[time-nuts] How do I know my GPS stabilized oscillator is working?

Didier Juges didier at cox.net
Sun Jul 30 15:04:15 EDT 2006


Along those lines, I have a question.

I am planning to (finally) finish my GPS stabilized OCXO using a Jupiter 
receiver and an HP 10811-60102 OCXO recovered from a defunct HP 8672A.

The question that comes up is: how do I know my GPSDSO is working and 
how do I evaluate its stability?

I understand I could measure the control voltage from the phase detector 
to the OCXO and plot it on the computer easily enough, but that will 
only tell me the error between GPS and OCXO, which can be significant in 
the short term, and that won't tell me who is right (even though in 
theory, if the OCXO is any good, it should be the GPS, then within a few 
hours or half a day, the GPS should take over).

I have a Takeda Riken (Advantest) counter TR5823H that has the high 
stability oscillator option (not as good as the HP OCXO) also and Time 
Difference capability but no GPIB port, so I cannot get the output to a 
computer. If anyone has the schematics for that counter, I may be able 
to jury rig something to get the display to a computer.

I have a couple of HP generators that have their own OCXO, I believe 
they are also 10811 (a HP 8673M synthesizer and a HP 3586A with high 
stability option). I also have another HP 8672A (on loan for now, but I 
can get it back if needed) and I have an EIP 371 counter with an OVENAIR 
OCXO (part number unreadable until I remove the OCXO from the chassis, 
and I do not have the spec for it). Finally, I have a Tek 494P with the 
excellent OCXO Tek has put in it (I do not know who makes it). So any of 
these instruments could be used as a reference against which I could 
measure my GPSDSO, but how do I know who is right?

Aside from the fact that some instruments specify the stability in days 
and others in year does not help determine the best oscillator, which 
should I use and how should I interpret the results? For instance, is 
the Tek 494P at 1x10-7/year better than the HP 8673M at 5x10-10/day? 
Since the HP 3586 uses a 10811 OCXO at 2x10-7/year vs. 1x10-7/year for 
the Tek, it seems the Tek has better long term spec, but what does it 
mean for short term stability?

Should I go out and by an HP counter with time difference and GPIB?

Is there another alternative that will not require another piece of HP 
gear in my lab?

Here are the specs I found for the oscillators I have:

Tek 494P       1x10-7/year (after initial 6 months)
HP 8673M     5x10-10/day (after 10 day warmup)
HP 8672A      5x10-10/day (after 30 day !!! warmup)
HP 3586A      2x10-7/year
TR 5823H      5x10-8/day

Thanks in advance for any suggestion or comment.

Didier KO4BB

John Ackermann N8UR wrote:
> Hi Faisal --
>
> I use a time interval counter to compare 1pps signals -- the DUT goes to
> the start input, and the reference (here, GPS) to the stop input.  You
> measure the time interval between the two, and use the change in
> interval to determine noise and offset.  For the LORAN experiment, I
> used my HP 5370B though this application could do with a much simpler
> counter.  I use homebrew logging programs under Linux using GPIB to log
> and process the data.
>
> John
> ----
>   




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