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HARRIS
888-9058-001
WARNING: Disconnect and lockout AC primary power prior to servicing
159
NOTE:
The AGC unit can be completely bypassed by a BNC barrel and an attenuator
Pad of 6 to 8 dB.
6.4.3 Phase and Gain
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Refer to schematic 843-5285-391 (3 sheets)
The phase and gain unit provides three functions. First, it divides the RF drive signal
into two paths in order to drive two separate high power amplifier chains. After
amplification, the high power signals are combined to sum their powers. In order for the
powers to sum properly, the two signals must be equal in amplitude with 90 degrees
difference in phase. The Phase and Gain module also provides the amplitude adjustment
and relative phase adjustment of each drive path needed to compensate for imperfections
in the amplifier chains.
6.4.3.1 Detailed Circuit Description
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Input Hybrid:
Input hybrid HY5 splits the input signal into two equal outputs. The signal at P2 is 90
degrees lagging the P1 signal. The signal from each output of HY5 passes through two
additional hybrids. The first in each path, HY2 & HY3, are used as variable
attenuators, the second in each path, HY1 & HY4, are used as variable phase
shifters. Window comparators driving front panel LEDs DS4 & DS3 to indicate the status
of the adjustment ranges.
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Gain Adjustment:
The GAIN ADJUST (R2) on the front panel is a voltage divider between plus 12 volts
and ground. (located on the left side of the drawing of sheet 2). This adjustable voltage is
buffered by two opamps in U2. The section of U2 associated with the CHB attenuator
inverts the control signal change such that an increase in the control voltage to the CHA
attenuator circuit results in a decrease to the CHB attenuator circuit.
The voltages on the bases of Darlington transistors Q1 and Q2 control the emitter
currents which change the bias current through the HP3081 pin diodes. If little or no
diode bias current flows, the diodes present a high impedance to the RF signal emitting
from attenuator hybrids (HY2 & HY3) and only the impedance of the paralleled 56.2
ohm resistors is seen. Since this impedance is close to the hybrid characteristic
impedance, most of the RF signal is absorbed in the 56.2 ohm resistors resulting in
maximum attenuation from input to output of each attenuator hybrid. However, if
current is flowing in the pin diodes, the total impedance presented to the signals from
HY2 & HY3 is less than that of the 56.2 ohm resistors since the diodes are paralleled
across the resistors. The resultant impedance mismatch causes the RF signal to be
reflected back to the hybrid and appear at the output of the hybrid. This appears as a
reduction of attenuation from input to output of the attenuation hybrid.