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

wb6bnq wb6bnq at cox.net
Mon Oct 27 19:37:34 EDT 2014

Hello Karen,

I think you are confusing and/or mixing the terms "stability" and 
"accuracy" with respect to your project.

It all depends upon your measurement period and the property being 
measured.  A GPSDO will never beat a truly, very high quality OCXO on a 
short term basis in the stability department.  For that matter most 
Rubidium's that you find on auction sites won't either.  Accuracy, on 
the other hand, is another matter.  A good and properly setup GPSDO will 
provide long term accuracy (years) with a Rubidium (months), calibrated, 
in second place and the very high quality OCXO (up to days) is last, 
again, with respect to accuracy.


Karen Tadevosyan wrote:

>Many thanks for all your recommendations. 
>Let me provide more details for understanding of my task. 
>I am playing with a GPSDO project on base of uBlox NEO-7M (http://www.ra3apw.ru/ublox-neo-7m-ocxo-gpsdo/) - sorry, text in Russian.
>One of the main step – ADEV measurement of a developed GPSDO. 
>My ADEV measure stand consists of a frequency counter Pendulum CNT-91 with TimeBase option 19 + GPIB interface + KE5FX TimeLab software (TNX again John).
>As option for CNT-91’s reference source I can use a homemade GPSDO on base of G3RUH design. 
>IMHO, in this condition a frequency stability of my GPSDO project should be higher than a stability of CNT-91’s  reference OCXO.
>Taking into account that rubidium source has a better short range stability than OCXO or GPSDO I hope to find an external rubidium as 10 MHz reference source instead of internal OCXO of counter.
>If my reasoning is not right could you please correct them as I am not an expert in this area.
>Karen, ra3apw    
>>Date: Sun, 26 Oct 2014 14:45:00 -0700
>>From: Brooke Clarke <brooke at pacific.net>
>>Hi Karen:
>>The ones you mention are all stand alone Rb oscillators that need to be
>>calibrated to set their frequency.
>>This was the historical way that crystal oscillators were calibrated every year
>>or so.  The great advantage of Rb over crystal oscillators is that their drift is
>>specified in months instead of days.
>>A much better - more modern idea - is the GPS Disciplined Oscillator (GPSDO).
>>It keeps the oscillator "calibrated" in real time.
>>A popular crystal based GPSDO is the Trimble ThunderBolt:
>>Another crystal based GPSDO is the HP Z3805:
>>There are many more commercial GPSDOs and this list has discussions that
>>show they can be a do it yourself project for under maybe $10, but require a
>>number of sophisticated skills.
>>I have the just released LTE-Lite GPSDO Evaluation Kit with 10MHz TCXO on
>>order.  Seems to offer good performance for the dollar.
>>The only advantage of a Rb GPSDO over a crystal GPSDO is for the case
>>where the GPS updating has not happened for some time.
>>This might be due to a power failure lasting some days or that the oscillator
>>will be used where there's no GPS access and it only gets "calibrated" then
>>used much later.
>>The Stanford Research PRS-10 Rb oscillator can be used stand alone where it
>>time stamps an external 1 Pulse Per Second input, or as part of a GPSDO
>>where an external GPS receiver supplies it with a 1 PPS input.
>>The Thunderbolt can be custom modified to drive an external Rb oscillator,
>>like the ones you mentioned, but that requires some technical sophistication.
>>Note the ThunderBolt and Z3805 are complete GPSDOs in a box, just connect
>>power and a GPS antenna.
>>The PRS-10 requires an external GPS receiver and antenna.  A a practical
>>matter that means it's more work to maintain the PRS-10 because there's
>>more opportunity for problems like disconnecting a cable.
>>PS Stanford Research offered a version of their SR620 Time Interval counter
>>that included a Rb oscillator (not a GPSDO) that some government agencies
>>purchased, but for normal use you really don't need a Rb oscillator, so the
>>CNT-91R appears to be a similar way so sell it to a government with a lot of
>>money to spare.  So don't feel pressured to use an Rb oscillator.
>>Mail_Attachment --
>>Have Fun,
>>Brooke Clarke, N6GCE
>time-nuts mailing list -- time-nuts at febo.com
>To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>and follow the instructions there.

More information about the time-nuts mailing list