[time-nuts] Testing the TAC, and a question about ADEV

Fabio Eboli FabioEb at quipo.it
Fri Dec 28 20:39:18 UTC 2012

Little update about the PA6H GPS.
I was having problems with periodic abrutp change in drift.
I didnt like the psu of the Rb, so modifed them and started
logging temperature and voltages.
Now I have some early evidence of the suspect: the GPS receiver
seem upset by rapid changes in temperature.
When I have a temperature variation on the GPS, I see a drift
change in the PPS logged signal (against Rb reference).
It seems that the drift changes by a magnitute proportional
to the rate of change in temperature (dT/dt).

Is this something I should expect from a GPS receiver or is
a "feature" of my model?  What process could generate this problem?
It's like if there is a big delay in the pulse inside the module,
that is temperature dependent and changes with unit temperature,
driving the PPS back and forth several 100's of nS when the temperature

I should "ballast" thermally the GPS, or I will have to thermostabilize 
it :(

Hello Magnus,

> It should be a good way to sweep through the various delays in a
> sequence, which with fairly simple post-processing would reveal both
> the jitter and non-linearity.

I've seen that with the scope I can set the two
Rb to march the edges almost exactly.

> Noise jitter adds like power, so same noise would give you a 1.41
> multiplication up.

So if my units are like the one in that test,
I will have somewhere around 90pS for 95% of the
samples. Not Bad.

> So, you just want to produce a 1 PPS, so a PIC divider such as TADD-2
> would be suitable if you don't have PPSes on your 5680s.

Fortunately my units have the PPS, I'm also
thinking to mod them and remove the LP filter
on 10MHz so I can have the logic level edge
without squaring the signal.

>> soldering iron, solder and scrap electronics :)
> You could do a bit of damage with that :)

No need of these things to to damages, when
I was child some fertilizer and too much curiosity was enough...
I survived a pair of "near miss" events before I changed interests :)

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