[time-nuts] 60 Hz frequency and phase measurement

[Tom Van Baak]

Bob,

Several of us do long-term measurement of mains frequency. We tend to 
time-stamp cycles and then compute period or frequency, rather than 
measuring frequency or period directly. Traditional counters in gated 
frequency or time interval mode have dead time and this will skew results.

In my case I just run a 5 VAC wall-wart through a 10k resistor directly 
to the input pin of a PIC. No scaling, no filtering, no opto, no ZCD, no 
nothing. If I measure every cycle I get 155 million samples per month. 
If I extract one cycle each second (decimate by 60) it's only 2.5 
million samples a month. Many months there is not a single glitch in the 
data in spite of all the FUD about power line noise. Once in a while a 
month contains an extra or missing sample but the beauty of timestamp 
data is that this can be detected and repaired as part of data 
processing with no loss of phase.

Here's a page where Kevin (in New Mexico) and I (in Seattle) both used 
picPET's to measure mains for a few days and then we compared the 
results. Although thousands of miles apart, we're both on the same grid 
so the agreement was astonishing. It was milliseconds in time and ADEV 
down to e-8 over a day:

http://leapsecond.com/pages/mains-cv/

See also: http://leapsecond.com/pic/mains-adev-mdev-gnuplot-g4.png

/tvb


On 7/2/2019 10:09 PM, Bob Albert via time-nuts wrote:
>   I have tried to measure the power line frequency with spotty success.  My best results came from a period measurement, as many periods as the counter can accumulate.  Due to noise, one is never sure at quite what point the source is measured.  Perhaps a brick wall filter would clean it up for a more reliable measurement.
> Of course, at 60 Hz the period is 16-2/3 milliseconds.  So the counter should properly show a 1 followed by a row of 6s, with the last digit bouncing between 6 and 7 most of the time.
> If there is a filter used, it will not only remove noise but also short term variations.  But generatlly speaking you don't want to measure those, unless you are trying to evaluate a rotary generator.
> Getting this reading can be a challenge.
>      On Tuesday, July 2, 2019, 10:01:03 PM PDT, jimlux <jimlux at earthlink.net> wrote:
>   
>   On 7/2/19 4:09 PM, Dana Whitlow wrote:
>> I've always noted that casual attempts to pick up 60 Hz with small antennas
>> etc see more harmonics and other trash than actual line frequency.  But if
>> you're in an office environment, why not plug something in?  It's quite easy
>> to build a simple passive diode clipper/filter that will plug into a wall
>> outlet and
>> which will provide a sort of soft (but clean) squarewave at a voltage level
>> convenient for lab instruments and with good protection against big spikes
>> and
>> other trash riding on the line.
>
> Safety approvals are one obstacle (of course one could use a AC wall wart).
>
> Actually, it's because someone asked me about a science experiment where
> you'd place them in a neighborhood outdoors.
>
>
>
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