All,
If you want a reason for logging the mains frequency, see the following
link to a news item which appeared on a BBC news program a few weeks ago
here in the UK.
http://www.bbc.co.uk/news/science-environment-20629671
There was also a full program about it which you can listen to at the
following link
http://www.bbc.co.uk/programmes/b01p7bxw
John G3UUT
On 23/02/2013 08:15, Tom Van Baak wrote:
So it is not correct to measure one point to a gnat's nose
hair and call it "the grid frequency."
Bill,
Yes, and this is true for any source of frequency. That's why when we specify
stability the averaging interval is critical; the x-axis of a log-log ADEV plot.
One might look at every cycle to measure stuff like jitter, not so much to
measure frequency over a tau of 0.016 seconds. The nice thing about ADEV is
that a single plot can convey frequency stability for all intervals from as
short as a single cycle to as long as days or months or more. Now with a 10 MHz
standard we don't normally start an ADEV plot at a single (100 ns) cycle. But
for 60 Hz it's perfectly natural to do so.
A histogram of period is another way to show variations in cycle time. This
gives more information than a single ADEV point. But to show variations as a
function of averaging time, a whole set of separate histograms, or overlaid
histograms, are required.
It might be more accurate to put a flywheel on a synchronous
motor and measure its speed, because the time constant of that
system is a whole lot closer to that of the real grid frequency.
I too was suspicious of digital or PLL or filtered methods of monitoring 60 Hz
phase. To validate the digital methods I compared against an old synchronous
wall clock. In the following animated GIF, a photo was taken exactly every 900
seconds (15 minutes):
http://leapsecond.com/pages/tec/mains-clock-ani.gif
It turns out the zero-crossing microprocessor digital time-stamping method
exactly agreed with the old mechanical synchronous motor/inertia method.
Satisfied with this result, I do all my mains phase/frequency logging using the
digital time-stamp method (picPET).
/tvb
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