Hi,
> What have you observed, out of the GPS, with the temperature variations?
The test was simple. A couple of seconds of warm air from a hair dryer
on low setting (the air coming out is 15C above ambient, so not very
much heat at all) through a paper tube blowing on the GPS. That little
change causes the phase to shift about 200nS in just a few seconds.
That is, the GPS PPS phase compared to the OCXO phase shifts 200nS, and
it happens over only a few seconds, literally almost instantly. It's an
enormous change!
I have checked that it's not electrical noise from the hair dryer, and
I have repeated the test multiple times.
> Also, when you say "the GPS temperature," what, exactly, are you
> measuring/reporting? Is the GPS's own time base varying with
> temperature?** And how much thermal isolation is there between the
> various subsystems (GPS, PLL circuitry, OCXO, etc.) in your test setup?
>
> ** I'm assuming that the GPS does not use the OCXO output for its
> own timing. That is such a good idea, I can't understand why more
> designers don't do it.
Currently to measure the GPS temperature I have a K type TC taped to
the top of the GPS RF shield. Since the temperature sensitivity was
noticed, the GPS has been sitting under a wool sock with a 1Lb roll of
solder sitting on that (That does help a bunch). Since the TC and GPS
are under the wool sock the TC gets pretty good coupling to the GPS
module itself. The GPS is mounted to a pine board right now, so has
insulation underneath. The rest of the stuff (OXCO, GPSDO board, linear
regulators/heatsink etc.) is sitting out in the open on the bench.
Exposed to every possible air current, and even the front door being
left open by my kids as they run in and out.
The small temperature cycles on the GPS are about 5 to 7 minutes long.
The HVAC cycles are about 45 minutes apart. So I believe these two are
not related. I can clearly see the HVAC cycles and the 'ramp'
waveform. As for what's causing the ramp in temperature on the GPS
module, I have no idea. Any guess as to what is going on would be just
that. It was just an interesting thing to note.
The setup is a prototype and pretty ugly, but things are spaced out
enough to be able to blow hot air on them with out hitting the other
components. I can also cover and uncover individual components with a
wool sock if need be. I've tried blowing the hot air on every other
part of the system, and the GPS is the one that responds now.
Previously the GPSDO DAC itself caused a similar response, but that has
since been resolved.
I'm well aware of the difference between cause/effect and
correlation. (Everyone who ever eats Broccoli will die, you know. ;) )
It's the reason I've been been blowing hot air on stuff, to be sure
there is a cause and effect relationship...
> It adds to the cost. If the end-user only needs XO or TCXO quality
timing, there's no incentive to increase the size and cost and power of
the GPSDO product with a OCXO.
>
> But, you're right, it *is* a really good idea, and of course we all
know the Trimble Thunderbolt does it this way. One reason why it's
always the #1 favorite GPSDO among time nuts.
>
> Note that most high-end GNSS timing receivers go one better and
simply have an external input for the clock. That way you feed your own
lab clock into the receiver. If you have Rb/Cs/maser you would use that
as the reference. It's what the national timing labs do, along with
dual-frequency and post-processing and all the other tricks of the
trade.
I think it would be agreat idea also. It's a wonder that more of the
'timing' receivers don't have that external clock option! I wonder what
these Ublox parts use for a clock? Is it something frequency
compatible with a 10Mhz source??? (Hmm, can we pry one apart to figure
it out! ;) )
As for the GNSS units, are these receivers something that an average
person can afford to get their hands on, or do you have to sell your
house to buy one? :)
Thanks,
Dan
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