Dr. Kirby, et al, Remember that a GPS-disciplined Rb has two loops, hence two time constants, to consider:
a) the loop which locks the Rb's internal OCXO to the atomic transition, and b) the GPS disciplining loop. I surmise that the internal loop (a) is fast enough to thoroughly suppress drifts of the unit's internal OCXO in the face of local temperature variations etc. But the other loop (b) can be, and should be, pretty slow (time constant of hours or more), so as to prevent the disciplined source being jerked around by GPS "noise". This is why the Rb can advantageous over the OCXO- the Rb is inherently stable enough to permit using a very slow loop for disciplining by GPS. In my own experience to date, the one GPSDO I have with user-adjustable loop bandwidth suffers greatly from ambient temperature variations when I make the disciplining loop TC large enough to effectively remove GPS noise. I can easily see HVAC cycling, for example, when I use a long time constant. I've played with numbers ranging from 5 sec up to 500 sec, and think the best compromise is around 40 sec. This with an old Trimble T'Bolt which was apparently built around 2004 or thereabouts. My other GPSDO is a CNS Clock II, purchased new about a year ago. It does *not* have provisions for user adjustment of the disciplining loop, which is fixed at some rather short time constant (I'm estimating roughly 5 sec). So its OCXO is kept pretty honest in the face of ambient temperature variations; however GPS noise jerks it around very considerably. in the short term. I'm finding this device's greatest utility in looking at phase drift in my Rb sources over a period of several hours at a time, mainly for purposes of frequency setting. My two Rb's are an old (telecom-modded) PRS-10 and an old L-PRO, neither of which has GPS disciplining capability. I continue to agonize over whether or not I should buy new (standard-featured) PRS-10- I've been trying to make this decision for about 2.5 years so far :-) I do not have access to the revered HP 10811A, so can't speak to its value for my interests. I'd dearly love to borrow one, as it would undoubtedly provide a good learning experience for me. Dana On Tue, Sep 17, 2019 at 3:11 AM Dr. David Kirkby < drkir...@kirkbymicrowave.co.uk> wrote: > On Tue, 17 Sep 2019 at 04:00, Dana Whitlow <k8yumdoo...@gmail.com> wrote: > > > The nice thing about a Rb is that its short term stability (seconds to > > minutes and perhaps > > even longer) is much better than that of a GPS timing receiver. The bad > > news is that Rb > > standards exhibit long term frequency drift in the neighborhood of a few > > parts in 10^11 > > per month. A pretty fair compromise is to use an Rb standard that is > > disciplined by GPS > > PPS pulses with a loop time constant on the order of a day or so. > > > > Dana (K8YUM) > > > Is there any advantage in using a GPS Rb disciplined oscillator vs a GPS > disciplined high quality OCXO like the HP 10811A? I can’t understand why > there should be, as a Rb source would use an OCCO in its output stage > Therefore in each case > > * Short term stability depends upon the quality of the OCXO > * Long term stability depends upon GPS. > > Perhaps there’s is period over which the the overall stability can be > improved by adding a rubidium oscillator. I would be interested to know if > that is the case or not. > > Dave > > > -- > Dr. David Kirkby, > Kirkby Microwave Ltd, > drkir...@kirkbymicrowave.co.uk > https://www.kirkbymicrowave.co.uk/ > Telephone 01621-680100./ +44 1621 680100 > > Registered in England & Wales. > Company number 08914892. > Registered office: > Stokes Hall Lodge, > Burnham Rd, > Althorne, > Chelmsford, > Essex, > CM3 6DT, > United Kingdom > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.