[time-nuts] Home made GPS disciplined atomic clock

[Esa Heikkinen]

> 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.

-- 
73s!
Esa
OH4KJU