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Audio pre-amp, IC5
Now we will check the audio pre-amp, IC5. The following trace shows IC5 pin 1 (upper trace, Ch 1) and IC5 pin 7 (upper
trace, Ch 2). These are the famous "I" and "Q" channels. Notice that I have changed the vertical scale to 5V/division. The
waveforms are horrendously distorted. Why? Simply because these pre-amps have 36dB of gain. The small input signal
results in a large output signal, after 36dB of gain... and the output amplitude cannot be represented accurately by the
operational amplifier, whose outputs cannot stray beyond its supply voltage rails (0V and 12V in my case). So you get this
nasty clipping.
I'm using 500us/horizontal division now - but either 500us or 1ms is fine for looking at 700Hz audio frequencies.
Since we have now come to a stage of the receiver where we have some gain applied, we need to now reduce the
amplitude of the signal injected into the input. I removed the 330-ohm resistor in parallel with R43 and instead, soldered in
a 10K resistor.
Now the vertical scale on the 'scope is 500mV/division and you can see the 1.1V peak-peak amplitude is a nice 700Hz
sinewave without any visibly evident distortion. Again the I channel (IC5 pin 1) is top ('scope channel 1) and the Q channel
(IC5 pin 7) is bottom ('scope channel 2).
