[time-nuts] Fury Interface Board: 5MHz needed?

[SAIDJACK at aol.com]

In a message dated 11/7/2007 13:07:32 Pacific Standard Time,  
tvb at LeapSecond.com writes:

>After that, oven current is relatively constant by  comparison.
>In a GPSDO the EFC is part of a closed loop. So even if  there
>is a small voltage offset in the EFC control or return path due  to
>heater current, it seems to me that it still has no effect on  either
>the accuracy or the stability of the GPSDO. So we ignore  that
>case too.

>Is the worry, then, about the minor ambient  temperature related
>variations in steady-state oven current? Or  something else?



Hi Tom,
 
yes, the ambient temperature variations will cause a significant heater  
current change, especially if they are very fast due to AC kicking-on etc.
 
Current changes for a typical Euro-can size OCXO can vary from 0.09A to  
0.16A due to these thermal changes in an ambient environment (lab bench). If the  
heater is exposed to the outdoors, this can be even worse.
 
Any voltage induced into the EFC ground return due to these current  changes 
can reduce the short term stability of the OCXO and cause phase  drift. It's 
one more error source in the system.
 
We see only typically 5 to 10 microvolt variations in the EFC  voltage due to 
diurnal temperature changes (aging offset removed) on the  best double oven 
OCXO's we have seen mounted onto Fury.
 
Putting the OCXO on a long cable and having the current go from 90mA to  
160mA can produce much more than 10 microvolts in EFC error, so the system's  
accuracy is reduced due to this.
 
bye,
Said



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