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.
Bill....WB6BNQ
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 <[email protected]>
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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