Simon, Good, you decided to make a time stamping counter instead of a frequency counter then?
On your spec -- 1e-5 precision of what? Of time accuracy? Or frequency accuracy? And over what time frame? The power line will give you 1e-5 stability at one second intervals; but this precision is destroyed as you average longer and longer (due to the rather significant variation in frequency that occurs in the grid). By one minute or one hour it's 1e-4. It takes a couple of days of averaging to get back down to 1e-5 precision. The way to see this is with an ADEV of the power line frequency: http://leapsecond.com/pages/mains/mains-picpet-23.gif I'm not sure about your +/- 1 Hz filtering suggestion. I don't do that here. But maybe your use for mains is different from mine. Do you want to use the power line as a reference time to slowly steer a drifting quartz oscillator over a month or year? Or do you want to make measurements of power line phase and frequency to make pretty real-time plots like Hal and I and others do? /tvb ----- Original Message ----- From: M. Simon To: time-nuts@febo.com ; t...@leapsecond.com Sent: Sunday, February 09, 2014 4:27 PM Subject: Re: [time-nuts] Line Frequency Tom, I was hoping for 1E-5 precision or better. My time stamping counter will have a 30 MHz clock (for convenience). My initial experiments will just set the 30 MHz clock to within about .1 ppm and ultimately I will sync it with GPS so I can track phase long term. In fact for quick, dirty, and cheap I will probably use a 10 ppm packaged oscillator with no adjustment. The ones I have checked out so far seem to be about +/- 2 ppm or so at room temperature. I like your idea (well you made me think of it) of filtering with time stamps. Only stamps within +/- 1 Hz of the line frequency are accepted (or what ever seems reasonable given grid limits). I would probably need a "loss of line voltage" circuit in addition. That is not a drawback. But "loss of line voltage" might be done in software as well. Simon Date: Sun, 9 Feb 2014 11:53:04 -0800 From: "Tom Van Baak (lab)" <t...@leapsecond.com> To: Discussion of precise time and frequency measurement <time-nuts@febo.com> Subject: Re: [time-nuts] Line Frequency Message-ID: <475bacc5-deef-43b7-a8ce-6416d997f...@leapsecond.com> Content-Type: text/plain; Charest=us-ASCII Hi Simon, 1) You can use the picPET to measure zero crossings; just bias the input. 2) With no external parts at all the picPET is a 2.5V crossing detector. 3) What you find when you start playing with 60 Hz is that this is not a problem. That's why few/none of us bother with it. 4) The "time" of 60 Hz varies so much minute to minute or hour to hour that line voltage effects are mostly irrelevant. 5) The picPET input is pin5, which is a Schmitt trigger input. 6) Schmitt trigger or not, line noise is no problem for a time-stamping counter anyway. What level of precision are you after? /tvb (i5s) > On Feb 9, 2014, at 8:04 AM, "M. Simon" <msimon6...@yahoo.com> wrote: > > Bill, > > I was hoping to avoid a device that was dependent on line voltage (rather than zero crossings) for timing as it will then be line voltage sensitive (or at least more than a zero crossing detector). Of course zero voltage (zero crossing) detectors have other problems. Susceptibility to line noise being one major one. > > Simon _______________________________________________ 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.
