RELEASED: 10/2/2006
Page 24
COMMUNICATI0NS
HSD SERIES OPERATION AND MAINTENANCE
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CR1 and CR2 to control the level of the input signal providing a constant level at the
output of CP1. The attenuator and its related circuitry are unaffected by the type of
modulation used. The attenuator output is coupled through C5 to a 23dB gain
amplifier U1, which is biased with 4.4 volts @ 50ma. This amplifier output is split by
CP1, a -3dB splitter. One of the splitter legs continues on through the transmit
chain; the other leg feeds the detector circuit formed around CR8, producing a
detected voltage to drive the input of U3B which exhibits a voltage gain of 11v/v.
The circuitry around U3B provides 3 outputs.
One output of amplifier U3B drives integrator U3A that provides control of the pin
diode attenuator, completing and locking the AGC loop. This is accomplished by
comparing the U3B output against a reference voltage applied to U3A.
The second output from U3B is connected to comparator U4B. It compares a
reference voltage against the voltage developed by CR8 and its following amplifier
U3B. The comparator output at J1 pin 4 then drives the outboard status circuitry in
the control board (indicating an IF lock) because the AGC loop is in lock. When this
output is high, at or above 2.7 volts, this signals the control board that the IF is in the
normal operating range.
The third output from U3B feeds through R41 to comparator U4A. This comparator
and its surrounding circuitry provide a mechanism to limit the transmitter from an
overshoot of the input AGC loop. If the voltage from U3B exceeds 2.7 volts (the
reference voltage) the output of U4A goes high; this then turns on the gate of Q1,
which in turn shorts out the voltage control driver (U5A) for the level control pin
diode attenuator. This causes the level control pin diode attenuator to impose
maximum attenuation, thereby limiting the drive to the following stages of the
transmitter.
The Operate/Standby control switching is accomplished through the circuitry of Q2
and Q3, followed by unity gain buffer U6A. If the Operate/Standby control is on
(Operate), the input to U6A is high, which in turn provides an output at U6A. This
voltage feeds the Power Adjust control R51. The output of the Power Adjust control
feeds another unity gain buffer U5B. The output of U5B feeds voltage amplifier U5A
which has a gain of 2.6v/v. This amplifier controls the action of the second pin diode
attenuator formed around CR3 and CR4.
The second pin diode attenuator is the level-adjust attenuator. The attenuator output
is followed by U2 which has the same gain and is biased the same as U1. The U1
output feeds through transformer T1 which doubles the voltage output. The circuitry
associated with the T1 output composes a gain expansion and correction circuit
known as the Linearity Corrector.
