A frequency filter for electromagnetic waves traveling in space or in a waveguide (i.e. wires, fiber optic cables, printed circuit boards, etc.) blocks unwanted frequencies.There are four ways to block unwanted frequencies and handpick which frequencies are passed along:
1) low pass filter - All frequencies above some frequency f are blocked; only those less than f are passed.
2 high pass filter - All frequencies above some frequency f are passed; all those below f are blocked.
3 band pass filter - We're interested only in the frequencies between f1 and f2; they are passed. All other frequencies are blocked.
4 band stop filter - We block all frequencies in the band between f1 and f2. All other frequencies are passed.
The reason that low pass filters are not just band pass filters with f1 = 0 (and similarly f2 = infinity for high pass filters) is that low and high pass filters require only a single mathematical pole to create the filter. To arbitrarily select a band at both f1 and f2, at least two poles are necessary (for example, combining a high pass filter with a low pass filter).
The characteristic curves of filters with one pole per corner frequency (plotted on a log-log plot) are flat up to their cutoff points, and then fall off at 3 dB per decade. Each of these filters may be constructed using simple electronic components, like resistors, capacitors, and inductors.