[time-nuts] Mains frequency

Bill Dailey docdailey at gmail.com
Tue Nov 19 01:03:33 UTC 2013


tom,

nice plots.  how do you figure out what the contribution of variability vs
noise? In other words there is a differential between the "ideal" and the
actual a dev curves... is there a way to tease out how much nose contribute
to that differential?  It does seem to me that there should be far less
short term variability (< 100s) than there appears to be.  Clearly in the
very short tau (< 0.1 s) the picPET can't tease that out but as the curves
diverge, how much of that is noise? between say 0.1s and 100s?  Being a
power plant operator I would say quite a bit although I am rethinking that
some due to the way the turbines push and pull each other.  I can envision
some fine whole grid oscillations due to that push and pull.

bill


On Mon, Nov 18, 2013 at 4:15 PM, Tom Van Baak <tvb at leapsecond.com> 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 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.
>
> 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.
>
> /tvb
>
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-- 
Doc

Bill Dailey
KXØO



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