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

Thu Jun 3 17:07:29 UTC 2010

Hi

Your loop for 10 hours would be around 1 or 2 hours. That's 60 X 60 X (1 or
2) seconds = 3,600 to 7,200 seconds. If GPS is "good" to +/- 20 ns out of
your receiver in your location then you would get 20 x 10^-9 / (3600 or
7200) = 2.7 to 5.5 X 10^-12 inside the loop. The Rb should be below that
level over the same time period. 

Simple answer - yes it should be good enough.

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 12:46 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Training period for a Rb clock using GPS

Yes, that makes sense.
I think that we can arrange things so that we train for 10-12 hours.
Do you not think that that is a long enough time for
a single loop to be effective?
Thanks again!
=Abhay

On Thu, Jun 3, 2010 at 9:37 AM, Bob Camp <lists at rtty.us> wrote:

> 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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