[time-nuts] Building a mains frequency monitor

[Chris Albertson]

Transformers really are nearly noiseless.  They can pick up magnetic fields
but most of that is the 60Hz you are trying to measure. (the problem is
normally with audio transformers picking up the 60Hz)

A transformer is by far the safest way to go.  I'd even go so far as to use
a split bobbin type so even if the transformer fails you are still safe.
So look for a _quality_ name brand plug-in transformer with AC output and a
"UL" stamp on it.

I think you really do want a zero crossing detector.  The biggest source of
error is going to be the non-constant AC line voltage that might go from
125 volts to 110 volts depending on the time of day and it might even cycle
along with the next door neighbor's air conditions unit or if your
microwave oven is running.

even with zero crossing I think the very best why to do this is to assume
the trigger is not perfect so you try and measure the imperfection then
remove it in post processing.   So perhaps you have two triggers that in
theory should be symmetric.  One measure the raising edge the other the
falling edge.  They should be exactly 180 degrees out of phase and have the
same period.  You compare what should be to what is measured and assume the
difference is some kind of imperfection in the trigger that needs to be
accounted for.

Then you do your best to fit a variable frequency sine wave to your data
points.

Are you only interested in frequency?  What about line transients?  For
that you simply run the AC into a A/D converter and log all the data points
at maybe 48K samples per second.  Sounds like a lot of data but with not
really, only a few hundred megabytes per hour and you can trash the data
after it is processed  You likely be looking at an FFT to get frequency
domain data from your sampled AC line.

On Wed, Apr 6, 2016 at 6:21 PM, 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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-- 

Chris Albertson
Redondo Beach, California