[time-nuts] How do I measure oscillator frequency using 1pps?

John Ackermann N8UR jra at febo.com
Sat Jul 9 17:16:33 EDT 2005

David Kirkby wrote:

> I've now got
> 1) Stanford PRS10 rubidium standard
> 2) Motorola M12+ timing GPS receiver with a 1 pps output.
> 3) HP 5370B time interval counter.
> I'd like to look at the drift of the rubidium before I try to steer it
> with the PLL. Can anyone explain how to do this with the 5370B?

If the PRS10 has a 1pps output, or you can kludge up a divider chain to
get to   1pps, it's pretty straightforward:

1.  Try to synchronize the PRS10 1pps so that it's (ideally) a few
microseconds early with reference to GPS.

2.  Feed the PRS10 1pps into the start input of the counter, and the
M12+ 1pps into the stop input.

3.  If you have GPIB capability in your system, you can record the time
difference and process in the PC.

4.  Alternatively, set the 5370 into averaging mode, preferably using 1k
samples.  Averaging over 1000 seconds will pretty well smooth out the
noise in the GPS 1pps.

5.  Record the mean time interval (and while you're at it, the standard
deviation, which will give you an idea of how solid your GPS signal is)
by hand, along with the time the data was taken.  Come back a day or so
later and record the values again.  Continue as long as you have
patience.  The larger the frequency offset, the shorter the measurement
time before you get reasonable results.

6.  The frequency offset is delta time interval over time -- a drift of
one microsecond per day is equivalent to 1.157x10e-11.   If the time
interval is increasing, that means that the Rb is running slow relative
to GPS; a decrease in time interval means it's running fast (assuming
you're starting on Rb and stopping on GPS).

If you don't have a 1pps available from the Rb, you can trigger from the
10MHz output, but cycle rollover can be a problem (at 10MHz, one cycle
is 100ns).

Hope this helps.


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