Audio Pwb (Optional) - SAC SS-800AVS Manual

2.2.3 Audio PWB (Optional)
The balanced 600 ohm audio input is routed through a switched input
attenuator SI, R47, R48, and R49 which allows operation with high
level or low level input signals. The signal is then transformer
coupled through high pass filter Cl, R2, C2, and Audio Input Level
control R6 to amplifier UIA and squelch gate U3. The output of
drives the audio meter through UIB and the squelch control circuit
UlC and UID. With R6 and input attenuator Switch S1 adjusted for
maximum sensitivity, the UID logic voltage at pin 7 will go to a
logic 1, or +12 Volts, as the input signal level rises above -28
dBm. The logic voltage from pin 7 will cause squelch gate U3 to
close and apply the audio signal to AGC circuit U2A, Q1, Q2, and
Q3. Q1 acts as a variable resistor at the input of U2A. The
resistance is controlled by the signal level fed back from pin 1
of U2A. As the signal level increases, the gate voltage on
decreases and the resistance decreases, thus lowering the audio
input level to U2A pin 3 and holding the average audio output level
constant. Switch $2 enables the AGC action to be inhibited if
desired. The audio output of U2A is amplified by U2C and applied
to symmetrical clipper CRT, CR8 which prevents overmodulation on
sudden audio peaks. R44 adjusts the point at which clipping begins.
The clipped signal is amplified and filtered by U2D and U2B, level
adjusted by R20, and fed out of the board at pin 9. For special
applications, the audio signal can be routed through optional special
filter and shaping circuits on an external board attached to Audio
PWB pins 7 and 9. The squelch signal at pin 13 is used to control
the automatic code tone level reduction during simultaneous voice
and code tone modulation.
SS-800AVS Southern Avionics
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