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

Thu Jun 3 17:29:27 UTC 2010

I should have said, we give it a 24hr time period to settle regardless 
of the time constant.

	Scott

On 06/03/2010 12:23 PM, Scott Mace wrote:
> 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.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
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