[time-nuts] time-nuts Digest, Vol 121, Issue 65

Bob Camp kb8tq at n1k.org
Sun Aug 24 18:32:52 EDT 2014


Hi

It is not what is done in the Efratom Rb’s. Their pps input is set up to get things on frequency / on time quickly. The assumption is that you plug it into a pps to get it “right” and then take off on your mission. That takes them into the short (for a Rb) time constant region. 

Bob

On Aug 24, 2014, at 5:56 PM, Brooke Clarke <brooke at pacific.net> wrote:

> Hi Bob:
> 
> I think that's what's done in the SRS PRS10
> http://www.prc68.com/I/PRS10.shtml
> 
> Have Fun,
> 
> Brooke Clarke
> http://www.PRC68.com
> http://www.end2partygovernment.com/2012Issues.html
> http://www.prc68.com/I/DietNutrition.html
> 
> Bob Camp wrote:
>> Hi
>> 
>> If you lock an Rb to GPS, you need / want / should do it with a *very* long time constant. Numbers in the one day to several days range are commonly seen. If you lock it up with a tighter (shorter time constant) loop, it will just wander around as it follows the GPS input. That’s what would happen if you hook your Rb to your Trimble and turn on the disciplining on the Rb. It will significantly degrade the stability of the Rb.
>> 
>> If you have a temperature stable environment (or create one) you can get some very good results with an (good) Rb locked to a (good) GPS via a proper long time constant setup. It’s not easy, but it can be done.
>> 
>> Bob
>> 
>> 
>> On Aug 23, 2014, at 6:31 PM, Ed Palmer <ed_palmer at sasktel.net> wrote:
>> 
>>> Hi Dave,
>>> 
>>> On 8/23/2014 3:51 PM, Dave M wrote:
>>>> Thanks for that suggestion, Ed.  After a bit of reading in the X72 Reference Guide, it appears that the X72 does have a 1PPS input.  That would be considerably easier than trying to interface the Rb into the GPSDO.  Still trying to understand what the manual is telling me. Next thing is to determine if my unit has that option enabled (firmware option).  That will be a chore for after the holiday... really busy next week.
>>>> 
>>>> What would that (1PPS disciplining) do for me... in terms of maintaining the Rb frequency accurately set?  Would it be as accurate as having the Rb disciplined via the EFC input?
>>> It's kind of overkill, but by connecting the 1 PPS from the NTBW50AA to the X72, the X72 will be disciplined to the 1 PPS so the frequency will be accurate.  The question is how well will it be disciplined, i.e. what will the Allen Deviation graph look like.  I have a few X72 and SA-22c (X72's cousin), but none of them have that option.  I don't know of any published data on it.  Maybe you can tell us how well it performs.
>>> 
>>> In general, I just don't see the point of disciplining a Rb standard to GPS.  I don't understand what will be gained by doing it.  I have a Z3801A and a Tbolt plus a free-running FRK as a house standard.  I occasionally compare the FRK to the Z3801A but the drift is so low (~1e-12 per month over 9 months) that I see no reason to link them.
>>> 
>>> One exception that I recently discussed on another forum was a guy who lives in a ground floor, north-facing condo.  He might need to have a disciplined Rb standard due to poor GPS visibility.
>>> 
>>> Ed
>>> 
>>>> Thanks,
>>>> Dave M
>>>> 
>>>> 
>>>>> Message: 5
>>>>> Date: Fri, 22 Aug 2014 16:19:45 -0600
>>>>> From: Ed Palmer <ed_palmer at sasktel.net>
>>>>> To: Discussion of precise time and frequency measurement
>>>>> <time-nuts at febo.com>
>>>>> Subject: Re: [time-nuts] EFC info on Trimble 34310-T OXCO
>>>>> Message-ID: <53F7C201.5070206 at sasktel.net>
>>>>> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>>>>> 
>>>>> Have you checked your X72 to see if it has the 1 PPS discipline
>>>>> option?
>>>>> That would be a lot easier (and probably better) than your proposed
>>>>> transplant.
>>>>> 
>>>>> Ed
>>>>> 
>>>>> On 8/22/2014 12:39 PM, Dave M wrote:
>>>>>> Does anyone have any info on the OXCO in the Nortel/Trimble
>>>>>> NTBW50AA-17 GPSTM receiver?  The OXCO is labeled as Trimble 34310-T.
>>>>>> I see some Trimble 34310-T oscillators on Ebay with pinouts labeled,
>>>>>> but no other info.
>>>>>> 
>>>>>> Specifically, I'd like to know the EFC characteristics for it.  I'm
>>>>>> thinking of the possibility of pulling the OXCO out of the GPSTM and
>>>>>> subbing in a 10 MHz Rubidium, and using the GPSTM to discipline the
>>>>>> Rubidium.  My Rubidium is a Symmetricom X72, recently purchased.  It
>>>>>> seems to be working well.
>>>>>> Does anyone know the differences between the three OXCOs used in the
>>>>>> GPSTM receivers ("T", "T2" and "Oak")?
>>>>>> 
>>>>>> Thanks for some insight,
>>>>>> Dave M
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> 
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