1-13
1-56.
A voltage sample of the forward power is applied through potentiometer R56 and resistor
R183 to integrated circuit U63D. Potentiometer R56 allows the forward power circuit to
be calibrated. U63D operates in association with diodes D35/D36 as a full-wave rectifier.
The full-wave rectified output from U63D is applied to a third order high-pass filter
consisting of U63A, R60, R61, R67, C63, C64, and C88. The dc output of the filter is
applied to comparator U61D. U61D compares the forward power sample to a ramp
voltage generated by integrated circuit U40C. U61D will output a square-wave signal
equal to the forward power voltage sample. The output of U63D is applied to switch
U41B. U41B functions as a feedback control device designed to convert the forward
power voltage sample into a signal proportional to power. The square-wave output from
U41B is applied to a low-pass filter consisting of resistor R70 and capacitor C89. The
output of the low-pass filter generates a dc voltage which is proportional to the forward
power. The voltage is amplified by operational amplifier U64A. The output of U64A is
applied to: 1) the fault detection circuitry, 2) a +5 volt full-scale meter output, and 3)
buffer U64B. Buffer U64B operates in association with programmable jumper P2 to
provide a +2.5 volt full-scale meter output. Jumper P2 enables or disables the +2.5 volt
full-scale meter output.
1-57.
FOLDBACK PROTECTION.
The controller circuit board is designed to monitor several
operating parameters for problem conditions. Several of the monitored conditions are
routed to a foldback circuit. The foldback circuit will automatically reduce the
transmitter RF output power to an acceptable operating level to prevent damage to the
transmitter. The controller monitors: 1) the cabinet temperature for over-temperature
conditions, 2) reflected power for a high reflected power condition, 3) forward power for
high forward power conditions, and 4) lightning detector for lightning conditions. When
an over-temperature, high reflected power, or a high VSWR condition occurs, the foldback
indicator will illuminate to indicate the transmitter is in a foldback condition.
1-58.
If the controller detects a high reflected/forward power, a high VSWR, lightning, or an
over-temperature condition, the foldback circuit will automatically reduce the transmitter
RF output power to an acceptable operating level to prevent damage to the transmitter. If
the controller detects a high reflected power or VSWR condition, the antenna status
indicator will illuminate as described below to indicate the problem.
ANTENNA STATUS INDICATOR
CONDITION
YELLOW
1.2 : 1 VSWR or greater.
RED
High reflected power condition. A reflected
power condition equal to 400 watts for AM-10A
models or 240 watts for AM-6A models.
FLASHING RED
Reflected power emergency condition. A
reflected power condition equal to 2000 watts for
AM-10A models or 1200 watts for AM-6A
models.
1-59.
High Forward Power Circuitry
. High forward power conditions are monitored by a high
forward power fault detection circuit. Integrated circuits U66A and U66B are configured
as comparators designed to monitor forward power conditions. U66B functions as an
output power monitor. Integrated circuit U66B compares a forward power sample from
U64A to a power control sample. U66A compares a forward power sample from U41B to a
voltage reference.