[time-nuts] Fury Interface Board: 5MHz needed?
SAIDJACK at aol.com
SAIDJACK at aol.com
Fri Nov 9 02:26:52 UTC 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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