Re: Product detector circuit
A product detector is a mixer, therefore any mixer circuit can be modified to act as a product detector. You just have to change the output to handle audio instead of RF, and probably low-pass filter the remaining RF away. One thing to bear in mind is that a product detector is probably handling larger signals than a front-end mixer (except in a direct-conversion receiver) so signal handling is more important than noise.
If you are familiar with broadcast receivers, then one thing to try is an ECH81 product detector. Input from IFT, oscillator triode uses single-tuned IFT (or half an IFT), output is a resistor instead of an IFT. Unusually, this product detector can even have AGC too!
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