[time-nuts] Beginner's Atomic Clock

[Dana Whitlow]

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 <
drkirkby at kirkbymicrowave.co.uk> wrote:

> On Tue, 17 Sep 2019 at 04:00, Dana Whitlow <k8yumdoober at 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,
> drkirkby at kirkbymicrowave.co.uk
> https://www.kirkbymicrowave.co.uk/
> Telephone 01621-680100./ +44 1621 680100
>
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