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

SAIDJACK at aol.com SAIDJACK at aol.com
Thu Nov 8 21:26:52 EST 2007


 
In a message dated 11/8/2007 17:57:09 Pacific Standard Time,  
bruce.griffiths at xtra.co.nz writes:

>With  some ingenuity and an opamp or 2, either sign can easily be  be
>accommodated.
>Using a length of PCB track for the current  sense resistor may provide a
>degree of temperature compensation for the  tempco of the internal ground
>wiring resistance, provided that the  internal ground wiring resistance 
>tempco tracks that of copper.  However the finite thermal gain of a
>single oven OCXO will in most  cases dominate the OCXO thermal drift so
>an adjustable tempco sensing  resistor may be more useful.
>  


Hi Bruce,
 
yes an opamp or two and a microprocessor or two can do the tempco  
compensation of course.
 
The point of my post was that if carefully matched, this change of EFC  
voltage versus heater-current/temperature can actually be designed to cancel out  
OCXO tempco.
 
 
You are right of course, higher heater current will result in higher IR  
losses, resulting in lower EFC voltage (not higher EFC voltage).
 

This simple compensation can be done without the traditional way of  current 
sensing, computing, and adding error offset.
 
It could be done simply by choosing the right cable lengths, and generating  
deliberate ground current loops.
 
Of course the OCXO would have to have the correct Tempco sign for this to  
work.
 
bye,
Said



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