Dave Tweed wrote: > Olin Lathrop wrote: > >>A switched capacitor filter just switches in/out elements of cascaded R/C >>lowpass filters. >> > > No, Olin, a switched-capacitor filter replaces each resistor in a filter > network (and not just low-pass configurations) with a capacitor and two > switches. Since the capacitor transfers a fixed amount of charge between > two circuit nodes on each cycle of the clock, it functions as a precise > amount of conductance that's directly proprotional to the clock frequency. > (Or think of it as a resistance that's inversely proportional to the clock > frequency.) Multiple such elements within a network track very closely, > making it possible to build high-order filters that maintain their > performance over a wide range. > < snippity> But if you operate a SCF type filter, you have to know the desired output frequency and adjust your filter accordingly.... Why not do it all backwards: An external sine wave oscillator is locked to the 50% duty cycle PWN (aka reference frequency), and presto! here is your sinewave. The phase detector could be a 4046 or something a bit more sophisticated, and then a 8038 type ic can be used. Use the lock detect pin to mute the audio - and there you are. Most industrial sine wave sources (synthesizers) do it this way.... The only thing you will really have fun with is the loop filter ;-( Greets Jochen Feldhaar DH6FAZ -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.