[time-nuts] Effects of filter delay

SAIDJACK at aol.com SAIDJACK at aol.com
Sun Dec 24 15:10:03 EST 2006


Hi Bruce,
 
yes the heater circuit is problematic, and will affect stability a  lot.
 
For example, some OCXO's don't have separate (Kelvin sensed) ground pins  for 
the heater, and EFC voltage.
 
The result is that any changes in the heater current will cause a voltage  
change in the ground pin of the OCXO from the OCXO body to the PCB ground plane  
(due to the finite resistance of the ground pin).
 
This I2R loss is very small, but will superimpose onto the EFC voltage, so  
it's as if this voltage is placed onto the DAC output.

We have measured this change of voltage to be in the order of 10's of  
microvolts for some OCXO's, and that translated into 10's of EFC DAC steps  worst 
case (during power-on for example when the heaters are running at maximum  
current).
 
Again the problem is not the steady-state heater current, but changes in  
heater current due to thermal tracking, and loop stability of the heaters, this  
in turn causing changes in EFC voltage. We are talking about 1E-012 or  better 
being desirable stability, so these marginal effects almost sudden become  a 
real issue.
 
The best approach is to separate the ground return current of the heater  
circuitry completely from the DAC ground circuitry. But that's not possible on  
many OCXO's...
 
Bye,
Said 


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