UHF Analog Driver/Transmitter/
Chapter 4, Circuit Descriptions
Translator
LX Series, Rev. 3
4-13
8 power raise and TB30-9 power lower,
on the rear of the exciter/amplifier
chassis. The ALC voltage is set for .8
VDC at TP4 with a 0-dBm output at J42C
pin 1C of the module. A sample of the
ALC at J42C pin 11C, is wired to the
Control Monitoring/Power Supply module
where it is used on the LCD display and
in 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.
The ALC circuit senses this drop at U10A
and automatically decreases the loss
through the pin-diode attenuator circuit
therefore increasing its gain maintaining
the same output power level.
The ALC action starts with the ALC
detector level monitored at TP4. The
detector output at TP4 is nom.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 complemented, by
current 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 level at IF input to this board
should drop, which normally means that
the output power will decrease, the null
condition would no longer occur at U10A,
pin 2. When the level drops, the output
of U10A, pin 1, will go more positive. If
SW1 is in the Automatic position, it will
cause the ALC pin-diode attenuators CR1,
CR2, and CR3 to have less attenuation
and increase the IF level; this will
compensate for the decrease in the level.
If the ALC cannot increase the input level
enough to satisfy the ALC loop, due to
the lack of 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 causes the
red ALC Fault LED DS2 to light.
4.3.1.10 Fault Command
The board also has circuitry for an external
mute fault input at J42 pin 10C. This is a
Mute command that protects the circuits of
high-gain output amplifier devices against
VSWR faults. This action needs to occur
faster than just pulling the ALC reference
down. Two different mechanisms are
employed: one is a very fast-acting circuit
to increase the attenuation of the pin-diode
attenuator, CR1, CR2, and CR3, and the
second is the reference voltage being
pulled away from the ALC amplifier device.
An external Mute is a pull-down applied to
J42 pin 10C, which completes a current
path from the +12 VDC line through R78
and R139, the LED DS4 (Mute indicator),
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, and pulls down the
reference voltage. This is a fairly slow
action controlled by the low-pass filter
function of R81 and C61. When the
transistor section of U11 is on, -12 VDC is
also connected through CR22 directly to
the pin-diode attenuator circuit. This
establishes a very fast muting action, by
reverse biasing CR3. This action occurs in
the event of an external VSWR fault.
4.3.1.11 ±12 VDC Needed to Operate the
Board
The ±12 VDC connects to the board at
J42C. The +12 VDC connects to J42C pin
16C and is filtered by L30, L41, and C80
before it is applied to the rest of the board.
The -12 VDC connects to J42C pin 18C and
is filtered by L31 and C81 before it is
applied to the rest of the board.
The +12 VDC also connects through R261
to the zener diode VR3 that connects to
ground, which generates the +6.8 VDC
output to the rest of the board.
The +12 VDC also connects through R265
to the diodes CR30 and CR31 provide a .9
VDC reference output voltage VREF that
