Tayloe Quadrature Product Detector | Hugonweb Annotated Link Bibliography

This simple circuit directly converts from RF down to baseband. Four square waves operating at $4 \times F_{RF}$ and shifted 90° from each other control a multiplexer thus producing four quadrature signals. These can be combined to give I and Q signals. A copy of the PDF is here.

I found this while reading about the Pi Pico Rx, which uses this circuit and a Raspberry Pi Pico to make a software defined radio.