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From: strong@tc.fluke.COM (Norm Strong)
Newsgroups: sci.electronics
Subject: Re: Getting timing from line cycles (was Re: Sidereal Time Clock)
Message-ID: <1991Jan10.153533.3894@tc.fluke.COM>
Date: 10 Jan 91 15:35:33 GMT
References: <5972@mint17.UUCP> <2137@otc.otca.oz>
Organization: John Fluke Mfg. Co., Inc., Everett, WA
Lines: 50

In article <2137@otc.otca.oz> brendan@otc.otca.oz (Brendan Jones) writes:
}in article <5972@mint17.UUCP>, wilner@motcid.UUCP (Corey S. Wilner) says:
}> From what I understand, using the 60Hz line as a reference for timing is not
}> a good practice if you want any accuracy.  I have heard that cycles can be
}> longer or shorter than 1/60th of a second and some cycles can be lost completely.
}> Anyone care to comment on this with more substance other than my glaring
}> generalities?!
}
}Although a given instant of the line frequency may not be exactly 60 Hz (or
}50 Hz over here in Australia) you should find that over the long term the
}*average* frequency is exactly 60 Hz, as the power companies speed up and slow
}down the cycles in the long term to maintain the line frequency exactly.
}
}In Australia, the power companies maintain their line frequency to the atomic
}time standard in Melbourne, so that's pretty damn accurate.
}
}I built a clock that uses nothing more than counting cycles of 50 Hz mains
}frequency as its timebase - no crystals, no battery backup, no oscillators.
}I have found that although its accuracy varies by up to (but no more than)
}+/- 3 seconds per day, that over the long term (ie weeks and months) it is
}*exact* to the second.  I never have to set the thing! (unless there's an
}outage!!)
}
}For building clocks, using line frequency as a timebase is one of the best
}things you can do, I reckon!

I suppose it depends on what kind of "clock" you have in mind.  If a quick
visual check of the time of day is what you want, the power line is just
the ticket. 

If you want to keep track of the time precisely, for navigation, &c, make
a crystal oscillator; the power line is--by the actual testimony above--
useless. 

How you can say that the line frequency is exact over a period of time,
but may vary by +/- 3 seconds per day is beyond me.  If it can be off by
that much in any one day, it can be in any other day!  In other words, it
can be off by 3 seconds any time.  

True, this doesn't seem like much over the course of a year (10E-7), but
it's miserable for one day.  My $15 wristwatch is several times more
accurate than that--short term.  Check your numbers again.  I can't
imagine the power company letting 150 cycles get away from them in one
day.


-- 

Norm Strong  (strong@tc.fluke.com)
2528 31st S.   Seattle WA 98144