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

ernieperes at aol.com ernieperes at aol.com
Thu Jun 3 16:19:55 UTC 2010

```Hi,

the simple stupid answer is:  24/365

Ernie.

-----Original Message-----
From: Bob Camp <lists at rtty.us>
To: 'Discussion of precise time and frequency measurement' <time-nuts at febo.com>
Sent: Thu, Jun 3, 2010 6:07 pm
Subject: Re: [time-nuts] Training period for a Rb clock using GPS

Hi
If you have an 18 hour time constant you would need a training period of 5
o 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
inute range.
Bob
-----Original Message-----
rom: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
ehalf Of Abhay Parekh
ent: Thursday, June 03, 2010 12:02 PM
o: Discussion of precise time and frequency measurement
ubject: Re: [time-nuts] Training period for a Rb clock using GPS
Hi Hal,
hanks so much for the detailed post. I have a follow up question: What is
he relationship between
he training time and the appropriate value of the time constant (currently
et at 18 hours)? The time constant isn't the size of
moving average window is it?
hanks again for your help. We are a bit clueless here but trying to
earn...
Abhay

n 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
s
> 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
hen
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
f
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
et
12 hours.

--
These are my opinions, not necessarily my employer's.  I hate spam.

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