I started with the stupidest version I could think of: one BJT used as a mixer.

HackRF provides the LO, an FM broadcast signal goes into the transistor, and I look at whatever comes out of the collector.

The first circuit was too crude even by the standards of this project. I later added a capacitor and resistor around the transistor and got somewhat more sensible behavior, but I want to keep this version here because this is where the experiment actually started.

Long wires that could pick up everything around are an essential part of this project.

This first version didn't involve much RF engineering. The point was mostly to answer one question: will this thing produce a measurable downconverted signal at all?

And apparently, yes.

I tried different LO frequencies and checked the output both on the oscilloscope and by capturing samples for FFTs. The signal is tiny - only a few millivolts - and my oscilloscope isn't particularly helpful at that level, but something is clearly happening at the expected difference frequency.

Moving the LO also moves the interesting part of the spectrum, which is a good indication that I'm seeing mixing rather than random setup noise.

At this point I don't have an FM radio yet.

I have a shitty mixer producing a very small signal that seems to contain a downconverted FM station.

The next step is amplification / better biasing and getting those samples into the Pico. Then we'll see if DSP can turn this mess into audio.