[time-nuts] Considerations When Using The SR620

Eric Garner garnere at gmail.com
Tue Dec 4 20:21:01 UTC 2012

I would be interested in those scripts as well. as soon as the replacement
TCXO for my sr620 arrives I can put it back together and this would be fun
to try.


On Tue, Dec 4, 2012 at 5:11 AM, Tom Van Baak <tvb at leapsecond.com> wrote:

> > We are using the SR620 to measure the interval between 1PPS signals from
> > two clocks.  One is the Septentrio PolaRx4 GPS receiver and the other is
> a
> > Rubidium clock.
> >
> > Many Thanks,
> > Paul
> Hi Paul,
> 1) If you are making frequency measurements, the warm-up of the internal
> oscillator is the major factor limiting accuracy. This doesn't mean you
> have wait 12 hours. For example, if all you need is 6 digits of precision,
> a one-minute warm-up may be sufficient. For each further digit of precision
> you wait longer. You can probably get 9 digits with 1/2 hour of warm-up. It
> depends on the oscillator. Plotting digits of precision as a function of
> warm-up time would make a very educational graph you could tape to the top
> of your SR620.
> 2) If you are making time interval measurements and using an external
> standard, the warm-up time will also affect the accuracy of your TI
> measurements, but to a far lesser degree. Here are informal results for TI
> (time interval) mode after a 5 minute power-down (see also attached plots):
> - if you need 1 ns accuracy, you can use the SR620 immediately after
> power-up
> - if you need 100 ps accuracy, wait 2+ minutes
> - if you need 10 ps accuracy, wait 15+ minutes
> - if you need 1 ps accuracy, you need a seriously stable lab environment
> or a different counter.
> Given that you plan to use the SR620 with high-end GPS gear I would
> suggest you try this quick experiment for yourself to see what *your* SR620
> does, with *your* inputs, in *your* environment. Your numbers will come out
> different than mine; but the methodology is the same. Your procedures can
> then be based on measurement and confidence instead of guesswork and
> folklore.
> Note also if your data collection is automated, there's really no reason
> to wait after power-up at all. Just collect data as soon as you can and
> skip the predetermined number of samples. I can send you the SR620 GPIB
> scripts I used for my test. This way you have a record of the warm-up
> settling time itself, which further gives you confidence in the data that
> follows.
> /tvb
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Eric Garner

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