[time-nuts] 10MHz Rubidium reference source for frequency counter

Adrian rfnuts at arcor.de
Thu Oct 30 18:32:48 EDT 2014


Karen,

as by the datasheet, the time interval resolution of your counter is 50 ps.
This translates into an ADEV value of 5E-11 at tau = 1 sec, 5E-12 at tau
= 10 sec etc,  and represents the theoretical measurement limit of the
counter.
It can be measured by feeding the same signal into both inputs (start
and stop) at the same time. A short delay (additional length of coaxial
cable) at the stop input keeps the counter from producing phase wraps
(jumping between a full input signal period and zero).

A good MV89A can be below 1E-12. For measuring two good MV89A's against
each other, a TI resolution of 50 ps is insufficient because up to 100
sec, to the most part of it, the actual ADEV of the oscillators can be
below the measurement limit of the counter.

Adrian

Karen Tadevosyan schrieb:
> Thanks again for your explanations and advices.
> I raised this question as a ADEV measurement result of my GPSDO/NEO-7M is about 1E-9 for tau = 1sec http://www.ra3apw.ru/wp-content/uploads/2014/10/NEO7M_OCXO_MV89A_10M_2h0_ADEV.jpg and it isn't meet my expectation. 
> In my understanding (before I published my question here) the reason for this is not a good enough short range stability of my frequency counter's reference OCXO that's why I had an idea to use external rubidium source.
> Now I see that even my internal reference OCXO provides a good ADEV value 1E-10 for tau =1 sec (according Pendulum CNT-91 with opt.19 datasheet).   
> If so what can be a reason of not a good ADEV result of GPSDO/NEO-7M?
> Karen, ra3apw
>
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