The expansion slots are meant to accommodate more complex instruments, and a lock-in amplifier is one of the add-ons I hope to make. Phase correlated sine waves are part of the package already, so only a multiplier and a low pass filter are needed.

As a first step, I played around with the AD633 analog multiplier, and tried out amplitude modulation using only the tools available on board.

The schematic is in the examples section of the AD633 datasheet. The red trace is the modulated output. A carrier of 10KHz was modulated with 150Hz.

A quick Fourier transform of the modulated wave shows the side bands.

Accurate Fourier transformations require a sampling period much greater than the largest wavelength that makes up the input waveform, so I'm polishing a live streaming protocol that won't rely on handshakes, and will send ADC readings at the maximum rate possible without dropped packets.

Here's a screencast of an old streaming app showing the modulated waveform, followed by an FFT of the same. The timebase shown is incorrect, and I'll be fixing it.