> It sounds rather like you are seeing harmonics of the step signal

Actually, no. The stepper will put out noise without it moving. Just
lightly tapping it or the bench, even fingerprint ridges across the shaft,
or a loud clap nearby, will cause a small AC voltage on the windings

Most nuisancesome. Put a motor on the scope and try it

I'm having some success by mounting it on a foam cushion, putting
10k across each winding and using a bridge of Schottky diodes
as a rectifier. This somewhat removes that low-level noise which was
being amplified by the following comparator

Still tinkering to do on the comparator, as the AC output from a slow
step is not very sinusoidal. Looking at filtering and gain/hysteresis

The idea has potential and there could be a product in the making.
Getting it to work with these particular motors (9.6V, 0.4A, 23 ohm)
would be just great, as I have a lot of them and they're free. We'll see

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