On Wed, July 1, 2015 12:55 pm, Bob Albert via time-nuts wrote: > I am missing something here. How does one use a 1 PPS signal? > I see how I can use a 1 MHz or 10 MHz for a time standard but > the 1 PPS usage eludes me.
Actually you have it backwards. A 1MHz or 10MHz can be a frequency standard, but how do you use it as a time standard? Time implies a clock (time of day) reference, and with just e.g. 10MHz you have 10 million edges per second, you do not know which edge delineates the beginning of a second, so you cannot use that 10MHz to derive time. > Unless the pulse is extremely sharp, a minor uncertainty in the > shape or amplitude will have profound effects on the timing. The pulse is sharp, but there are other noise sources present on the PPS output from GPS, so you need other tricks such as averaging to make it useful for more than just delineating the second in an approximate sense. > I use an HP 5328A with the OCXO and wonder how I can > improve its accuracy. The typical way GPS is used is by using the PPS to (effectively) gate a counter of a 10MHz source, and the long term average count is used to steer the 10MHz oscillator so that on average there are 10 million edges between every PPS. As I pointed out the PPS from GPS is kind of noisy (in the time sense, it moves around a bit and isn't actually exactly 1 second apart each time) so you need a lot of averaging. That is what takes place in a GPS disciplined oscillator. The 10MHz output from the GPS disciplined oscillator can then be used as an external reference to the 5328A. If you only have a GPS receiver, and not a full GPS disciplined oscillator, then you can have the 5328 counter measure the frequency of the PPS output, and average that over a long period of time. The long term average will be 1 Hz, so the difference from 1 Hz is the offset of your counter time base from the GPS referenced accurate frequency standard. Again, you would need to take the long term average frequency because there are things such as receiver inaccuracies and inherent atmospheric noise sources which cause small period variations in the time between each PPS output, but those "should" be random and cancel out with averaging. -- Chris Caudle _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.