[sdiy] Mysterious ground noise

[Ingo Debus]

Antti Huovilainen wrote:
> Turned out, that the scope shows the same noise
> (about 4mV p-p) even if the probe tip is connected to probe ground lead
> and I touch the tip/ground (doesn't matter which one, since they're
> connected) to circuit ground. The noise goes away if I remove power from
> the digital section (PIC, UART & 3 latches).

Perhaps this isn't noise at all. Try a very short sweep time on your
scope and play a little with trigger level or trigger mode settings.
Perhaps you'll find the frequency of the quartz of your microprocessor,
or of a RF station. 4mV is quite low and can easily be picked up with a
shorted scope probe. With low sweep time settings it looks like noise.

Ingo