Glenayre Document Number: 9110.01002
GL-T8541 PA
09/10/99
THEORY OF OPERATION
t8541
p6.
fdk
Copyright © 1997 Glenayre
Page: 6-17
6.7.8 AGC Circuit
Refer to Figure 6-9 for a simplified circuit diagram of the metering board AGC circuit. The
heart of the AGC circuit is comparator U9-A. The output of comparator U9-A is a variable
error voltage (the AGC signal) that sets the RF output power of each PA board. The AGC
voltage has an approximate range of -13 Vdc to +6 Vdc. The AGC voltage is approximately
-13 Vdc when the transmitter is unkeyed, and is above 0 Vdc when the transmitter is keyed.
During normal operation, the AGC voltage should never approach the +6 Vdc limit.
The AGC reference signal is compared to the RF output sample signal in the U9-A compar-
ator to produce the AGC signal. The RF output sample signal is proportional to the
transmitter’s RF output power level, and is obtained from the forward directional coupler.
The AGC reference signal is generated in the DSP exciter to set the transmitter RF output
power level. The exact AGC reference signal level is established during the forward power
calibration.
When the transmitter is unkeyed, Q12 conducts. This action grounds the AGC reference
signal, which sets the comparator’s AGC output voltage to -13 Vdc. The -13 Vdc AGC
drives all of the PA boards into cutoff.
When the transmitter is keyed, Q12 turns off, which allows a non-zero AGC reference to
be applied to the AGC amplifier.
Figure 6-9 Metering Board Simplified AGC Circuit
U9-A
Forward RF
Dc Sample
Directional
Coupler
R89 C3
R64
R39
PA Key input
From
DSP Exciter
AGC Reference
Signal From
DSP Exciter
AGC Voltage Output
to all PA Boards
From
Q12
R78
