thanks daniel. i've actually decided to give the Qprox QT300 chip a go, since it basically does what i'm trying to achieve in my attempted schematic: ie, charging and discharging a cap really quickly, and turning the results into 16-bit data for processing. and if that fails, yep - i'll try the f to v converter. i've been hesitant to try them in the past because i thought they'd be too sluggish in response time, but maybe if i run the oscillator at a relatively high frequency...? anyway, i'll post the results as soon as qt300's arrive and i can get something happening. oh and thanks for the theremin world link - i've been using the site for a while now and think it's a really good resource!
jon.