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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