The task of a filter is to allow certain frequencies to pass with low loss and attenuate other frequencies as much as possible. Attenuation varies by design and the filter is either narrowband or wideband. A bandpass filter is a combination of a low pass- and high pass filter that allows frequencies with a defined bandwidth to pass. Sometimes the purpose of a filter is to attenuate a specific frequency. Then a band-stop filter is used to block the specific frequency and to let other frequencies pass. A sharp transition between the passband and the attenuation is often requested and depends mainly on the number of filter elements.
Examples of different types of filters are crystal filters, ceramic filters, cavity filters and MEMS-filters. Crystal filters are used for sharp transition between passband and attenuation. Cavity filters are used for higher power. Cavity filters as use in transmitters and receivers to limit the bandwidth to the allowed frequency band and frequency spectrum. Cavity type diplexers has three ports and are used in transceivers to separate the transmitting frequency band from the receiver frequency band. With a diplexer a single antenna can be used for transmitting and receiving. Specific filter characteristics and frequency response is achieved by using inductors, capacitors, resistors and resonators with different values in different configurations. A certain reactance (imaginary part of the impedance) is achieved by using inductors and capacitors in the filter. Together with the resistance (real part of the impedance) the total impedance is determined as a function of frequency. Different impedance affects different frequency bands, some frequencies pass and other frequencies are attenuated.
Inductors, capacitors and resistors can be designed in different ways. The most simple inductor is a copper coil around a ferrite core, but it can also be integrated into a printed circuit board. Inductive reactance increase with frequency and capacitive reactance decrease with the frequency. Resonators have a mechanical structure with complex response on impedance that varies with differences in frequency.
Filters are often customized as much as possible to certain applications. The choice of filter topology is made depending on power handling capabilities, frequency band, noise level, physical size and cost.
A DC block stops DC voltage and other low frequencies from entering a system, while it let other RF signals through. This way a DC block can be considered as a high pass filter only letting RF frequencies passing through. Most of the time, DC blocks are build up with a capacitor in series with the transmission line. When the capacitor is placed on the inner conductor it is called an Inner DC block. There are also DC blocks with the capacitors on the outer conductor. An inner/outer DC block has capacitors in series on both conductors.