Hal is graphing seconds of offset and seeing 5 seconds worth of shift in one day. Worrying about phase shift across the transformer changing with temperature, is like rearranging deck chairs on the titanic :-)
Tim N3QE On Wed, Feb 5, 2014 at 5:40 PM, Tom Harris <[email protected]> wrote: > For your setup measuring mains there will be a large phase difference > across the transformer. This is due to very many physical properties of the > materials, the largest being the magnetic succeptability of the core. Now, > this does show a slight temperature dependance. So how do you know that you > are not getting a slow variation in the phase showing up as a frequency > shift, since you are measuring such tiny variations. I know that the > transformer is probably in thermal equilibrium with it's surroundings, so > is at a steady temperature, but this problem (of getting an accurate idea > of mains frequency & phase) has exercised me over the years. I currently > use an opto and voltage reference to get mains frequency, phase & and > voltage (computed by lookup table from pulse width) which I found was more > stable than a transformer. And cheaper as well, since this is for a > commercial product. > > I'm just surprised that you get such results with a cheap transformer. > > Just remembered, we got a tiny change in phase shift across a transformer > due to its orientation, we could turn it 90 Deg and get a tiny change (less > than a milliradian), we never got to the bottom of it, maybe the Earth's > magnetic field? > > > Tom Harris <[email protected]> > > > On 6 February 2014 04:39, Hal Murray <[email protected]> wrote: > > > > > [email protected] said: > > > Interesting.. I'm assuming the green graph is actual voltage and the > red > > > graph is..? > > > > The green is the frequency as measured over the last 10 seconds. > > > > The red is the long term clock offset in cycles relative to what it would > > be > > if the frequency was exactly 60 Hz. It's the error you would see if you > > looked at a clock that was tracking the power line. The 0 point is > > arbitrary > > since I can't see the reference clock the power system is using. For > those > > graphs, I used the start of the day/file as 0. > > > > > > > I've never done any mains monitoring/measuring and was wondering, > what's > > > your equipment setup? > > > > It's simple. The hardware is an AC wall wart and a couple of resistors > as > > a > > divider connected to a modem control pin. I forget which one. It's the > > one > > that ntpd expects to use with a PPS input. > > > > There was a discussion on that topic here a year or 3 ago. It's in the > > archives, but I couldn't find it with a quick look. > > > > The software is a simple python hack. It runs on Linux. > > http://www.megapathdsl.net/~hmurray/time-nuts/60Hz/pps.py > > > > Linux has a back door to the PPS info. Things like > > /sys/class/pps/pps0/assert give text like this: > > 1391619268.999925084#1125070 > > The number left of the # is the time of the last PPS. The number to the > > right is the pulse count. The software above just waits 10 seconds, > grabs > > another sample, and writes a line of text to a log file and switches to a > > new > > file every day. It's 1/2 megabyte per day. > > > > If you have FreeBSD or NetBSD rather than Linux, it shouldn't be too hard > > to > > use the same API as ntpd uses. I don't know how PPS works on Windows. > > > > Another approach would be to feed it into the audio input and scan for > zero > > crossings. I captured the raw binary for a while when I was chasing some > > noise glitches. It's a lot of data. > > > > > > -- > > These are my opinions. I hate spam. > > > > > > > > _______________________________________________ > > 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. > > > _______________________________________________ > 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. > _______________________________________________ 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.
