Tom,
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?
Volker
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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