Mixer
Audio In
OSC
IN
Filter
Inverted audio
out
AMPLITUDE
Desired
Spectral display of audio inputs and outputs. Sum components are filtered, difference components are used.
Filtered
Difference Sum
Audio Osc
FREQUENCY
The audio input is amplified and filtered by the IC, then "mixed" with the carrier to produce both the sum and difference frequencies. For example, a 500 Hz input mixed with the 3 KHz carrier will produce signals at both 2500 Hz and 3500 Hz and a 1KHz signal will produce a 2 KHz and 4 KHz result. The outputs are now low pass filtered to remove the unwanted signals above 3 KHz. Final result; the low 500 Hz and slightly higher 1 KHz inputs produce 2500 Hz and 2 KHz outputs respectively. Voila, inverted audio!
The inverted audio is then amplified by either Q1 or Q2, depending on the path. On the RX path, J4 jumper position selects either speaker level audio from the
Front panel controls include power on/off switch S1, scramble/bypass switch S2, and RX input level select switch S3 which allows the SS70A to be used with either line or speaker level inputs. Potentiometer R11 controls audio level when speaker level output is selected.
SS70A • 5