[time-nuts] GPS: ADEV or MDEV?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Apr 22 02:07:40 EDT 2007


Dr Bruce Griffiths wrote:
> Tom Van Baak wrote:
>   
>> Bruce, I'm getting the idea you don't like _any_ of the
>> hobbyists GPSDO's that have come out in the last ten
>> years...
>>
>> So I'm curious what then would qualify as a well designed
>> GPSDO in your opinion? (and please don't bring up the
>> Quartzlock thing; it's a hundred times more expensive than
>> the GPSDO's we're talking about here).
>>
>> /tvb
>>   
>>     
>
>   
Additional desirable feature.

12) provision for measuring OCXO case temperature and power supply 
current to allow compensation for variable internal wiring voltage drops 
for those OXCOs that do not allow Kelvin sensing of the EFC varactor 
voltage.

Some OCXO's (FTS1200 etc) do not have independent sensing of the EFC 
varactor differential voltage, so the temperature dependent voltage in 
the internal "ground" wiring drop due to the variable oven current will 
modulate the OCXO output frequency when the DAC ouput voltage is fixed.  
If this effect is modelled sufficiently accurately then the DAC output 
can be adjusted to compensate.

For example with 0.1 ohms resistance in the ground wire shared by the 
EFC varactor and the heater a variation of say 1mA in the heater current 
will alter the effective EFC varactor voltage by 100uV, which is far 
from negligible. In any case a heater current stable to 1mA or better is 
somewhat unlikey unless the ambient temperature is also controlled to 
better than 1 degree.

Bruce



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