[time-nuts] Considerations When Using The SR620

Tom Van Baak tvb at LeapSecond.com
Tue Dec 4 13:11:42 UTC 2012

> 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.

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