[time-nuts] FE5680 GPS Disciplined Controller Update

Don Latham djl at montana.com
Tue Jun 24 22:12:00 UTC 2014


bang-bang servos do depend on inherent low-pass filtering by the controlled
device.
Don

SAIDJACK at aol.com
> Tom,
>
> 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.
>
> bye,
> Said
>
>
> 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
> and
>> 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?
>
> /tvb
>
>
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-- 
"The power of accurate observation is commonly called cynicism by those who
have not got it."
 -George Bernard Shaw

Dr. Don Latham AJ7LL
Six Mile Systems LLC
17850 Six Mile Road
Huson, MT, 59846
mail:  POBox 404
Frenchtown MT 59834-0404
VOX 406-626-4304
Skype: buffler2
www.lightningforensics.com
www.sixmilesystems.com





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