Hi “Back in the day” we used WWV and the kitchen clock for that sort of thing……
Bob > On Mar 11, 2018, at 8:32 AM, Tom Van Baak <t...@leapsecond.com> wrote: > > Bob, > > Correct, measuring mains frequency to a couple of digits is not hard. What > makes an interesting challenge is to monitor mains, "kitchen clock", phase > drift. And to do it with cycle accuracy; no slips. Note that to measure down > to 1 cycle over 1 day is 0.2 ppm. Over a month, 6 ppb, and over a year, > 5e-10. So the numbers add up and you see why we use atomic standards or GPS > or even NTP as a long-term reference for this. > > Your measurement system needs to have short- and long-term stability ~10x > better than: > http://leapsecond.com/pic/mains-adev-mdev-gnuplot-g4.png > > Again, that's not asking a lot. But it makes a really fun project. Much of > what you ever need to know about time & frequency metrology can be done by a > student with $10 in parts and a 60 Hz outlet. > > /tvb > > p.s. Yes, it's very early here on the west coast, but I had to check how > badly my WWVB clocks handled DST a few hours ago. > > ----- Original Message ----- > From: "Bob kb8tq" <kb...@n1k.org> > To: "Discussion of precise time and frequency measurement" > <time-nuts@febo.com> > Sent: Sunday, March 11, 2018 4:53 AM > Subject: Re: [time-nuts] Recommendations for Mains Power Monitor / Logger > > > Hi > > So, how good is “good enough?”. My first attempt ran a counter with a 1 us > period resolution. > (remember, it was tube based …). That turned out to be major overkill in > terms of line frequency > measurement. 60.123 Hz is doing pretty well in terms of line frequency. Even > to get that level, you > will be doing a bit of filtering (or you are just watching the last two > digits pop around randomly). > > Your typical time base in a PC is good to a few hundred ppm. That’s giving > you an error in the > fourth digit of your measurement. With a bit of luck, your sound card > timebase may be 5X > more accurate than your system clock. (or it may be worse …) it depends a bit > on how fancy > your audio setup is. > > Adding NTP to your PC will correct for any long term errors. In a rational > environment it should > get you into the “few ppm” range short term and zero error long term. > > A GPS gizmo will get you into the parts per billion (or better) range. It > might be 100’s of ppb, but it’s > still *way* better than your CPU clock. The usual auction sites have lots of > candidates in the sub $50 > range.There are also places that are happy to sell you shields with GPS > devices on them. > > A fancier yet solution is a GPSDO. We are well into overkill at this point. > The advantage to using > one is that it may be the time / frequency standard for your entire lab > setup. You are up in the > $100 to $500 range for most of them. They will get you into 10’s or 100’s of > parts per trillion. > > There are indeed *lots* of different time sources you could use. The number > of alternatives is > *much* larger than what’s on the list above. > > Bob > >> On Mar 10, 2018, at 11:46 PM, Tom Van Baak <t...@leapsecond.com> wrote: >> >>> I've done some Googling and have found any number of designs. >> >> Pat, >> >> 1) Safety. I usually use a low voltage step-down transformer. This gives >> isolation and safety. Anything from 3 VAC to 24 VAC is fine. >> >> 2) Trigger. There are dozens of schematics on the web for capturing the >> zero-crossing of a low-voltage sine wave. You can easily go overboard on >> this. Or just keep it simple and feed the signal through a resistor directly >> into a microprocessor input. The internal clamping diodes do their thing. A >> Schmitt trigger input is helpful but not necessary depending on how your >> software makes the measurement. >> >> 3) Timebase. Given the long-term accuracy of mains (seconds a day, seconds a >> year) you don't need an atomic timebase. If you collect data for a couple of >> days any old XO will be fine. If you plan to collect data for months you may >> want a OCXO. Most of us just use cheap GPS receivers. >> >> 4) Measurement. There are many ways to measure the signal. You can measure >> frequency directly, as with a frequency counter. You get nice data but it >> may not be perfect long-term due to dead time or gating effects in the >> counter. >> >> So what most of us do is measure phase (time error) instead. One way is to >> make time interval measurements from a given mains cycle to a GPS 1PPS tick >> or vice versa, from each GPS/1PPS tick to the very next mains cycle. Either >> way you get about sample per second. If you're in search of perfection it >> gets a bit tricky when the two signals are in a coincidence zone. >> >> The other approach is not to use a frequency or time interval counter at >> all. Instead you timestamp each cycle, or every 60th cycle. Unix-like >> systems have this capability. See Hal's posting. I use a picPET, a PIC >> microcontroller that takes snapshots of a free-running decimal counter >> driven by a 10 MHz timebase (OCXO or GPSDO). >> >> The advantage of the timestamp method is that you don't ever miss samples, >> you can time every cycle (if you want), or throw away all but one sample per >> second or per 10 seconds or per minute, etc. And best of all, timestamping >> avoids the hassles of the coincidence zone. >> >> 5) CPU. A plain microcontroller, or Arduino, or R-Pi can be used. Or if >> you're on Windows and have a native or USB serial port try this simple tool >> as a demo: >> >> http://leapsecond.com/tools/pctsc.exe >> http://leapsecond.com/tools/pctsc.c >> >> 6) An assortment of mains links: >> >> http://leapsecond.com/pages/mains/ >> http://leapsecond.com/pages/mains-cv/ >> http://wwwhome.cs.utwente.nl/~ptdeboer/misc/mains.html >> http://leapsecond.com/pages/mains/mains-adev-mdev-gnuplot-g4.png >> http://leapsecond.com/pages/tec/mains-clock-ani.gif >> http://leapsecond.com/pages/ac-detect/ >> http://leapsecond.com/pic/picpet.htm >> http://leapsecond.com/pic/pp06.htm >> >> 7) Final comments. >> >> It is tempting to worry about the design, as they are so many out there on >> the web. Which is best? What are the pitfalls? What about noise immunity? >> What about precision and accuracy? My recommendation is not to over-think >> this. Just throw something together and see what you've got. Most of the >> work is with handling the data you get, doing the math, making plots, etc. >> If after the first day you see odd-looking 16 ms jumps in your data then you >> know you need to pay more attention to trigger level or noise issues. >> >> 8) A sound idea. >> >> We need someone to try out the sound card method. Send the isolated low >> voltage AC into the L channel and a GPS 1PPS into the R channel. "The rest >> is just software." Note that because you have access to the entire sine wave >> there's a lot you can do with this method besides making charts of time >> drift or frequency deviation from the zero-crossings. >> >> For an even cheaper solution, forget the GPS receiver and the R channel -- >> since the PC (if running NTP) already knows the correct time. And skip the >> AC transformer too -- instead just hang a foot of wire off the L channel >> input. There's mains hum everywhere. It would be the one time in your life >> where the ever-present audio hum actually has a good use. >> >> /tvb >> >> ----- Original Message ----- >> From: "Patrick Murphy" <fgdhr...@gmail.com> >> To: <time-nuts@febo.com> >> Sent: Saturday, March 10, 2018 2:53 PM >> Subject: [time-nuts] Recommendations for Mains Power Monitor / Logger >> >> >> All this talk of varying mains power frequency aberrations has me >> curious what is happening in my own back yard here in Tulsa in the >> USA. Can some recommend a reasonable "introductory level" solution for >> this? (As a fledgling Time-Nut, those two words were hard to say.😀) >> At the least I would like to watch voltage and frequency, with a >> configurable monitoring and logging interval. I can provide precise >> timing as needed for synchronization and time-stamping. Expanded >> ability to also monitor amperage, various power factors, etc is a plus >> but not required at this point. >> >> I've done some Googling and have found any number of designs. What I >> can't tell is how well they work. I am pretty handy with my hands and >> do not at all mind a DIY solution. >> >> So what do the Oracles say? >> >> Thanks! >> >> -Pat >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions > there.<mains-adev-mdev-gnuplot-g4.png>_______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
