[time-nuts] time-nuts Digest, Vol 51, Issue 8

Murray Greenman Murray.Greenman at rakon.com
Sun Oct 5 17:37:47 UTC 2008


Steve,

The suggestion of using the mains frequency as a reference is not
practical. There will be no certainty, even after counting for a week
that the mains frequency will give you a reliable reference. You said it
yourself - "the instantaneous frequency of the mains is not always spot
on".

The mains frequency specification is probably set down in a regulation
somewhere, and it probably accounts only for a mean number of cycles per
day, and so the counting period required to lock a reference would be
too long. Essentially it is a reference with immense low frequency phase
noise. Heaven knows, it's a challenge locking 10MHz to 1Hz, so locking
to 1/86400Hz would be very difficult.

An early mains frequency controller for power stations used a pair of
synchronous clock motors (one on the mains, one driven by a
crystal-controlled source), driving a differential gearbox which drove a
pointer calibrated in 'seconds fast/slow', so potentially you could do
the electronic equivalent, but I doubt that it would be capable of
better performance than the OCXO on its own.

If you want a cheap and cheerful 50Hz reference in NZ, use the frame
frequency of TVNZ's TV1 or TV2 transmissions. They have a Trimble
Thunderbolt GPSDO driving the sync generator. Just pull the sync out of
an old video recorder. It's all been done before. Electronics Australia
about 1976!

73,
Murray ZL1BPU




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