[time-nuts] Mains frequency

Tom Van Baak tvb at LeapSecond.com
Mon Nov 18 22:15:14 UTC 2013


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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