Almost Accurrate Clock

[Darrel VanBuer]

Power line frequencies are generally pretty accurate (long term), in part
because they use the National Bureau of Standards as a reference (e.g. WWV).
It's essential to have consistent frequencies when you run a national
power sharing grid.  Then can notably run slow during high load conditions
(e.g. hot summer afternoons, but will (usually) eventually catch up.
WWV provides standards for both power and frequencies (their carrier frequencies
are all exact multiples of 2.5 MHz.
The long term stability of WWV comes from a battery of atomic clocks with
accuracies approaching 1 in 10^12 (and short term stability better by a couple
orders of magnitude.
The big advantage of WWV over any power line clock is with the proper
equipment (Heath clock or voice recognition -- your head) it's an absolute
standard, rather than relative.
We had a serious problem with a powerline clock on our PDP 11/70 after
our motor-generator power conditioner was installed; it ran about 58 Hertz
output, so our processor ran 40 minutes off each day!

If you ever want a really accurate, cheap frequency standard, try your
color TV.  For the color to come out right, it has to stay quite close to
the color burst frequency at the transmitter, so is resyncronized every
60 microseconds to keep it within a tenth of a cycle, or so.  The major networks
maintain theirs against NBS.  (it's 315/88 MHz).

-- 
Darrel J. Van Buer, PhD
System Development Corp.
2500 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,orstcs,sdcsvax,ucla-cs,akgua}
                                                            !sdcrdcf!darrelj
VANBUER at USC-ECL.ARPA