[time-nuts] Building a mains frequency monitor

[Jeremy Nichols]

I've monitored line frequency with one of my old HP frequency counters. A
filament transformer with a potentiometer across the secondary allows me to
dial a safe voltage for the counter, about 1 V RMS.

My ancient counters are limited in their data collection abilities; I'm
sure your picPET device (whatever that is!) will do a better job.

Jeremy
N6WFO


On Wednesday, April 6, 2016, Jay Grizzard <elfchief-timenuts at lupine.org>
wrote:

> Since it seems to be a week for new projects on time-nuts... ;)
>
> So I've been wanting to set up a power line frequency monitor for a while,
> and now(ish) seemed to be a good time for me.
>
> So initially, I was planning on doing a simple design that was posted here
> a couple of years back, which basically works out to:
>
>   mains -> simple 9v ac/ac power brick -> dropping resistor -> picPET
>
> I have a good 10MHz reference to feed the picPET, so this seems like it
> would make a good first shot. But, of course, I eventually want to do
> better than just a first shot. So, I have questions!
>
> Q1: Assuming the schmitt trigger in the picPET triggers at a consistent
> point in the waveform, the frequency at any given cycle is easy to
> calculate: 1.0 / (timestamp2 - timestamp1)    ...but, is there a better
> way? That method just feels... naive, for some reason.
>
> Q2: What are the sources of noise in this design? Assuming the picPET is as
> accurate as my 10MHz reference is, I can think of a few potential places
> that phase noise could creep into the measurements:
>   - Whatever is in the power brick beyond the transformer (I don't think a
> step down transformer alone would add phase noise, right?)
>   - The dropping resistor will slowly change the amplitude of the waveform
> (and thus the point in the cycle that the schmitt trigger fires) due to
> thermal and aging effects, if we're measuring anything that's not the exact
> zero crossing
>   - The point at which the schmitt trigger in the picPET fires will change
> over time for the same reasons. Also potentially due to picPET input
> voltage, depending on how the comparitor is built
>   - Am I missing any?
>
> Q3: The open-ended question: How do I improve on this? I suspect the main
> place for improvement will be in the trigger, but I'm not sure where to go
> with that.  Most designs I've seen involve a schmitt trigger, generally
> with reference voltages set by things like voltage dividers. This seems
> dubious at best, to me, since that means the reference voltage will be
> affected by the same effects I'm calling out above. Is there a *specific*
> design (rather than "make a zero crossing detector!" or something similarly
> vague) that someone can point me to, that would minimize this kind of
> trigger noise?
>
> Q3.1: Is there a better way to get mains voltage down to something I can
> work more directly with? I saw at least one design that just used a couple
> of megaohm resistors inline -- does that introduce appreciably less phase
> noise than random AC/AC power brick?
>
> I apologize if any of this is overly basic. I've actually read everything
> I could find both in the time-nuts archives and the internet at large about
> this kind of project, but I've still found myself left with the questions
> above.
>
> I appreciate any comments / feedback / pointers!
>
> -j
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