[time-nuts] FE5680 GPS Disciplined Controller Update

SAIDJACK at aol.com SAIDJACK at aol.com
Tue Jun 24 17:07:30 EDT 2014

airflow, or changes in airflow are typically much worse for ADEV than  
actual ambient temperature changes in still air.
In still air for example a typical Eurocan DOCXO will have a case temp of  
about 55C to 60C at 25C ambient.

Turn on any kind of significant airflow over that part and the air  
temperature will be nearly the same as before, but the case of the  DOCXO is now 
very quickly cooled to 30C to 40C by the airflow.
This affects mostly TCXO and single-oven units (including Rb's etc), it  
does very little to DOCXO units typically because the outer oven takes the 
brunt  of the temp change and insulates the inner oven. There are other 
components on  the PCB though next to the DOCXO that will be affected such as the 
DAC, DAC  reference, EFC filters, current ground loops due to changing 
heater-current on  the ground pin, etc.
This even affects TCXO's because they are typically heated by themselves  
and other components on the PCB, and any kind of airflow change will cause an 
 instant temp change on the TCXO due to cooling.
So in my opinion a bang-bang controlled fan near any kind of oscillator is  
about the worst thing one can do for short term stability.
In a message dated 6/24/2014 13:35:21 Pacific Daylight Time,  
tvb at LeapSecond.com writes:

>  However it is disappointing that no one has stepped up to tackle the   
> temperature problem. how many have looked at the temperature  attachment 
> clicked on the N5TNL link. Let me make it clear  that yes the GPSDO will 
work but 
> there will be one or two orders of  magnitude degradation without active 
> fan  temperature control  unless the internal temperature compensation is 
>  disabled.

Can you clarify the "two orders of magnitude" claim? That's  hard for me to 
believe, I think, without seeing the ADEV plots or actual lab  report.

I mean, even a cheap XO or TCXO or OCXO can be disciplined  against GPS and 
achieve superb results. Temperature (or rather, temperature  rate of 
change) has little effect short-term. Temperature also has little to  zero effect 
long-term. So it's only in the, what, tau 100 to 1000 or maybe  10000 second 
range that temperature even matters. As long as the LO is locked  to GPS; I 
assume you're not talking about holdover.

Obviously you'd  want a slightly shorter loop time constant for a 
non-temperature-controlled Rb  than a fancy temperature-controlled Rb. But does this 
really make a one or two  *orders of magnitude*  difference?


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