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

Bob Camp kb8tq at n1k.org
Fri Oct 31 22:12:43 EDT 2014


Unless the OCXO’s you have are broken, there is something wrong with your measurement. I have bought enough broken OCXO’s to know that you can get quite a few bad ones for every good one you find.

The CNT counters are good instruments. They will do about as well as any frequency counter out there. They will not get to 1.0 x 10^-12 at one second. Depending on your signals and your setup, they may not get much past 1.0 x 10^-10 at one second. One of the things that will drive them a bit nuts are sub-harmonics. If your OCXO’s are 5 MHz doubled to 10 MHz, they will have sub-harmonics (5 MHz, 15 MHz, 25 MHz etc). These “extra” signals are not what the counter is expecting to see on a 10 MHz signal. 

Since your measurements are different for each OCXO, and go down by square root of tau, it’s got to be something unusual. The interaction of sub-harmonics with the CNT series frequency approximation algorithm may be it. 


> On Oct 31, 2014, at 3:57 PM, Karen Tadevosyan <ra3apw at mail.ru> wrote:
> Thanks All for your comments and clarifications.
> The next OCXOs have been tested today:
> 	* Morion MV89A 10.000MHz  #1
> 	* Morion MV89A 10.000MHz  #2
> 	* Morion MV89A 10.000MHz  #3
> 	* Isotemp 131-191 10.000MHz #1
> 	* Pilot C3Z 10.000MHz 
> 	* Bliley NV26R891 100.000MHz
> 	* Giacint-M 5.000MHz 
> Measurement set up:
> 	* Pendulum CNT-91 with TimeBase opt.19 and 10
> 	* Prologix GPIB-USB 
> 	* TimeLab V1.2 (x64)
> 	* Windows7 Pro (x64)  
> 	* PC i5 /3.4GHz / 16GB 
> ADEV results https://dl.dropboxusercontent.com/u/21338179/hamradio/OCXO_Adev/ocxos_adev_test_20141031.jpg
> TimeLab configuration https://dl.dropboxusercontent.com/u/21338179/hamradio/OCXO_Adev/timelab_adev_config.jpg
> All OCXO's TimeLab TIM files can be download https://dl.dropboxusercontent.com/u/21338179/Hamradio/OCXO_Adev/OCXO_Adev.zip
>> A good MV89A can be below 1E-12.
> As you can see ADEV of all my tested OCXOs are not better than 4E(-10) for tau =1s.
> Question: can I be more or less happy with this result or something wrong in my measurement?
> Karen
>> Date: Thu, 30 Oct 2014 23:32:48 +0100
>> From: Adrian <rfnuts at arcor.de>
>> 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
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