[time-nuts] Mains frequency

Didier Juges shalimr9 at gmail.com
Sat Nov 16 22:32:35 EST 2013


"Turns out that professional gear for this does not do time-stamping in
this regard. Rather, they I-Q demodulate the signal with a reference
signal at the nominal rate, low-pass filter it and pay attention to
details of filtering like group-delay and compensation thereof."

It makes sense to me. That way, you use the entire signal instead of just the zero crossing. 
You know that the signal is generated as a sine wave. Therefore, essentially you synthesize a local sine wave that is a best fit to the input signal and you measure the zero crossings of the synthesized signal.

 Didier KO4BB


Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
>Tom,
>
>On 11/17/2013 03:02 AM, Tom Van Baak wrote:
>> Charles, et al.
>>
>> I think we agree. Just to clarify...
>>
>> I rely on no hardware and no software filters when I use a
>time-stamping counter such as a sub-nanosecond Pendulum CNT-9x or
>sub-microsecond picPET. An electrical zero-crossing happens when it
>happens. If you "filter" you're just trying to change history: spikes
>are spikes; noise is noise; history is history. Deal with it. Record
>it, don't filter it away.
>>
>> The beauty of the time-stamping method is that you capture any and
>all positive zero-crossings. If there is "noise" all it does is create
>unexpected and obvious artificial too-early or too-late samples --
>which are trivial to analyze or eliminate in software.
>>
>> Some call them "outliers" and ignore them. This is correct. However,
>if one "filters" or "averages" them, you give validity they may not
>deserve. Bogus data should be eliminated by *logic*, not attenuated
>with pseudo-analog *filtering*.
>>
>> You can either focus on the signal, or the noise. That's two separate
>plots. An extraneous time-stamp happens to me a couple times a month;
>they are easy to spot and ignore. Similarly, a couple times a year I
>might miss a 60 Hz sample; these are also easy to spot and repair. For
>best time & frequency results, never "divide by =60"; instead "decimate
>by ~1 second".
>Standard wide-bandwidth counters isn't really ideal for signals like
>this.
>
>When you measure the mains signal, nominally 60 Hz in this case, spikes
>etc. is noise which is local and not of interest when comparing over a
>large area. Inter-area oscillations have much slower properties.
>If you go the time-stamping way, you *should* remove such noise.
>
>Removing or padding over it by logic will mean dropping data, which is
>not helpful.
>
>Turns out that professional gear for this does not do time-stamping in
>this regard. Rather, they I-Q demodulate the signal with a reference
>signal at the nominal rate, low-pass filter it and pay attention to
>details of filtering like group-delay and compensation thereof. It's a
>rather wise approach for the type of conditions you have.
>
>Cheers,
>Magnus
>_______________________________________________
>time-nuts mailing list -- time-nuts at febo.com
>To unsubscribe, go to
>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>and follow the instructions there.

-- 
Sent from my Motorola Droid Razr 4G LTE wireless tracker while I do other things.


More information about the time-nuts mailing list