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

Bob Camp kb8tq at n1k.org
Thu Oct 30 19:13:49 EDT 2014


Hi

The gotcha with translating ADEV directly to a measurement is the nature of ADEV. You are looking at a standard deviation measure, so the result will be some sort of “one sigma” kind of measure. 

Bob

> On Oct 30, 2014, at 6:32 PM, Adrian <rfnuts at arcor.de> wrote:
> 
> 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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