Questions about Power-Line-Cycle (PLC) synchronization (precise DC instruments)

[RLiangCN]

Hi,

I'm making a precision pA meter. I found instruments (e.g., Keithley 6430 and Keysight 2985) have PLC synchronization function.
Data are read-out per N times PLC cycles.

I understand if the integration time is perfectly matches N PLC cycles, then the 50Hz (or 60Hz) PLC noise are averaged to Zero.
My question is:
1. Is it possible to precisely match the PLC cycle? or is PLC frequency 50Hz is stable?
    How these instruments achieve the synchronization automatically and precisely? (I can't find out from their handbooks)
2. What if the sync is not good, will this effort still reduce the PLC noise error?

Thank you!

[EC8010]

The short answer is; no, yes.

It's much better to make your electrometer free of hum. Keep an oscilloscope (triggered from line) on its output and improve your design and construction until there is no hum. And yes, it is possible.

[Phil1977]

The PLC synch just reduces the PLC influence. The better it is synchronized, the more PLC noise will be cancelled out, but its never reduced to zero.

It´s not very complicated to synch to mains with better than 1% phase and frequency stability. And that accuracy is more than enough to get a significant noise reduction.

[SanderMTI]

I'm totally with EC8010 on this.
If you want to measure picoamps accurately, you get better results by keeping hum out to begin with than by trying to hide it.
Also.. it's not just hum which interferes at picoampere levels, so having a look with an oscilloscope can be very educational.

Sander

[Kleinstein]

There are 2 parts to the power line syncronization. One is getting the frequency very good. A few meters use a PLL to really synchronize the ADC clock to the mains frequency. So they can compensate for the small (usually < 0.1% variations in the mains frequency). Besides a PLL a more simple frequency measurement and digital adjustment of the integration time could also be done. The better the frequency is matched the less the starting phase has an influence.

The other point is starting the conversion always at the same phase of mains, this suppresses the possible beat frequency from an residual hum effect. There can still be an error from hum, but that error would be largely constant. This would be effective for hum effecting the DMMs ADC directly as this would than be subtraced in the AZ cycle. However it only changes from beat frequency to constant error for hum coming in with the signal. This is usually the case with hum at a pA meter.  So the constant start phase may be only for the looks.

With meter where you have the choice it is usually the 2nd part, so the phase relative to mains. It may come with a slightly reduced reading rate, waiting for nearly one more mains cycle.
An option I have not seen / decribed so far would be doing averaging over multiple starting phases. This would especially be a thing when there is averaging over many shorter conversions to simulation longer integration anyway (e.g. > 10 PLC with many meters).