[time-nuts] Question for my new GPSDO

[Tom Van Baak]

Hi Tobias,

Very interesting project you have. Not just that it's a fresh GPSDO, but 
interesting because you seem to be taking so much care in the design. A 
couple of comments:

1) The need for ultra low tempco is more important if you expect your 
GPSDO to be in holdover often; less so if it is permanently locked to GPS.

One approach is to spend the money for low tempco devices and trust 
them; another is to monitor the temperature and apply calibrated 
corrections in s/w. I can't predict which is better, but don't discount 
the latter approach.

Still, either method will have to be verified with actual experiment, 
strip charts, correlation analysis, and ADEV plots. So I hope you have 
the test equipment at hand to measure these subtle effects in a GPSDO.

2) I suspect one can spend a great deal of time picking and testing 
optimal parts in a GPSDO based on tempco specs. But at what point is it 
cheaper to just control the environment of the GPSDO board itself?

You mention opening lab doors and windows and such. Wouldn't it be 
simpler to spent time to design a box that is wind-proof, or 
fan-controlled; maybe even oven controlled? That way you can relax all 
your worries about exotic passive and active components and just build a 
controlled enclosure.

That could not only take care of temperature, but humidity and pressure 
as well if they were found to be sources of instability. One advantage 
of this approach is that the same box design could then be applied to 
any other T&F projects like distribution amps or phase / frequency 
counters that you design in the future.

3) About sensors.

 > many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4) show info 
about the OCXO temperature

The Trimble Thunderbolt (TBolt) uses an onboard DS1620 temperature 
sensor. I don't recall it ever being called "OCXO temperature"; it's 
ambient board temperature. Perhaps because the board is inside a larger 
aluminum enclosure the board temperature is related to OCXO case 
temperature. But as far as I know the reported temperature value from a 
TBolt is definitely not the temperature of the crystal resonator itself.

/tvb