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

Brooke Clarke brooke at pacific.net
Sun Oct 26 17:45:00 EDT 2014

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. http://www.prc68.com/I/TandFTE.shtml#SR620
Mail_Attachment --
Have Fun,

Brooke Clarke, N6GCE
Karen Tadevosyan wrote:
> Hello All,
> Can I have your recommendation regarding a choice of 10 MHz rubidium source
> (available now on eBay like FE-5680; LPRO-101; LPFRS; FRS etc.) as a
> reference signal for my frequency counter Pendulum CNT-91.
> Thanks in advance.
> Karen, ra3apw
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