[time-nuts] Strange GPS behaviour

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


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.


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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