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

Bob Camp kb8tq at n1k.org
Wed Oct 29 21:28:43 EDT 2014


Hi

Straight sawtooth corrected GPS starts out with (say) a 1x10^-9 ADEV at 1 second. That ADEV likely drops as 1/tau. If you go to 10 seconds it’s 1x10^-10. 

Your Rb starts out with a (say) 1x10^-11 ADEV at 1 second. That ADEV likely drops as 1/ square root (tau). If you go to 100 seconds it drops to 1x10^-12

Your OCXO starts out with a (say) 1x10^-12 ADEV at 1 second. That ADEV likely is the same as tau goes up to 10 or 100 seconds. 

Both the OCXO and Rb are sensitive to temperature. They may change by may parts in 10^12 per degree or per ten degrees. It depends a lot on how good a unit you have. In a room with a lot of drafts (temperature changes) this may impact ADEV even at 100 seconds. 

Nothing ever gets better and better forever. All of these standards level out at some point. GPS is unlikely to get past 1x10^-14. Most Rb’s struggle to get to 1x10^-13. An OCXO is unusual if it’s below 1x10^-12 at 1,000 seconds. 

No matter what sort of standard you have, it will perform better if you keep it powered up all the time. 

Bob

> On Oct 29, 2014, at 2:09 PM, Karen Tadevosyan <ra3apw at mail.ru> wrote:
> 
> Charles,
> Thanks a lot for the detailed explanation and recipes. Understood and will check this weekend.
> All my Morion 10MHz MV89A OCXOs are used and from eBay's. That's why the first version of broken surplus MV89A looks realistic. 
> Best regards,
> Karen
> 
>> Date: Wed, 29 Oct 2014 10:02:44 -0400
>> From: Charles Steinmetz <csteinmetz at yandex.com>
>> 
>> Karen wrote:
>> 
>>> I raised this question as a ADEV measurement result of my GPSDO/NEO-7M
>>> is about 1E-9 for tau = 1sec 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.
>> 
>> The Pendulum is a well-respected instrument for this sort of measurement,
>> as you have now discovered by reading the datasheet.  Much better than the
>> ADEV you are getting.  (In fact, it is probably better than the datasheet
>> specification.)
>> 
>>> 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?
>> 
>> (1) poor performance of the particular OCXO you have, or (2) poor GPSDO
>> design or construction.
>> 
>> To check (1), remove the OCXO from the GPSDO and measure its ADEV by
>> itself.  A good OCXO should be better that 1e-11 at 1 second, certainly better
>> than 1e-10.  I believe you said you are using a Morion MV-89.  Many of the
>> surplus MV-89s available in the US are broken and have poor ADEV or lots of
>> spurs.  The situation may be better where you are.
>> 
>> If the ADEV of the oscillator by itself is only 1e-9, then you need [at least] a
>> different oscillator.  If the oscillator by itself is OK, then the GPSDO is not
>> doing a good job of disciplining it.
>> 
>> The purpose of a GPSDO is to let the oscillator run without interference at
>> tau where the oscillator stability is better than the GPS timing signal, then to
>> cross over at longer tau and let the GPS control the oscillator at averaging
>> times where the GPS stability is better than the oscillator.  Usually, this will
>> be somewhere between
>> 100 and 1000 seconds.  So the control loop must be VERY slow, with a time
>> constant in the hundreds or thousands of seconds -- it should be doing
>> essentially nothing at tau = 1 second.  The ADEV of the GPSDO at 1 second
>> should be the same as the ADEV of the OCXO at 1 second, or very close to it.
>> 
>> Best regards,
>> 
>> Charles
> 
> 
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