[time-nuts] Re: Low cost synchronization, kitchen appliances

Chuck Harris cfharris at erols.com
Sun Aug 21 12:19:21 EDT 2005

Let's see, your graph shows for that small time interval that the frequency
is within 0.05% of being correct.  If it continued at that rate of error,
it would result in an error of about 42 seconds in 24 hours.    Sounds pretty
bad, until you factor in a few other variables.  One, the accuracy of the spectrum
analyzer you used to measure the plot, and the other is the fact that the powerline's
variation is cyclical by design.  Your spectrum analyzer appears to be soundcard
based.  The absolute accuracy of such a system is not so good Soundcards don't
even trim their crystals for frequency.  I would venture that the typical accuracy of
a sound card's clock is certainly no better that +/-0.01%.  If you plot the
powerline frequency over  a longer period, I believe you will find that it keeps coming
back to reality.  The usual pattern for summer time is to lose time during the day, and
to gain it back during the night.  Winter is usually the opposite.

In a synchronization problem such as the one the OP was trying to solve, you would
use the best characteristics of a xtal oscillator, and the power line in arriving
at your solution.  Since the powerline is cyclical in its error, and the crystal is usually
a very slow long term drift you should base your correction on perhaps weeks of
observation of the powerline signal.

-Chuck Harris

Alberto di Bene wrote:
> FWIW, this is a plot of the 53rd harmonic of the 50 Hz of the mains at
> my house, measured not more than half a hour ago. Nominally it should be
> 50 x 53 = 2650 Hz, but it ain't...
> http://sundry.i2phd.com/mains.html
> 73  Alberto  I2PHD
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