sks am trs antenna measurement
TRANSCRIPT
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
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OUTLINE
Principle of modulation in old Generation/
new generation AM Transmitters
Basic blocks.
Differences in MW & SW Trs.
AM Antenna Measurement parameters.
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Block diagram
Oscillator TRPA Driver PAFilter
network
Audio Filter Pre amp Corrector AF Driver Oscillator
Mod. Transf
Audio
Control
SupervisionCooling
VentilationDC Supplies
Power
distribution
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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.
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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
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CLASS A AMPLIFIER
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CLASS B AMPLIFIER
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CLASS C AMPLIFIER
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CLASS D AMPLIFIER
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GRID CIRCUIT OF TYPICAL CLASS D RF AMPLIFIER
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OUTPUT NETWORK OF TYPICAL CLASS D AMPLIFIER
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CLASS D AMPLIFIER
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CLASS D AMPLIFIER
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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
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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.
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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%).
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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.
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Block Diagram of modern AM Transmitter
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FEATURES OF MODERN
TRANSMITTERS FULLY SOLID STATE
MODULAR POWER AMPLIFIERS
DIGITAL AMPLITUDE MODULATION
REDUNDANCY OF AMPLIFIERS
LOW DISTORTION
HIGH EFFICIENCY
LOW NOISE
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FEATURES Contd.. NO WARM UP TIME
POWER FOLD BACK ON O/L
VARIABLE POWER LEVEL
CAPABLE OF TAKING DIGITAL
TRANSMISSION
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Summary---Basic Analogy of a PA
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0V
+V
HALF QUAD
SWITCHES A A
Voltage at -- A
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FULL QUAD
COIL
All these switches are operated alternately at RF
Output at the coil
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COMBINING MANY AMPLIFIERS
FULL QUAD PA
Combining by
Transformer
action
RF Switching signal
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Combining 2 PAs
ONE PA
ONE PA
TWO PAs
ONE PA
NO PA ON
2PA
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Low Pass Filter
Square wave is shaped to smooth sine wave
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Step By Step Modulation
STEP BY STEP
Modulation
Distortion filled by
SMALL step voltages
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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.
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I.I.George,Dy.Director, STI(T)
Vector Modulation
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Thank you