[time-nuts] Mains frequency

[Bob Camp]

Hi

There is no way to come up with the noise floor of the picPET from that plot. In fact coming up with the floor of a single channel device like the picPET is not all that easy. First you need an ideal noise free sine wave signal …. I’ve spent more than a few hours on that particular project with other list members involved as well. As always we kept it off list to keep from offending those who place a high value on their bandwidth. 

Bob

On Nov 18, 2013, at 9:11 PM, Bill Dailey <docdailey at gmail.com> wrote:

> I meant ideal at the noise floor of the picPET (i.e in this case the
> generated 60Hz).
> 
> 
> On Mon, Nov 18, 2013 at 8:08 PM, Bob Camp <lists at rtty.us> wrote:
> 
>> 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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> 
> 
> 
> -- 
> Doc
> 
> Bill Dailey
> KXØO
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