User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-14
transmitter is accomplished, 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, in turn, the
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 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 passed on to the front panel
meter and the AGC circuits.
The 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, which the ALC
circuit senses at U10A, automatically
lowering the loss of the pin-diode
attenuator circuit and increasing the
gain to compensate.
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
the ALC detector is offset, or
complimented, by current that is taken
away from the summing junction. In
normal operation, U10A, pin 2, is at 0
VDC when the loop is satisfied. If the
recovered or peak-detected IF signal at
IF input jack J7 of this board should
drop in level, which would normally
mean that the output power is
decreasing, the null condition would no
longer occur at U10A, pin 2. When the
level drops, the output of U10A at pin 1
will go more positive and, if jumper W3
on J6 is in the Automatic position, it will
cause ALC pin-diode attenuators CR1,
CR2, and CR3 to have less attenuation.
This will increase the IF level and
compensate for the decrease in level. If
the ALC cannot sufficiently increase the
input level to satisfy the ALC loop,
because of not enough range, an ALC
fault will occur. The fault is generated
because U10D, pin 12, increases above
the trip point set by R84 and R83 until
it conducts. This makes U10D, pin 14,
high, and lights the red ALC Fault LED
DS2 on the board.
4.3.7.10 Fault Command
The ALC board also has circuitry for an
external mute fault input at J19, pin 6.
This is a Mute command and, in most
systems, it is part of the interface for
the protection circuits of high-gain
output amplifier devices. The Mute
command is intended to protect the
amplifier devices against VSWR faults.
In this case, it is required that the
actions occur faster than just pulling
the ALC reference down. Two different
mechanisms are employed: one is a
fast-acting circuit to increase the
attenuation of pin-diode attenuator
CR3, CR1, and CR2, and the second, as
just described, is the reference voltage
being pulled away from the ALC
amplifier device. An external mute is a
pull-down applied to J19, pin 6, that
provides a current path from the +12
VDC line through R78 and R139, the
Mute Indicator LED DS4, and the LED
section of opto-isolator U11.
These actions turn on the transistor
section of U11 that applies -12 VDC
through CR21 to U10A, pin 3, which
pulls down the reference voltage. This
is a fairly slow action that is slowed
down by the low-pass filter function of