[time-nuts] Strange GPS behaviour

Fabio Eboli FabioEb at quipo.it
Fri Dec 28 22:27:34 UTC 2012

Like I mentioned in a precedent message
(answering Magnus) I'm seeing some temp
effects on my GPS module, see this message:

In this graph there are the FE5680 voltages
and temperatures, and the temperature sensed
on the PCB near the GPS:

At time 20000s I heated the GPS receiver directly
with hot air gun and the drift started to change
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):
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.


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