[time-nuts] Mains frequency

[Tom Van Baak]

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".

/tvb

----- Original Message ----- 
From: "Charles Steinmetz" <csteinmetz at yandex.com>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Saturday, November 16, 2013 4:54 PM
Subject: Re: [time-nuts] Mains frequency


> Chuck wrote:
> 
>>In the case of a 60Hz mains derived signal, most of the noise is
>>going to be riding on the signal, and will be amplified with your
>>gain stage.
> 
> The potential evils of bandpass filters in a timing chain are well 
> known, but as long as you can accept the delay of a filter (or 
> correct for it, which should be trivial with a PIC or other uC), you 
> may be much further ahead with a noisy signal like the AC mains if 
> you use a sharp bandpass filter on the incoming 60 Hz then amplify & 
> clip the signal to increase the slew rate.  Active filters with fast, 
> quiet op-amps should do the job well.  For the lowest jitter, a 
> Collins-style multi-stage zero cross detector may be helpful.
> 
> Best regards,
> 
> Charles