[time-nuts] Strange GPS behaviour

Said Jackson saidjack at aol.com
Sat Dec 29 20:18:56 UTC 2012


Fabio,

Happens in all the GPS receivers we have tested here. The difference between receivers is how fast they can recognize this error and how fast they can re-aquire once they shut off the 1PPS output due to tcxo instability.

There was a recent thread here about effects of adding a fan to a Z380x and the behavior you have seen is one of the reasons why that is a bad idea.

Some receivers like Rockwell, Trimble etc allow for external 10MHz input from an ocxo or atomic clock, and some GPSDOs make use of that feature. Having a very stable and accurate 10MHz reference for the GPS is also supposed to reduce time to second fix.

Bye,
Said

Sent From iPhone

On Dec 28, 2012, at 14:27, Fabio Eboli <FabioEb at quipo.it> wrote:

> Like I mentioned in a precedent message
> (answering Magnus) I'm seeing some temp
> effects on my GPS module, see this message:
> http://www.febo.com/pipermail/time-nuts/2012-December/073310.html
> 
> In this graph there are the FE5680 voltages
> and temperatures, and the temperature sensed
> on the PCB near the GPS:
> http://www.flickr.com/photos/14336723@N08/8318815981/
> 
> At time 20000s I heated the GPS receiver directly
> with hot air gun and the drift started to change
> rapidly.
> At 25000 I heated the FE5680#2 I was using
> as reference, but no visible effects, (apart
> the slight variation in it's voltage :)
> At 30000 33000 35000 seconds I heated the
> GPS with a resistor placed near the PCB,
> this generated more gradual temperature
> variation on the GPS.
> 
> Here can be seen the results of the heating
> on the drift, (logging GPS PPS against Rb):
> http://www.flickr.com/photos/14336723@N08/8318816213/
> the hotair generated so much variation, that
> the script was unable to unscrable the data.
> The resistor heater generated slower temperature
> variation on the GPS, it's visible a glitch
> everytime there was a temperature variation,
> and the drift magnitude seem to follow the
> variation of the temperature in time (dT/dt).
> 
> I will try to reduce temperature sensivity
> incrementing the thermal capacitance and
> isolating the GPS from the ambient.
> 
> Is this normal or it's a defect ("feature") of
> my unit? I'm also curious about what internal
> structure can generate this wander in PPS.
> Like I said before it's like if the PPS pulse
> (for intervals of few 100's of nS) depends on
> something that is very temperature dependent.
> 
> Thanks,
> Fabio.
> 
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