easy to generate but caution necessary

High Electric or Magnetic field strengths = $$$$$$ and potentially lethal

Radiated Susceptibility Low Frequency E-Fields

Two Primary techniques:

Transmission Line

E-Field Radiator

Radiated Susceptibility High Frequency E-Fields

Two Primary techniques:

Direct illumination by an Antenna

Indirect illumination in a Reverb chamber

Radiated Susceptibility Very High Frequency E-Fields

Two Primary techniques:

Direct illumination by an Antenna

Indirect illumination in a Reverb chamber, typical upper frequency limit of 40 GHz

Radiated Susceptibility H-Fields

Three Primary techniques:

Partial illumination by a Loop Antenna

Partial illumination by a floor mounted loop Antenna

Complete emersion in a series of coils e.g. Helmholtz

May need to cancel Earths Magnetic Field

Beware of Ferrous Materials

Radiated Susceptibility H-Fields

Radiated Susceptibility - Test Philosophies

IEC Uniform Field Plane i.e. EN61000-4-3

Real Time Illumination and control by field sensor i.e. MIL STD 461F

Substitution of EUT by Field Sensor then use forward power during the test i.e. DO-160G

Reverb Mode-Stirred or Mode-Tuned

Radiated Susceptibility - EN61000-4-3

Establishes a uniform field plane by measuring up to 20 points on a matrix and deriving a single power level replayed during testing

Places the EUT face to be tested in the UFP

Requires repositioning of the EUT to expose all sides

CW and Modulated fields different: Modulation 5.1 dB higher

Radiated Susceptibility - MIL STD 461F

Places one or more field sensors in the test set up and monitors/controls the test in real time

Test is run with modulation applied

CW and Modulation field amplitudes the same

Radiated Susceptibility - RTCA DO-160G

Places one field sensor in the test set up where the EUT normally resides

Replays forward power during the EUT test

Different modulations applied

CW and Modulation field amplitudes the same

Radiated Susceptibility - Reverb

Mode Stirred: No attempt made to position the “paddle” that stirs the field inside the chamber

Mode Tuned: The paddle is positioned in small rational increments, dwelling long enough to expose the EUT through a complete operational cycle

Very high fields possible with low power amplifiers.

Software really desirable to control test.

Radiated Susceptibility EMP

One Primary technique:

Transmission Line

Very high voltage Generator

Very fast pulse

Field measured by D-Dot probe

Radiated Susceptibility DC/Damped Sine H-Field

One Primary technique:

Multiple Coils

Very BIG coil

Very High Current

May need to be field portable

Radiated Susceptibility ESD Magnetic Field

One Primary technique:

Loop

Very high voltage Generator

Very fast pulse

Field measured by B Dot probe

EMC TESTING PITFALLS

Common EUT Pitfalls that ruin EMC Tests

Wrong EUT build, wrong software running

Unrepresentative wiring harness

Wrong wire type, length

Pigtail shields, foil shields

Inappropriate support equipment

Poor monitoring means

Requesting wrong test standard

EMC DEBUGGING TECHNIQUES

EMC Debugging Techniques

Close field Probes

AL Foil and Copper Tape

Ferrite Beads

Current Probes to isolate CM and DM noise

“Tee” filter to aid noise direction

SA 0Hz Span

SA Sweep Rate change

Capacitively coupled sense probe

EM Scan

ACCREDITATION

Laboratory AccreditationA 3rd party review of a test laboratory to assure a minimum performance level

Assessed against the requirements of ISO17025

Requires a management system

Requires MU budgets

Requires Accredited Calibrations

Assessment 1 to 4 days every 2 years

Assessors need to technically challenge the lab on over 3800 current standards (NVLAP)

Little value to a good lab

Raises status of a mediocre lab

Eliminates a bad lab

Required only for a few programs

QUESTIONS?

THANK YOU FOR LISTENING!

DEREK WALTON NCE #438 815 566 5655 WEB:WWW.LFRESEARCH.COM EMAIL:DEREK@LFRESEARCH.COM LINKED-IN: HTTP://WWW.LINKEDIN.COM/IN/DEREKNWALTON/EN