[time-nuts] 10MHz Rubidium reference source for frequencycounter

Tom Van Baak tvb at LeapSecond.com
Mon Oct 27 23:48:25 EDT 2014

Thanks for the explanation. In this case you should buy both a cheap Rb and good OCXO from eBay. This will allow you to explore the resolution of your CNT-91 as well as make several pair-wise plots using TimeLab.

Remember that no one frequency reference will suite all occasions. Sometimes the ones with best phase noise do not have the best ADEV. The ones with best short-term stability may not have the best long-term stability. The one with best long-term stability (e.g., sawtooth corrected/GPS 1PPS) needs to be sampled over long time scales. So a variety of time/frequency sources is nice to have.

A NEO-7M GPSDO sounds like a fun project. I agree with Said that a Morion would be a nice oscillator to discipline. The Rb may be useful to validate the mid-term performance of the GPSDO and help determine the tuning of your algorithms.


----- Original Message ----- 
From: "Karen Tadevosyan" <ra3apw at mail.ru>
To: <time-nuts at febo.com>
Sent: Monday, October 27, 2014 1:01 PM
Subject: Re: [time-nuts] 10MHz Rubidium reference source for frequencycounter

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:
> http://www.prc68.com/I/ThunderBolt.shtml
> Another crystal based GPSDO is the HP Z3805:
> http://www.prc68.com/I/Z3805A.html
> 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.
> http://www.ebay.com/itm/171504585820
> 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.
> http://www.prc68.com/I/PRS10.shtml
> 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.
> http://www.prc68.com/I/TandFTE.shtml#SR620
> Mail_Attachment --
> Have Fun,
> Brooke Clarke, N6GCE
> http://www.PRC68.com
> http://www.end2partygovernment.com/2012Issues.html
> http://www.prc68.com/I/DietNutrition.html

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