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I have two Chinese Pixie kits that I've assembled. I think they are pretty interesting, from a QRP / tinkering stand-point.

Looking at the schematic below, where is the beat frequency coming from?

I know Q1 is part of the crystal oscillator and Q2 is essentially the power-amp during transmit, but what is mixing with the incoming signal to create the CW tone instead of the simple hissing you'd hear? I simply cannot see where this is happening. I do understand all of the other areas of the circuit (power supply area, keying, audio amp and low-pass filter on the antenna).

Pixie schematic Pixie schematic

I'd like to do more tinkering with this, but before I do, I really would like a firm knowledge of what is acting as the BFO here.

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Q1 forms a Colpitts crystal oscillator. It uses the reverse biased D2 as a varicap (variable capacitor) to pull the crystal frequency by 700 to 800 Hz, This frequency beats with the received frequency to produce the audible CW tone. The adjustability of the pull through W1 also allows a simple, limited range RIT function during receive.

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  • $\begingroup$ I didn't even catch the varicap setup, but indeed, there it is. Thank you so much! $\endgroup$ – Will B - AF7EC Feb 4 '18 at 20:16

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