SAC SS-800AVS Manual page 29

With switch Sl in the IDENT position and no code being produced,
capacitor Cl begins charging through resistors R3, R4, and R5 and
reaches approximately 7 volts in 3.5 seconds. When Cl reaches this
level, unijunction transistor Q3 fires, creating a trigger pulse
through C2, R7, Q4, C3, and RI0. The trigger pulse activates the
first Letter PWB of the identification sequence (see Letter PWB).
The Logic Output of the Letter PWB's causes Q1 and Q2 to turn ON
during keyed code tone output periods. When Q1 and Q2 are ON, the
potential on Cl is discharged through CRI, R4, and R5. At the end
of the identification sequence, Cl begins to charge as described
above, and the identification sequence begins again.
When Sl is in the CONT position, Cl is prevented from charging and
initiating the ident sequence. Cl is discharged through CRI, R4,
R5, and to ground by Sl. The identification code or continuous
tone logic signal is taken from the collector of Q2 through R23,
to pin 3 of integrated circuit U2 (Quad Transmission Gate).
Integrated circuit Ul (Operational Amplifier) and its associated
circuitry function as an audio oscillator to provide an identifi-
cation tone (1020 Hz or 400 Hz) to pin 1 of U2. The passage of the
tone from pin 1 to pins 2 and 3 of U2 is controlled by logic signals
produced on the optional Audio PWB. Without an Audio PWB installed,
R20 pulls up U2 pin 13, the transmission Gate A control line, and
the audio signal passes through Gate A without attenuation. The
passage of the tone from pin 3 of U2 through transmission gate B
to pin 4 of U2 is controlled by the code signal present at U2 pin
6, with 0 volts at pin 6, and thus 14 V at pins 5 and 9 allowing
tone passage through transmission gate B. The audio signal passes
through PWB pin 14 or 17 to the audio input, pin 22 of the AM
Oscillator PWB. When the Audio PWB senses the presence of a voice
input, squelch control input PWB pin 15 will go low to 0 volts.
This will cause U2 gate A to stop conducting. The audio tone will
bypass gate A through RI9 at a reduced amplitude, causing the code
amplitude to be about 20% during voice signal presence.
SS-800AVS Southern Avionics
27
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