[time-nuts] Mains frequency

[Bob Camp]

Hi

An “ideal” curve would go to the bottom of the scale as soon as the plot started. Anything that shows on the ADEV curve is by definition noise. The slope of the ADEV curve can help you determine what sort of noise it is. The slope(s) on an modified ADEV curve can do that slightly better.

Bob

On Nov 18, 2013, at 8:03 PM, Bill Dailey <docdailey at gmail.com> wrote:

> 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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