[time-nuts] Cs standard stability

Bob Stewart bob at evoria.net
Sat Mar 4 11:58:12 EST 2017

Hi Bruce,
I'm aware that the GPSDO is better at low tau than the 5370.  This is born out by its low tau ADEV being right at the specs for the 5370, as well as by measurements done some time ago on a Timepod.  Unfortunately, I don't have a Timepod, so I have to make some guesses based on what I can see; limited as that is by the 5370.  But, given that the GPSDO tests validate the 5370, when I see an ADEV that is worse than the 5370's specs (i.e. worse than the result of the GPSDO test), I take that as an indication that the additional noise is from the DUT.  Of course, I don't know anything more than that.  Correct?


      From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
 To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Saturday, March 4, 2017 3:44 AM
 Subject: Re: [time-nuts] Cs standard stability

That doesn't seem inconsistent with the specs given the contribution of the 5370A to the measured ADEV is likely to be of the same order as the PRS45A spec. The ADEV of the GPSDO also needs to be taken into account. The contribution of the 5370A to ADEV will gradually become insignificant compared to the PRS45A specs as Tau increases. To achieve an accurate measurement of ADEV for Tau=1s you will need something better than the 5370A together with a low ADEV source (for Tau = 1s).

> On 04 March 2017 at 18:26 Bob Stewart <bob at evoria.net> wrote:
> I hooked up my PRS-45A yesterday, just to do some ADEV comparisons with my GPSDO.  Unfortunately, the GPSDO seems to be better by almost two points at all tau as measured on my HP 5370A and plotted by Timelab.  So, can I infer from that that the OCXO in the PRS has a problem, or could it be something as stupid as a bad cap?  Or do I just not understand the specs and the PRS isn't up to my expectations?  The PRS docs show the unit as 3E-11 at 1s tau, and I'm seeing about 4.25E-11 at 1s, so maybe it's the latter.
> NOTE: The test setup for this is 1PPS from a well-aged GPSDO to the 5370's start input, and 10MHz from either a different GPSDO or the PRS to the stop input.
>  As best as I can tell looking through the holes in the case (still running the tests), the PRS has an MTI 250-0827 5MHz OCXO.  I've got one of the Lucent KS units that I don't care much about, so maybe the MTI 260-0624 5MHz in that will work?  Both are 5MHz, but I don't know whether both are 12V at this point.  And, of course, I don't know whether making the swap to an MTI 260 OXCO will benefit the PRS.
> Any comments or educated speculations are welcomed.
> Bob - AE6RV
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