The power supply contribution is interesting. This might have been a useful
tool when a year ago I was playing with some very large inverters on a
microgrid. I had one inverter as master (in UF mode) and two others as
grid-connected slaves in PQ mode. The first slave would come online just fine
yet when the second was synched the entire microgrid would go unstable. It was
noise at the zero crossings but not enough to see on a scope.
With all the distributed generation coming online I would be wary of relying on
those zero crossings.
Peter
On 11/18/2013 5:15 PM, 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 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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