Assembly Instructions Chapter 4

10-kW UHF Transmitter with Chapter 4, Circuit
Feedforward Drive Descriptions
840A, Rev. 0 4-11
Following the bandpass filter, the signal
is split using L24, L25, and R89. The
signal passing through L24 is the main IF
path through the board. A sample of the
corrected IF signal is split off and
connected to J10, the IF sample jack.
The IF connects to jacks J27 and J28.
These jacks control whether a 6-dB pad
is included in the circuit by the
positioning of jumpers W9 and W10. The
6-dB pad-in is when jumpers W9 and
W10 are connected between pins 2 and 3
on J27 and J28. The 6-dB pad-out is
when jumpers W9 and W10 are
connected between pins 1 and 2 on J27
and J28. Normally, the pad is out. The IF
signal is then applied to a two-stage,
frequency-response corrector circuit that
is adjusted as needed.
Variable resistors R103 and R106 adjust
the depth and gain of the notches and
variable caps C71 and C72 adjust the
frequency position of the notches. The IF
signal is amplified by U13 and U14 before
it is connected to J12, the IF output jack
of the board. R99 is an output level
adjustment that is set to provide
approximately 0 dBm of IF output at J12.
A sample of the IF is fed to J11 to
provide an IF sample point that can be
monitored without breaking the signal
path and gives an indication of the IF
signal after the linearity and the
frequency-response correction takes
place.
ALC Circuit
The other path of the corrected IF signal
is used in the ALC circuit. The IF is wired
out of the splitter through L25 and
connects to op-amp U12. The output of
U12 is wired to jacks J8 and J9 on which
jumpers W4 and W8 control the normal
or encoded operation of the ALC circuitry.
For normal operation, jumper W4 on J8 is
between pins 1 and 2 and jumper W8 on
J9 is between pins 1 and 2. The IF signal
is applied to transformer T5; T5 doubles
the voltage swing by means of a 1:4
impedance transformation before it is
connected to the ALC detector circuit on
the board and amplified by U10B.
For normal operation, jumper W7 on J26
is between pins 1 and 2 and jumper W5
on J21 is between pins 1 and 2. The
detected ALC voltage is wired to U10A,
pin 2, where it is summed with the front
panel power control setting. The output
power adjustment for the transmitter is
achieved, if the (optional) remote power
raise/lower kit (1227-1039) is purchased,
by R75, a motor-driven pot controlled by
switch S1 on the board, or screwdriver
adjust pot R1 on the front panel of the
UHF exciter tray. An external power
raise/lower switch can be used by
connecting it to jack J10, at J10-11
power raise, J10-13 power raise/lower
return, and J10-12 power lower, on the
rear of the UHF exciter tray. S1, or the
remote switch, controls relays K1 and K2,
which control motor M1 that moves
variable resistor R75. If the (optional)
remote power raise/lower kit is not
purchased, the ALC voltage is controlled
only by screwdriver adjust pot R1 on the
front panel of the UHF exciter tray. The
ALC voltage is set for .8 VDC at TP4 with
a 0 dBm output at J12 of the board. A
sample of the ALC at J19, pin 2, is wired
to the transmitter control board where it
is used on the front panel meter and in
the AGC circuits.
This ALC voltage, and the DC level
corresponding to the IF level after signal
correction, are fed to U10A, pin 2, whose
output at pin 1 connects to the ALC pin-
diode attenuator circuit. If there is a loss
of gain somewhere in an IF circuit, the
output power of the transmitter will drop.
The ALC circuit senses this drop at U10A
and automatically lowers the loss of the
pin-diode attenuator circuit to
compensate by increasing the gain.
The ALC action starts with the ALC
detector level that is monitored at TP4.
The detector output at TP4 is nominally
+.8 VDC and is applied through resistor
R77 to a summing point at op-amp
U10A, pin 2. The current available from