Ramsey Electronics SS70A - Manual (Page 5)

SS70A • 5
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 LM-380 2 watt audio amplifier or line level audio output.
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.
Audio In
OSC
IN
Filter
Mixer
Inverted audio
out
Audio Osc
Sum Difference
Desired
FREQUENC Y
AMPLITUDE
Filtered
Spectral display of audio inputs and
outputs. Sum components are
filtered, difference components are
used.