In analog will be quite complicated because you need 3 bandpass filter. The good news is that frequency are far away one from each other so you will not need a very sharpen filter (meaning higest order). One solution might be the variable transconductance amplifier where you can move the central frequency of the filter with a DC signal. What probably is not cristal clear, is that even with FFT you need an antialising filter in front of your A2D, else you'll see a lot of frequencies generated by Nyquist. Vasile On 12/2/09, Bruno L. Albrecht wrote: > Yeah, I thought a lot about doing it all in some microcontroller, with FFTs > and Goertzel et al, but I'm trying to do it all in analog electronics. > > On Wed, Dec 2, 2009 at 6:05 PM, Marcel Birthelmer > wrote: > > > On Wed, Dec 2, 2009 at 9:01 PM, andrew kelley > > wrote: > > > Use a discrete fourier transform. > > > > > > http://www.dspguide.com > > > http://www.dattalo.com/technical/theory/dtmf.html > > > http://en.wikipedia.org/wiki/Discrete_Fourier_transform > > > http://en.wikipedia.org/wiki/Discrete-time_Fourier_transform > > > > > > Andrew > > > > If you're only looking for a single frequency, you don't have to do > > the full spectrum DFT... you can just calculate the frequency > > components in question. > > -- > > http://www.piclist.com PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist