[time-nuts] What is the best counter for a Time Nuts?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Oct 9 00:43:19 UTC 2008

Bruce Griffiths wrote:
> Brooke Clarke wrote:
>> Hi Bruce:
>> The Ref Output on the front panel of the SR620 is derived (1/10,000) from the 
>> EXT IN on the rear panel.  So when you apply a 10 MHz signal to EXT IN you get 
>> a 1 kHz REF OUT.  Appendix B in the PSR10 Rubidium source manual describes how 
>> to use this along with the arming function to average 1,000 measurements per 
>> second resulting in 2E-12 per second on the display.  i.e. very close to the 
>> HP53132A.  So no external hardware is required and no PC to get this result.
>> Have Fun,
> Brooke
> Now I am back on my own network, Here's is what I actually intended to post.
> That only works well when averaging a fixed time interval or phase error.
> In general with a frequency input, the unwrapped phase measures are 
> linear in time.
> You need to process the raw measurements to unwrap the phase and fit a 
> linear regression line to the unwrapped phase.
> You also need to divide down the frequency to produce an output 
> frequency that is less than the maximum measurement transfer rate  to 
> the PC.
> If I remember correctly the 5370 (in binary mode) can transfer more 
> measurements per second than the SR620.
> So in this case the older hardware has a speed advantage although it 
> needs a pair of external frequency dividers (the extra one is used to 
> divide down your frequency reference to a convenient frequency for the 
> STOP input greater than the divided down input frequency connected to 
> the START input.).
> Grenhall's picket fence algorithm can then be used to unwrap the 
> measured time intervals into the corresponding time stamps.
> Bruce

It also can be made to work well for the case where there is only an 
occasional single phase wrap per averaging period.
This applies to the case when the 2 frequencies being compared are 
within 1Hz or so of one another as in the case discussed in the PRS10 
manual appendix.
When there are many phase wraps per averaging time it becomes 
difficult/impossible to reliably account for such phase wraps without 
access to the raw measurements.


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