RF and microwave filter

Radio frequency (RF) and microwave filters represent a class of electronic filter, designed to operate on signals in the megahertz to gigahertz frequency ranges (medium frequency to extremely high frequency). This frequency range is the range used by most broadcast radio, television, wireless communication (cellphones, Wi-Fi, etc.), and thus most RF and microwave devices will include some kind of filtering on the signals transmitted or received. Such filters are commonly used as building blocks for duplexers and diplexers to combine or separate multiple frequency bands.

Four general filter functions are desirable:

In general, most RF and microwave filters are most often made up of one or more coupled resonators, and thus any technology that can be used to make resonators can also be used to make filters. The unloaded quality factor of the resonators being used will generally set the selectivity the filter can achieve. The book by Matthaei, Young and Jones provides a good reference to the design and realization of RF and microwave filters. Generalized filter theory operates with resonant frequencies and coupling coefficients of coupled resonators in a microwave filter.

The simplest resonator structure that can be used in rf and microwave filters is an LC tank circuit consisting of parallel or series inductors and capacitors. These have the advantage of being very compact, but the low quality factor of the resonators leads to relatively poor performance.

Lumped-Element LC filters have both an upper and lower frequency range. As the frequency gets very low, into the low kHz to Hz range the size of the inductors used in the tank circuit becomes prohibitively large. Very low frequency filters are often designed with crystals to overcome this problem. As the frequency gets higher, into the 600 MHz and higher range, the inductors in the tank circuit become too small to be practical. An inductor of 1 nanohenry (nH) at 600 MHz isn't even one full turn of wire.

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