[time-nuts] Mains frequency

Magnus Danielson magnus at rubidium.dyndns.org
Wed Mar 12 11:39:50 EDT 2014


On 18/11/13 23:15, Tom Van Baak wrote:
> Magnus,
> I'm going to push back a bit on your mains sampling claim. Mostly, I'd like to see the results of the professional I-Q demodulated gear that you mentioned. Can you post raw data, or a sample plot?

I don't have much of that myself. I do recommend you to check the 
presentations of the NASPI conference (naspi.org). There is plenty of 
plots there.

> I agree that looking at power line voltage with 16- or 24-bits at 1 Msps is going to reveal interesting amplitude and phase noise information. But see how well a $1 PIC can do.

Well, I should toss that over to the good folk at NIST doing 
synchrophasor calibrations. Should I grab them now that we are in the 
same room?

Have a look at IEEE C37.118.1 for measurement methods.

> Attached is a plot made using TimeLab + picPET just now. The picPET is fast enough to capture the zero-crossing of every 60 Hz cycle with 400 ns resolution; the TimeLab plots have tau0 of 16.67 ms.
> -- The blue trace was simply plugging a 9 VAC wall-wart into the picPET though a 10k resistor.
> -- The pink trace was adding a 10 nF cap across the input.
> -- The green trace was unplugging my laptop switching power supply from the same outlet!
> -- The red trace is replacing the mains wall-wart with a hp 33120A set to 9VAC at 60 Hz, a tentative noise floor measurement of the picPET when used this way.
> My conclusions are that at least here in the US, or at least at my house, the short-term stability of mains hits about 5e-6, at about tau 0.2 seconds. The attached short-term plot is also not-inconsistent with the long-term plot at http://leapsecond.com/pages/mains/
> My other conclusion is that the picPET (a simple PIC-based time-stamping counter) is doing a pretty good job measuring this. Note, no software or data filtering was used. This is just raw serial/USB data going into TimeLab.

Well, if you are happy with that, fine. But there are many things 
happening on the grid which needs deep analysis and the tools for it has 
been developed to provide both resolution and removal of noise which is 
not part of the measurments. Just calibrating the trigger noise for a 
PMU requires care, as the S/N required for a straight comparator for the 
applications is several tens of dBs away from a good conditions, so they 
have had issues with doing that.

Doing your own time-stamping like you have done is naturally fun, but do 
not confuse it with the experience and processing that have been shown 
needed by an industry.

BTW. WECC, who has a large network of PMUs, and that covers where you 
have your house and measurement point, can't release detailed data to me 
or you just for fun. It always needs to be cleared from a security point 
of view.


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