N6QW -- Left Coast Radio

Passing only a narrow band of frequencies  -- sounds easy enough. Well Not! I we fail to design the Band Pass Filter (BPF) properly we risk having out of band signals, spurs and maybe a problem with the FCC. This is where a simulation tool like LT Spice is worth its weight in gold.   The Band Pass Filter (BPF) is such a critical circuit element yet often is done in a haphazard manner. At a bare minimum you only need 7 components consisting of five capacitors and two inductors. Now that sounds pretty easy, but the problem begins because often the values may not be integers or standard parts. For instance you may have a section coupling capacitor of 1.5pF or an inductor like 2.304uH.   A 20M Band Pass Filter (PSSST)     So what happens -- not having an extensive junk box, you slap a 5pF cap in there and what you actually wound on the toroid is  3uH. Sure it will limit the signals outside the band pass but the range may be way off, and the response curve resembles a camels back!   I often