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

[Karen Tadevosyan]

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