[time-nuts] Considerations When Using The SR620

Volker Esper ailer2 at t-online.de
Tue Dec 4 20:14:18 UTC 2012


I agree. Since Paul want's to use an SR620 I presume he needs precision. 
Otherwise almost any TIC with a fairly stable osc would do, for example 
one with a battery backup. So I further presume that he needs nearly the 
full accuracy / stability. But that's just speculation, surely Paul can 
answere this question?


Am 04.12.2012 14:11, schrieb Tom Van Baak:
>> 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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