User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-41
Overtemperature LED DS3 on the front
panel is lit.
4.8.12.1 Phase Control
The phase control pot on the front of
the tray connects to J8-1 (+26 VDC),
J8-3 (phase I/P), and J8-4 (RTN) on
the board. The phase-control output
connects from J7 on the board to the
input of the phase-control circuit on the
variable gain/phase board (1265-
1425).
4.8.12.2 Automatic and Manual Gain
Control Circuits
The amplifier control board contains the
AGC function for the UHF amplifier tray
in which it is mounted. An AGC
reference-level input from the UHF
exciter tray is applied to J3, pin 1, and
is amplified by U3B. The output of U3B
is connected to J5, which is wired to
the front panel gain pot that sets the
output power level of the tray when the
AGC is in the Auto position. The voltage
at the arm of the front panel gain pot is
amplified by U2D and is compared to a
sample of the output power of the tray
in U2A. The error voltage from U2A is
sent through Auto/Manual switch S1 to
J10, which connects to the pin-diode
attenuator circuit on the variable
gain/phase board. A sample of the AGC
voltage level is connected to position 1
on the front panel meter switch. The
tray can also be operated in manual
gain by switching S1 to the Manual
position and adjusting R16 for the
desired output power level.
FETs Q1 and Q3 delay and slowly
reapply the AGC voltage to the variable
gain/phase board when the system is
switched on or when the board is
switched from Auto to Manual (or back)
to prevent the overdriving of the tray.
4.8.12.3 Overdrive Circuit
A sample of the output of the single
stage amplifier assembly, class A, from
the coupler board assembly connects to
J11 on the amplifier control board. The
sample is peak detected by CR7 and
U3A and the output is connected to
U3D. If the input drive level increases
above the overdrive threshold
reference set by R71, which is the drive
level needed to produce 110% output
power, the output of U3D goes high
and is split three ways. One of the
highs is connected through R38 and
CR3 to U3C, causing its output to go
high and lighting the red Overdrive LED
DS2 on the front panel. Another of the
highs is connected through R74 and
R75 to Q6, which is biased on, causing
its output to go low. The low is
connected through J10 to the variable
gain/phase board and its output power
is cut back.
The final high from U3D is connected
through CR2 and R37 to U1D, which is
biased on, causing its output to go
high. The high is connected to U2A,
whose output decreases and its output
power is cut back. If this path is not
present, the AGC, because the forward
power decreases, will try to drive the
variable gain/phase board harder,
creating a positive feedback loop that
could damage the amplifier tray.
4.8.12.4 Metering Circuits
The +26 VDC that is connected to the
board from the switching power supply
is applied to jack J6, pin 1, of the board
and connected through R63, R96, R66,
R65, and R76 to the front panel meter
for monitoring. R65 can be adjusted to
calibrate the voltage reading to +26
VDC on the front panel meter, bottom
scale. This calibration was completed at
the factory and should not need to be
adjusted at this time.
A forward power sample of the output
of the tray is applied to jack J1-1 and
J1-2 of the board from the dual peak
detector board, single supply. The
forward power sample is connected
through R1 and R2 to U1A, a buffer