> Would 50Hz mains AC be accurate enough for LF calibration?

Depends on where you live.  In civilized parts of the world, the power
line frequency is controlled far more accurately than you can see on a
scope.

> and is there
> a fairly precise higher frequency waveform that I can use in or from
> something else??

Any crystal oscillator will be far more accurate than any error you can
see on a scope.


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