microwave filter[′mī·krə‚wāv ‚fil·tər]
A two-port component used to provide frequency selectivity in satellite and mobile communications, radar, electronic warfare, metrology, and remote-sensing systems operating at microwave frequencies (1 GHz and above). Microwave filters perform the same function as electric filters at lower frequencies, but differ in their implementation because circuit dimensions are on the order of the electrical wavelength at microwave frequencies. Thus, in the microwave regime, distributed circuit elements such as transmission lines must be used in place of the lumped-element inductors and capacitors used at lower frequencies. This can make microwave filter design more difficult, but it also introduces a variety of useful coupling and transmission effects that are not possible at lower frequencies.
The majority of modern microwave filters are designed by using the insertion-loss method, whereby the amplitude response of the filter is approximated by using network synthesis techniques that have been extended to accommodate microwave distributed circuit elements. A general four-step procedure is followed: determination of filter specifications, design of a low-pass prototype filter, scaling and transforming the filter, and implementation (conversion of lumped elements to distributed elements).
Microwave filters are implemented in many ways. Waveguide cavity band-pass filters have very low insertion loss, making them preferred for frequency multiplexing in satellite communication systems. Coaxial low-pass filters, made with sections of coaxial line with varying diameters, are compact and inexpensive. Planar filters in microstrip or stripline form (see illus.) are important for integration with hybrid or monolithic microwave integrated circuits. While planar filters are usually more cost effective than waveguide versions, their insertion loss is usually greater. Computer-aided design procedures are used in the synthesis of more sophisticated amplitude and phase responses, and active microwave devices (field-effect transistors) are used to provide filters with gain or tunable response characteristics. See Coaxial cable, Computer-aided design and manufacturing, Microwave solid-state devices