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HARRIS
888-9058-001
WARNING: Disconnect and lockout AC primary power prior to servicing
160
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Gain Monitoring:
The comparators in U3 monitor the bias voltage from Q2, comparing it with two fixed
and one variable reference voltage. The comparators with the fixed voltages drive red
LEDs in DS4. The right red LED illuminates when the gain reaches maximum in the
“B” amplifier path, the left for maximum “A” path gain. This will alert the operator that
the adjustment limit has been reached. The yellow LED in the center is a reference
indicator that shows where the setting of the GAIN ADJUST pot was when the high
power combiner was last properly set up for minimum reject load power. The indicator
has an adjustable reference voltage. After proper amplitude balance has been achieved,
the GAIN BALANCE INDICATOR ADJUSTMENT may be used to illuminate the
yellow LED. Further small adjustments of amplitude balance that may be needed from
time to time will not extinguish the yellow LED. However, if larger amplitude changes
become necessary, the LED will extinguish. This will alert the operator to check for a
gain loss in one of the amplification paths that may be failure related.
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Phase Adjustment:
In a manner similar to gain adjustment, the PHASE ADJUST control is used to increase
the phase delay in one path while decreasing it in the other path. The shift in each phase
hybrid is controlled by the voltage bias on the KV3901 varicap diodes. The varicap
diodes are in series with 120pf capacitors C27,C33, C31 and C38. As the varicap diode
bias is increased, the varicap capacitance value increases presenting a different reactive
impedance to the RF signals emanating from hybrids HY1 and HY4. The reflected
signal from this reactive impedance is combined in HY1 or HY4 and appears at the
output of the hybrid with a phase different than the signal applied to the input. A change
in adjustment of the PHASE ADJUST control causes the varicap bias in one path to be
increased while the bias on the varicaps in the other path is decreased.
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Phase Monitoring:
The A-B PHASE LEDs show the position of the PHASE ADJUST control by
comparing the control voltage to fixed voltages established by voltage divider resistors
at the input pins of each section of U1. The illumination of the LED indicates the center
of the range of adju/-10 degrees (approximately) from quadrature. Adjustment
from the center of the range that causes the LED to extinguish should alert operators to
a change in the system that could indicate a failure. The red LEDs indicate end of the
adjustment range.
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Gain Stages:
RF amplifiers U4 and U5 are used to overcome losses sustained in the phase and gain
circuits. Each amplifier is capable of approximately 14db of gain. The attenuation value
of the resistive pad installed at the input of each amplifier is chosen to provide
approximately unity gain between the Phase & Gain assembly input and both outputs.
