Looking for specifics on designing an AC harmonic filter....thinking this is just a bandpass filter with a highQ ? Want it to be able to pass 45Hz to 90Hz...or even make it somewhat adjustable. The AC is being fed into an optocoupler to measure the frequency of an alternator. --------------------------------- Food fight? Enjoy some healthy debate in the Yahoo! Answers Food & Drink Q&A. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist