[time-nuts] Line Frequency

[M. Simon]

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)" <tvb at leapsecond.com>
To: Discussion of precise time and frequency measurement
    <time-nuts at febo.com>
Subject: Re: [time-nuts] Line Frequency
Message-ID: <475BACC5-DEEF-43B7-A8CE-6416D997F299 at leapsecond.com>
Content-Type: text/plain;    charset=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" <msimon6808 at 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
 



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