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DATA TRANSMISSION AND TELEMETRY

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INTRODUCTION Telemetry is presentation of measured values

at location remote from site of measurement. Greek words ‘Tele’: remote, ‘meter’: measuring. e.g., doctor analyzing data of patient from remote location

Telemetry involves three steps:a. converting measured quantity to signalb. Transmission of that signal over proper

channelc. Its reconversion to actual data for recording,

displaying(CRT) for graphical analysis and further computation

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DATA ACQUISITION SYSTEM

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BLOCK DIAGRAM OF TELEMETRY SYSTEM

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FACTORS INFLUENCING TELEMETRY SYSTEM DESIGN

Primary criteria for choice and design is accuracy.

System is decided whether transmitted data is in Analog or Digital domain.

For digital data to be transmitted, error detection, recognition and correction capability make system accurate.

Selection of apt. bandwidth for data channel and data link to avoid crosstalk.

Power levels must be low to reduce noise generation.

S/N ratio of the system must be high.

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TYPES OF TELEMETRY SYSTEMS

Landline Telemetry System: Power Lines, Telephone Lines and Electrical Wires. Distance ranges from 50m to 1 km e.g., labs, industries. Types: Current, Voltage & position.

Radio-Frequency System: Radio links from1 km to 50 km at 4MHz. For distance >50 Km Microwave links are used 890 MHz to 30GHz. Repeaters are installed after every 30 to 60 km for long distance transmission.

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LANDLINE TELEMETRY SYSTEM

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VOLTAGE TELEMETRY SYSTEM Measured Variable is transmitted in form of voltage At transmitting end, Slide wire is connected in series

with battery. Slide wire is further connected to Bourdon tube for

pressure measurement. When pressure changes, slider actuates the slider of

potentiometer. Thus , change in Voltage is transmitted to rvr.

It is transmitted at the receiver end.

Circuit of Voltage Telemetry System

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CURRENT TELEMETRY SYSTEM

Circuit of Current Telemetry System

Working is almost same as of Voltage telemetry system

When pressure changes Borden tube moves sliding contact thereby value of current changes

This current passes through pair of wires and measured by milliammeter.

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DEMERITS AND MERITSMerits: Effective for short distance measurement V and I can be easily transmitted Circuitry required is simple Wide variety of primary sensing elements are

available to measure reqd. variable.Demerits: Demands high S/N ratio that is difficult to

calibrate. Need to be protected from EMI, noises and

distortions in the channel. Multiplexing is difficult Limited frequency response

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R.F. TELEMETRY SYSTEM

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R.F. TELEMETRY SYSTEM

No physical link between telemeter transmitter and receiver.

Link is established through radio links.

Examples : in spacecrafts, rockets and missiles corrective actions can be taken from stations with help of R.F. Links

In instrumentation the o/p of transducer is considered as modulating signal.

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R.F. TELEMETRY SYSTEM

Modulation schemes:When signal is in continuous form:Amplitude ModulationAngle ModulationWhen signal is in form of pulses: Pulse modulation

AM : amp. of high-carrier signal is varied acc. to instantaneous value of modulating message

signal m(t)Ac

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tFrequency

Phase

CALCULATING FM BANDWIDTH

For PM

For FM

t)ωsin(β(t) m

dt t)ωsin(β(t) m

m

d

f

Vfβ

FM: modulation index, is ratio of deviation, fd multiplied by amplitude of modulating signal divided by modulating frequency, fm.

PM : the phase shift is proportional to instantaneous amplitude of the modulating signal.

mp ek

ANGLE MODULATION

)f(f2

1)(βf2BW

devmax

max

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FREQUENCY TELEMETRY SYSTEM BLOCK DIAGRAM:

THE DISADVANTAGE OF FM TELEMETRY SYSTEM IS CAPACITY OF CHANNELS OFFERED IS LESS

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PULSE TELEMETRY SYSTEM

Pam telemetry system: Employs TDM technique

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TYPES OF PULSE MODULATION

PPM

PWM: Monostable multivibrator

PAM

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