[time-nuts] Home made GPS disciplined atomic clock

[Bruce Griffiths]

Esa

You can change the Thunderbolt recovery mode from holdover.

One test you can perform that should give an indication of the location
of the Allan intercept is to:

1) connect the receiver to an antenna.

2) let it run for a few days so the Kalman filter learns the drift,
tempco and other parameters.

3) manually disable the disciplining leaving the thunderbolt connected
to the antenna.

4) Log the Thunderbolt PPS offset (plus time stamp) for a day or more.

5) Analyse the resultant data to determine the relative Allan deviation
between the receiver and the 10MHz source.
Ulrich's Plotter is good for this (use the overlapping ADEV algorithm -
although TOTDEV and Theo1 are even better estimators of the Allan deviation)
All going well, you will see a minimum in the Allan deviation versus tau
plot.
In most cases the value of tau at the minimum will be relatively close
to the value of tau at the Allan intercept.

However to do this successfully your antenna will need a good view of
the sky.
For short tau the GPS receiver noise will dominate.
For long tau the 10MHz source noise and drift will dominate.


Bruce

Esa Heikkinen wrote:
>> Right, it all depends on what stability you're after. The OCXO
>> will have much better short-term stability than the LPRO -- the
>> LPRO is close to ten times worse.
>>     
>
> Basicly I'm seeking an accurate frequency standard for RF lab. It should 
> be always as accurate as possible, regardless the state of GPS receiving 
> etc.
>
> Before doing this modification I did some test runs Trimble versus LPRO 
> with phase comparator circuit. I noticed that Trimble is accurate as 
> long as it gets the GPS signal and phase change between LPRO and Trimble 
>   was changing evenly. It is even accurate after the GPS drops (holdover 
> mode) but after the signal comes back the things start to go badly 
> wrong. It starts to roll it's phase / 1 PPS back to alignment woth GPS 
> time and this caused very badly looking phase activity when compared to 
> LPRO.
>
> Another bad issue was that if there's a change in satellite receiving 
> (satellite hopping or some) it causes rapid change the PPS offset and 
> OCXO frequency starts to roll to get the 1 PPS back to alignment. So it 
> seems that Trimble's main principle is 10000000 pulses per PPS, with no 
> exceptions and when the PPS goes off the 10 MHz must also go off to get 
> the 1 PPS back to aligment. So there's no constant 10 MHz frequency 
> either. That's not acceptable because in normal use I should be always 
> aware of GPS receiving states - I'd just like to trust that I'm getting 
> accurate 10 MHz - any time!
>
> So I become to think that may be very slow loop dynamics will solve that 
> problem (if the DAC value isn't changed at every little change at 
> satellite reception). And for that purpose the rubidium sound better 
> than OCXO.
>
> I also got misunderstanding from this:
> http://www.ptsyst.com/GPS10RB-B.pdf
> It claims that rubidiums will have good short therm drift.
>
> My problem here is that there's no way to measure the different setups 
> because my only rb is now part of the experiment. All I can do is the 
> log them and look the change between PPS timing offset readings.
>
> When doing the GPS vs. LPRO phase comparison told before I noticed that 
> the changes of PPS offsets are correlated the phase changes between LPRO 
> and Tbolt output, when observed quite short time. So it seems that the 
> PPS offset is somehow accurate measurement of oscillator stability as well.
>
> I also done some noise measurements with spectrum analyzer between LRPO 
> and Trimble outputs. LPRO had lower noise floor around fundamental.
>
>   
>> See John Miles work to replace the Thunderbolt OCXO:
>> http://www.thegleam.com/ke5fx/tbolt.htm
>>     
>
> Hmm. May be the OCXO on my tbolt is then somehow bad if the LPRO should 
> be even worse? It has Trimble label on and the unit is manufactured on 
> 2005, in China.
>
> Is there any logs available with that better OCXO? It would be nice to 
> see the PPS offsets variance between readings with that oscillator.
>
>   
>>> http://www.amigazone.fi/files/gpsdo/tbolt-lpro-test.log
>>>       
>> I'll have a look at this; but it's not accessible for some reason.
>>     
>
> Oops.. Now you should get it.
>
>