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 <[email protected]> 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 > _______________________________________________ > time-nuts mailing list -- [email protected] <javascript:;> > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > -- Sent from Gmail Mobile _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
