[time-nuts] EFC tracking

[Steve Rooke]

I would like to track the EFC voltage in hardware using something
cheap and ready to hand. I was thinking of using a sound card as it
has good resolution but it's obviously only AC coupled so it would not
measure the DC of the EFC. I thought about modifying a sound card to
make it DC coupled but most of them seem to reference the 0V point to
some internal reference voltage hence there is a DC shift there. I
next thought about turning the DC into AC by chopping it, IE.
inverting 50% of the voltage via an oscillator. This way I could pass
the square wave directly into an unmodified sound card, take
measurements and then do an RMS calculation on them (really just need
to flip the sign on, say, the negative readings).

I wonder if anyone has done something like this before and could share
their experiences. I've attached a diagram image (hope it is accepted
by the list) which is my first go with Eagle so I'm not exactly very
familiar with it, sorry. The R's and C's in the astable would be set
to a clock frequency that enables this to work without bias given the
sampling frequency. I'm not sure if this clock should be slower than
the sampling frequency or higher, just haven't got my head around that
yet. The R's around the op-amp would need to be set in a ratio that
transforms the EFC voltage into the range that the sound card can
handle (that is yet to be calculated by measuring the limits). If you
have any suggestions or ways of doing this in a better way, I'd be
very grateful for the advice.

Thanks,
Steve
-- 
Steve Rooke - ZL3TUV & G8KVD
The only reason for time is so that everything doesn't happen at once.
- Einstein
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