Coherent Sampling vs Windowing for 50/60 applications Let's say I need to sample 50/60 Hz signals to find RMS. I can use for example 1kHz sampling frequency. That means 100ms sampling is 5 cycles of 50Hz and 6 cycles for 60 Hz. That's coherent sampling. But does this account for frequency variations? I don't have zero cross detection in hardware, maybe if I do it in software (microcontroller) it may help. I've heard that windowing the signal help reduce discontinuities in case I don't exactly get a integer multiple of the frequencies during sampling. Should I use windowing? What other approaches can be implemented?
That's coherent sampling.

Assuming this is a AC power application: it's only coherent sampling if you derive the sampling clock directly from the AC frequency using a PLL (Phase Locked Loop) or equivalent.

But does this account for frequency variations?

Depends on how you derive the sampling clock. If you use a PLL it will track the AC frequency, if you use an independent clock, the frequency will indeed drift.

Should I use windowing? What other approaches can be implemented?

If you really just want to measure the RMS of the signal, you don't need coherent sampling. Actually incoherent is better. You just need to make sure you have enough samples.