digital ultrasonic demodulator by undersampling

Started by palmering July 24, 2007
Hi!  I want to make an ultrasonic leak detector. Generally, they consist
mainly of an ultrasonic transducer and an analog demodulator with a
carrier of 40Khz which they enable to hear and to measure an audio signal
from 0 to 1.5 or 3Khz.   
How I can make that digitally?  I'm thinking about to make it through
undersampling. That is, to sample at 38.5Khz to obtain an spectra from 0
to 3Khz, from a ultrasonic signal of 38.5 to 41.5Khz.
I suppose that the ADC hold time must be less than 1/2*Fmax=1/80Khz.
Is it right?
Any advise will be greatly appreciated.