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

Scott Mace smace at intt.net
Thu Jun 3 17:23:26 UTC 2010 

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<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.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> time-nuts mailing list -- time-nuts at febo.com
>>>>> To unsubscribe, go to
>>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>>> and follow the instructions there.
>>>>>
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at febo.com
>>>> To unsubscribe, go to
>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>> and follow the instructions there.
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at febo.com
>>>> To unsubscribe, go to
>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>> and follow the instructions there.
>>>>
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>>
>>
>>
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>
>

More information about the Time-nuts_lists.febo.com mailing list