On Wed, 16 Sep 2009 22:27:13 -0300 John Coppens wrote: > Normal crystal have way too high Q to make a practical filter at that > frequency (say Q of 40000, would give a filter of only 1.5 Hz!) Again, if I'm not mistaken, Collins Radio was the manufacturer of those filters. http://www.airshowinc.com/about/additionalproducts/collinsfilters/mechanical_filters/index.html http://en.wikipedia.org/wiki/Mechanical_filter http://www.collinsmuseum.com/filters.html (This last one was one of the most popular models) John -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist