ECN Forum Forums Technical Discussion Re: Computers and the Internet Where Does the Time Come From?

Aside from Xtals and other sources mentioned one has not been dicussed (actually more than one), and that would be your plain ole 60 Hz power supplied by your POCO. Many of the older digital clocks took a sample via a capcitor to a frequency divider and simply divided it by 60 naturally.

dereckbc:

Long term this 50 or 60 Hz will be very accurate, Depending on loading on the overall grid the frequency can vary by up to quite a few seconds a day if not tens of seconds a day.

During peak demand the main frequency may be 59.687 Hz for example, for say a couple of hours when all generators in the system are heavily loaded and it is not warranted to start up extra generating capacity. Generally at night when the grid is lighter loaded the frequency will be raised a little like 60.206 Hz for a period of time to make up for lost cycles generated so that over a 24 hour period a "nil" error is in generated cycles.
60 Hz system . 24h*3600*60 = 5,184,000 cycles
50 Hz system . 24h*3600*50 = 4,320,000 cycles
The mains operated clock usually has a hysteresis motor, or synchronous motor which runs exactly in step with the mains frequency, so some seconds are lost during the day and made up for at night. Over a long period these clocks are very accurate.

Depending upon whereabouts in the world you're located, there's always the option of synchronizing with one of the broadcast atomic clock standards.

Here in the U.K. we have the National Physical Labs. broadcast from Rugby:
http://www.npl.co.uk/time/

Software is available to interface a suitable receiver, for example:
http://www.galsys.co.uk/atomic-clock/msf-rugby-atomic-clock.htm


In the U.S., there is station WWV, Fort Collins, CO, which provides a similar service on several HF transmitters:
http://tf.nist.gov/timefreq/stations/wwv.html


It's also possible to synchronize by internet:

http://tf.nist.gov/timefreq/service/time-computer.html

Or failing that, you can just call (303) 499-7111 and set the clock manually every day!



[This message has been edited by pauluk (edited 01-08-2006).]

I don't think that they normally let the grid frequency drift that much. Under normal conditions the grid operates at plus or minus 0.01 hertz. When time correction is needed for cumulative error correction the system is run at 60.02 or 59.98 hertz.
Don

In the U.K. the grid is specified as 50Hz +/-1%, so 49.5 to 50.5Hz allowable. They make tiny adjustments to the frequency to that in the long term it always averages out to exactly 50Hz to keep synchronous clocks accurate.