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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Nov 8 21:51:35 EST 2007


SAIDJACK at aol.com wrote:
> 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
>
>   
For those who were wondering, a suitable circuit schematic is attached.
This isnt the only suitable, or necessarily the best, circuit.
For clarity the opamp supply connections and supply bypassing is omitted.
The 7812 regulator is used to illustrate the principle of regulating the
OCXO supply voltage at its pins.
Other regulators may be more suitable depending on the required noise,
stability, voltage and current requirements.

Bruce
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