# [time-nuts] Training period for a Rb clock using GPS

Bob Camp lists at rtty.us
Thu Jun 3 16:37:34 UTC 2010

```Hi

That will give you the "best" answer with a simple loop. The problem is that
"best" may not be good enough to actually get your Rb on time / on
frequency. Something more sophisticated than a simple loop may be needed.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Abhay Parekh
Sent: Thursday, June 03, 2010 12:28 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Training period for a Rb clock using GPS

Ok, great. So if we can train for h hours we should set the time constant
somewhere between
h/10 and h/5. It would be safer to pick something closer to h/10 since when
the clock powers up
it might "start" in the wrong place so a smaller value helps the clock move
quickly into
the right area, but h/5 will act as a better buffer against hanging
bridges.
Is my reasoning correct?
Thanks
=Abhay

On Thu, Jun 3, 2010 at 9:07 AM, Bob Camp <lists at rtty.us> wrote:

> Hi
>
> If you have an 18 hour time constant you would need a training period of 5
> to 10 X 18 hours to get the system to settle.
>
> For a one hour training period the time constant should be in the 5 to 10
> minute range.
>
> Bob
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Abhay Parekh
> Sent: Thursday, June 03, 2010 12:02 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Training period for a Rb clock using GPS
>
> Hi Hal,
> Thanks so much for the detailed post. I have a follow up question: What is
> the relationship between
> the training time and the appropriate value of the time constant
(currently
> set at 18 hours)? The time constant isn't the size of
> a moving average window is it?
> Thanks again for your help. We are a bit clueless here but trying to
> learn...
> =Abhay
>
>
> On Wed, Jun 2, 2010 at 2:02 AM, Hal Murray <hmurray at megapathdsl.net>
> wrote:
>
> >
> > parekh at berkeley.edu said:
> > > I am a newbie at this, but have been playing around with 2 prs10s. For
> > our
> > > application we need to run the clocks without gps, but we do get to
> sync
> > it
> > > to gps *initially* for as long as we want. However, what we've noticed
> is
> > > that when we train it for short periods of time (< 1 hour a day) the
> > clock
> > > drifts for a few microseconds a day once we've disconnected gps, but
> when
> > we
> > > train it for say 12 hours, its drift seems to be much less (sub sub
> > > microsecond/day). We were wondering why this should be so!
> >
> > Look at it the other way.  How long should it take to train it?
> >
> > Let's use rough numbers.
> >  There are 1E5 seconds per day.
> >  Your "few" microseconds is 1E-6 seconds.
> >    That's an accuracy of 1 part in 1E11.
> >  Your "sub-sub" is 1/10 microsecond or 1E-7 seconds.
> >    So that's an accuracy of 1 part in 1E12.
> >
> > The data sheet says:
> >  Aging (after 30 days)  <5E-11 (monthly)
> > 5E-11 is 50E-12, so that's 2E-12 per day which is what you saw.
> >
> > The data sheet also says:
> >  The PRS10 can time-tag an external 1 pps input
> >  with 1 ns resolution. These values may be reported
> >  back via RS-232, or used to phase-lock the unit to an
> >  external reference (such as GPS) with time constants
> >  of several hours.
> >
> > There are 4E3 seconds in an hour and 1E9 nanoseconds per second.  So in
> an
> > hour, you can get close to 1 part in 1E12.  But that's assuming that the
> > input PPS signal is right-on.
> >
> > There are two types of GPS receivers.  Most use a free running clock and
> > generate the PPS pulse with the closest clock edge.  They typically have
> > noise on the order of 15-50 ns.  Fancy ones will tell you how far off
> they
> > think it is.  The really fancy ones will have a VCXO so they can slew
the
> > clock to the right offset.
> >
> > One magic word is "hanging bridges".  It comes up in discussions
> > occasionally.
> >
> > For lots of info on that area:
> >  http://www.gpstime.com/files/PTTI/PTTI_2006.pdf
> > 31 pages, lots of good stuff, aka time sink.
> >
> > More here:
> >  http://www.leapsecond.com/pages/vp/heater.htm
> > 2 or 3 screens, good stuff, a quick read.
> >
> > So with only an hour, it's not unreasonable that you are off by a factor
> of
> > 10, but you might have to get unlucky for a hanging bridge to get you.
> >
> > But there is another factor to consider.  What sort of filter is the
> > software
> > using between the PPS input and the knob that adjusts the frequency?
> >
> > More from the data sheet:
> >  When tracking an external input, the time constant can
> >  be set from 5 minutes to 18 hours.
> >
> > I think the manual says the default is 65K seconds.  That's 18 hours.
> >  Unless
> > you changed it, that explains why 1 hour wasn't enough.  It might get
> > better
> > if you give it more time and/or tweak the time constant if you can only
> get
> > 12 hours.
> >
> >
> > --
> > These are my opinions, not necessarily my employer's.  I hate spam.
> >
> >
> >
> >
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