SOUTHERN AVIONICS COMPANY
Model SA100 Dual2-14 Theory of Operation, Radiobeacon
2.3.4 Audio PWB (Optional)
The balanced 600 ohm audio input is routed through a switched input attenuator S1, 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 C1, R2, C2, and Audio Input Level control R6 to
amplifier U1A and squelch gate U3. The output of U1A drives the audio meter through U1B and
the squelch control circuit U1C and U1D. With R6 and input attenuator Switch S1 adjusted for
maximum sensitivity, the U1D 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 Q1 decreases and the resistance
decreases, thus lowering the audio input level to U2A pin 3 and holding the average audio output
level constant. Switch S2 enables the AGC action to be inhibited if desired. The audio output of
U2A is amplified by U2C and applied to symmetrical clipper CR7, CR8 which prevents over
modulation 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 pin13 is used to control the automatic code tone level reduction during
simultaneous voice and code tone modulation.
The timing for the return to high level IDENT modulation and/or the transmitter keyer may be
adjusted by changing the value of R34 according to the following table.
Time | Value | Comments
---------------------------------
5 sec | 470K | Default
8 sec | 820K |
11 sec| 1.2M |
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