[time-nuts] Did a member of time-nuts buy this?-

Bob Camp kb8tq at n1k.org
Wed Dec 10 20:25:35 EST 2014


Hi

> On Dec 10, 2014, at 8:02 PM, Mike Monett <timenuts at binsamp.e4ward.com> wrote:
> 
>>> What does the temperature controller do that degrades the ADEV?
>>> Can anything be done to modify it to improve the performance?
> 
>> On the lightweight Rb's they feed the correction into a DDS. The DDS
>> steps are big enough to show up above the ADEV. You get a "hump" in
>> the ADEV as a result. The solution is to disconnect the temperature
>> sensor that feeds the correction circuit.
> 
> OK, Thanks
> 
>>>> Lots of details to take care of. If you get them all right,
>>>> you'll beat any / all of the older Cs standards.
> 
>>> Is there a list anywhere of all the details that need to be
>>> addressed?
> 
>> Well, there's this list :)
> 
>> The things that need to be done are a "that depends" based on the
>> approach you take. A full listing of all you might do down all of
>> the roads you could take would be a wall full of books.
> 
> Yes, I was afraid of that!

Consider that a team of people likely work for a few years to bring out a fully tested and working design like this when they first start out. That is with a fully equipped lab and full time to focus on it. 

> 
>>> How about words or phrases that can be used to search google and
>>> the archives?
> 
>> GPSDO is a good place to start.
> 
> I searched google with the following:
> 
> https://www.febo.com/pipermail/time-nuts/ gpsdo
> 
> This gave 68 hits. I then used
> 
> site:www.febo.com/pipermail/time-nuts/ gpsdo
> 
> This gave about 2,310 results. Going to page 2 reduced the count to 136.
> 
> I had previously downloaded the archives from 2004 to 2013. A Boyer-Moore
> search gave the following:
> 
>  Files Searched : 63404
>  Matches Found  : 6198
>  Bytes Read     : 329,255,183
>  Elapsed Time   : 31,350.793 ms
> 
> So it seems google is not very useful.
> 
> It would appear the best approach is to simply start reading the posts at
> the beginning and save the interesting ones.

There are lots of interesting threads on Time Nuts ...

> 
>>>> One very cute addition would be to pull down the NIST GPS data
>>>> and use it to correct your system on an hourly / daily basis. If
>>>> you do that with common view satellites, you most certainly will
>>>> beat a surplus grade Cs standard.
> 
>>> How can we do this?
> 
>> First step is to be able to extract timing data from individual
>> sat's. Not all GPS modules do this correctly.
> 
> Can you tell me some of the ones that do?

I have yet to see one for under $2K that does it correctly. I don’t have the cash to buy ones at those sort of prices. Some have reported that the old Motorola UT’s will do it. The samples I have tried have not done very well. I may have not had them running right - who knows….

> 
>>> The NIST archives state
>> 
>>> "The archive is only updated once every 24 hours, so data are not
>>> available for today's date.
>> 
>> Thus the "daily" statement. There has been discussion on the list
>> that interchange with other list members at a more rapid rate might
>> make sense. Given the floor of most setups, daily updates are not a
>> bad thing.
> 
> OK, that makes sense.
> 
>>> Data from the previous day are added to the archive at about 1600
>>> UTC."
> 
>>> http://www.nist.gov/pml/div688/grp40/gpsarchive.cfm
> 
>>> Is there another page that has current data? If so, how do we
>>> incorporate it into the GPSDO?
> 
>> 1) Measure your local time offset by GPS sat number every hour and
>> save the data.
> 
>> 2) When NIST posts there data, pull it down.
> 
>> 3) Compare your data to theirs
> 
>> 4) Do a fit
> 
>> 5) Feed that into your control loop equation.
> 
> There's another term I need to research!

It’s not a simple control process, but it’s not all that terrible either. It just takes a bit of work to optimize. Figure a few months to a few years for the optimization depending on what you have for issues along the way. 

> 
>> The missing element is the per sat data.
> 
>> Bob
> 
> OK, so I figure out how to do this. How do I tell if this is making the
> gpsdo more accurate? In other words, how do I get the ADEV without having
> an H-Maser?

You get a Cs (or other atomic standard) or you compare several different GPSDO’s against each other. The preference would be for groups of three so you can rule out ones that are not doing what they should. 

You also need the measurement gear to resolve frequency down in the 1x10^-13 range. A normal counter will not do that. 

—————————

Backing off a bit - why do this?

You can buy commercial GPSDO’s on eBay.

You can build published GPSDO designs that are known to work. 

What is the objective here?

Bob

> 
> Thanks for your help!
> 
> Mike
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