sks am trs antenna measurement

8/8/2019 Sks Am Trs Antenna Measurement

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By S. K. Srivastava,

Dy. Director(Engg), STI(T)

AM Transmitter/Antenna/Measurement



OUTLINE

Principle of modulation in old Generation/

new generation AM Transmitters

Basic blocks.

Differences in MW & SW Trs.

AM Antenna Measurement parameters.



Block diagram

Oscillator TRPA Driver PAFilter

network

Audio Filter Pre amp Corrector AF Driver Oscillator

Mod. Transf

Audio

Control

SupervisionCooling

VentilationDC Supplies

Power

distribution



The POWER AMPLIFIER

IN BROADCAST TRANSMITTERS ,

THE EFFICIENCY OF THE RF POWER

AMPLIFIERS HAS TO BE VERY HIGH.

TO ACHIEVE A HIGH OVERALL

EFFICIENCY. A HIGH EFFICIENCY PA

REDUCES LOSSES IN THE PA ASWELL AS THE MODULATOR.



RF POWER AMPLIFIERSThe four basic classes of Amplifiers are:

1. CLASS A : 360 DEGREES OPERATING ANGLE

2. CLASS B : 180 DEGREES OPERATING ANGLE

3. CLASS AB : 180-270 DEGREE OPERATING ANGLE

4. CLASS C : < 180 DEGREES OPERATING ANGLE



CLASS A AMPLIFIER



CLASS B AMPLIFIER



CLASS C AMPLIFIER



CLASS D AMPLIFIER



GRID CIRCUIT OF TYPICAL CLASS D RF AMPLIFIER



OUTPUT NETWORK OF TYPICAL CLASS D AMPLIFIER



CLASS D AMPLIFIER



CLASS

OF P.A.

P.A.

EFFICI-

ENCY

DC

POWER

INPUT

TO P.A.

AUDIO

POWER

INPUT

TO P.A.

POWER

LOSS IN

P.A.

POWER

LOSS IN

MOD.

TOTALPOWER

LOSS

CLASS

C

65% 154

KW

77

KW

81

KW

77

KW

158

KW

CLASS

D

85% 117

KW

59

KW

26

KW

59

KW

85

KW

COMPARISON OF CLASS C AND D AMPLIFIER



PRESENT DAY PAs.. Are active devices that work as an ideal

switch, thereby providing the opportunity toreplace valve based PAs with solid statePAs.

The whole idea is to saturate the activedevice in order to reduce the devicedissipation & also design the output

network transient response in such a waythat harmonics in the output current arelimited below a certain desired level.



Active Device in saturation

region«..

Voltage andcurrent do

not existsimultaneously across thedevice. Atthe time of

switching,even the rateof change of voltage andcurrent iszero..

This accounts

for high

efficiency of

class E(90 to

95%).



CLASS C, D CLASS E

1.ACTIVE DEVICE IS USED AS A CURRENTSOURCE.

2.THE DEVICE OUTPUT IMPEDENCE IS

ALWAYS HIGH.

3.THE ACTIVE DEVICE IS NOT ALLOWED TO

SATURATE.

4.DURING TURNING ON/OFF , VOLTAGE AND

CURRENT EXISTS SIMULTANEOUSLY

ACROSS THE ACTIVE DEVICE.

5.ONLY THE INPUT DRIVE DETERMINES THE

CURRENT THAT FLOWS THROUGH THE

ACTIVE DEVICE DURING CONDUCTION.

6.THE VOLTAGE ACROSS THE ACTIVE

DEVICE WHEN IT IS CONDUCTING DEPENDS

ON THE VOLTAGE RESPONSE OF THE

OUTPUT NETWORK TO THE CURRENT

PULSES DELIVERED BY THE ACTIVE DEVICE

INTO THE OUTPUT NETWORK.

1.ACTIVE DEVICE IS USED AS AN IDEAL SWITCH.

2.THE IMPEDANCE IS VERY HIGH WHEN OFF AND

VERY LOW WHEN ON.

3.THE ACTIVE DEVICE IS MADE TO SATURATE

INTENTIONALLY.

4.VOLTAGE AND CURRENT DO NOT EXIST

SIMULTANEOUSLY ACROSS THE DEVICE AT ANY

TIME.

5.THE OUTPUT NETWORK DETERMINES THE

CURRENT THROUGH THE ACTIVE DEVICE DURING

CONDUCTION.

1. THE VOLTAGE IS ZERO AS THE ACTIVE DEVICE

SATURATES AND IS INDEPENDENT OF THE

OUTPUT NETWORK.



Block Diagram of modern AM Transmitter



FEATURES OF MODERN

TRANSMITTERS FULLY SOLID STATE

MODULAR POWER AMPLIFIERS

DIGITAL AMPLITUDE MODULATION

REDUNDANCY OF AMPLIFIERS

LOW DISTORTION

HIGH EFFICIENCY

LOW NOISE



FEATURES Contd.. NO WARM UP TIME

POWER FOLD BACK ON O/L

VARIABLE POWER LEVEL

CAPABLE OF TAKING DIGITAL

TRANSMISSION



Summary---Basic Analogy of a PA



0V

+V

HALF QUAD

SWITCHES A A

Voltage at -- A



FULL QUAD

COIL

All these switches are operated alternately at RF

Output at the coil



COMBINING MANY AMPLIFIERS

FULL QUAD PA

Combining by

Transformer

action

RF Switching signal



Combining 2 PAs

ONE PA

ONE PA

TWO PAs

ONE PA

NO PA ON

2PA



Low Pass Filter

Square wave is shaped to smooth sine wave



Step By Step Modulation

STEP BY STEP

Modulation

Distortion filled by

SMALL step voltages



MODULATION STRATEGY IN S7HP

The overall modulation management is done in the

encoder board.

The output amplitude of the modulated RF signal is

obtained by vector summing of the main anddedicated module output signals

For each clock pulse, the encoder determines the no.

of main and dedicated modules to be activated.

The no. of dedicated modules to be activated is

calculated from the no. of main and dedicated

modules activated at time t in relation to value to be

reached at time t+1.



I.I.George,Dy.Director, STI(T)

Vector Modulation



Thank you

sks am trs antenna measurement

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