We have noticed two things with the PRS10s and other Rbs. They need
about 24hrs to settle before they really start to perform well and if
there are any significant temperature swings, expect them to react to
it. We have a PRS10 in a Meinberg M900 that takes about 1 day to
recover from 2-3C temp swing in a datacenter. It will be off (as
compared to the other 1-PPS that are not in that room) by about
50-100ns. Normally it's within +-10ns when compared to the other
GPSDOs. We noticed this happened when the air-handlers would alternate
on about a 2-week cycle.
Scott
On 06/03/2010 12:09 PM, Abhay Parekh wrote:
Awesome. Thanks so much!
=Abhay
On Thu, Jun 3, 2010 at 10:07 AM, Bob Camp<li...@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-boun...@febo.com [mailto:time-nuts-boun...@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<li...@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-boun...@febo.com [mailto:time-nuts-boun...@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<li...@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-boun...@febo.com [mailto:time-nuts-boun...@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<hmur...@megapathdsl.net>
wrote:
par...@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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