[time-nuts] Strange GPS behaviour

Azelio Boriani azelio.boriani at screen.it
Fri Dec 28 23:25:38 UTC 2012


Yes, we have yet another proof that the oscillator in (cheap?) GPS
receivers can be a source of troubles... shall we start to design Ovenized
GPS Receivers (OGPR) too? Or ovens for GPS receivers.

On Fri, Dec 28, 2012 at 11:35 PM, Bob Camp <lists at rtty.us> wrote:

> Hi
>
> The GPS does an estimate against the local crystal frequency. It generates
> the PPS off of it's estimate. The less often it updates the estimate the
> more odd things you see as the crystal drifts.
>
> Of course, the crystal can have trouble all it's own. If the crystal has a
> rapid rate of frequency change over a narrow temperature range, the GPS
> simply can't keep up with the crystal.
>
> Bob
>
> On Dec 28, 2012, at 5:27 PM, 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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