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

Abhay Parekh parekh at berkeley.edu
Thu Jun 3 13:09:36 EDT 2010


Awesome. Thanks so much!
=Abhay


On Thu, Jun 3, 2010 at 10:07 AM, Bob Camp <lists at rtty.us> wrote:

> 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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