Assembly Instructions Chapter 4

10-kW UHF Transmitter with Chapter 4, Circuit
Feedforward Drive Descriptions
840A, Rev. 0 4-23
between J7-1 and J7-2 for the operation
of the diacrode (external reference).
The (A25) AGC control board (1137-
1201) compares a sample of the output
of the diacrode with a sample of the 3-
watt tray. The front panel gain control R2
adjusts the overall gain of the tray. The
output voltage of the comparator drives a
pin attenuator circuit that corrects for
drift by maintaining a constant output
even though temperatures, therefore
gain, of the amplifier devices in A3, A5,
or A7 may vary.
The AGC board also contains circuitry
that monitors the AGC level and provides
for an AGC override detector. If the final
3-watt amplifier stage is overdriven, the
AGC override LED DS2 on the front panel
will light to indicate a cutback of the AGC
that cuts the gain of the tray. The AGC
override detector is also used as the
system Mute (Standby, VSWR fault, and
other faults). There is also a ramp control
signal input to the AGC control board.
When the 3-watt amplifier is externally
enabled, the drive output of the amplifier
ramps up over a time frame of several
seconds. The output of the pin attenuator
is amplified to approximately 0 dBm and
fed to output jack J2 of the board.
The RF from the AGC board is fed to
(A27) the UHF phase shifter board
(1142-1315). The phase-corrected signal
is then amplified by (A3) the UHF
amplifier/regulator board (1007-1204)
that has a nominal gain of +17 dB. Next,
the signal (+17 dBm typical) is amplified
by (A5) the 3-watt amplifier board #1
(1007-1211) that has a nominal gain of
+9 dB. The operating drive current, no
RF input, for the 3-watt amplifier board
#1 is set by the (A6) opto-bias board
(1002-1109). The signal (+26 dBm
typical) is amplified further by (A7) the
3-watt amplifier board #2 (1007-1211)
that also has a nominal gain of +9 dB.
The operating drive current for the 3-
watt amplifier board #2 is set by (A8) the
opto-bias board (1002-1109).
The output (typically +35 dBm) of the
second 3-watt amplifier board is fed to
(A29) the overdrive protection board
(1142-1626). The signal enters the board
at J4 and is connected directly out of the
board at J5. A sample of the RF signal is
coupled off the input and fed to a
detector circuit to set up the AGC
override level. If the RF output level of
the transmitter increases to 120%, the
override circuit will cut back the output of
the 3-watt tray to prevent damage to the
amplifier devices.
The output of the overdrive protection
board is fed to (A9) a dual coupler
assembly (1007-1208) that supplies a
forward and reverse power sample to
(A10) the dual peak detector board
(1137-1510). The dual peak detector
drives the front panel meter for a %
Reflected Power reading and also
provides a forward power sample (inner
loop AGC) to (A25) the AGC control
board (1137-1201). The AGC control
board provides an output to the front
panel meter for the % Forward Power
reading and also provides an output to
the AGC board that is used as the AGC
sample of the output. If an outer loop
AGC is used from an external amplifier
tray, it is fed to the AGC control board
through J5 on the rear of the tray. The
output from the dual coupler is directed
to the RF output jack J2 on the rear of
the tray (+35 dBm).
Two power supplies are used in the tray.
The (A16) +24V power supply board
(1007-1207) supplies power for the
amplifier devices through the opto-bias
boards in the 3-watt amplifiers and the
UHF amplifier/regulator board. The (A22)
general purpose ±12V power supply
board (1062-1013) supplies voltage to
the rest of the boards in the tray.
AC is applied to the tray through J4 on
the rear panel and is connected to
terminal block TB2. MOVs VR1 and VR2
are provided as protection from
transients or surges on the AC input line.
When CB1, the 2.5-amp circuit breaker,