[time-nuts] Considerations When Using The SR620

Paul DeStefano paul.destefano at willamettealumni.com
Thu Dec 6 18:10:27 UTC 2012

On Tuesday, 4 December 2012, Tom Van Baak 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
> 1) If you are making frequency measurements, the warm-up of the internal 
> oscillator is the major factor limiting accuracy. ... 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.

Tom & Co.,

 	Thank you!  These plots are excellent and will be very helpful. 
You are quite right; we should do the test ourselves.  We will definitely 
do that.  Obviously, there is not need to worry, as we can characterize 
the instrument behavior ourselves, which is probably necessary anyway if 
we're going to publish these measurements with error values.

Many Thanks,

Paul DeStefano

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