Bandpass filter
A band pass filter is a component which is needed to prevent interference of signals and effectively utilize a frequency. In the microwave communication networks, the microwave frequency spectrum has become severely crowded and has been subdivided into a vast number of different frequency bands. There is a need to design microwave filters that have an output signal only at a precise (narrow) frequency band. Also, it is necessary that this filter can be tuned to a precise frequency band with there being a separate filter for each precise frequency band. Generally, a bandpass filter is the parts used at the mobile radio communication base station such as a cellular mobile telephone, a personal communications service (PCS) and a wireless local loop (WLL), and a radio frequency (RF) band. In wireless multiple access communications, a given frequency band must accommodate many wireless users. To accommodate so many users, stringent bandpass filtering requirements are required to minimize interference from communications occurring in neighboring frequency bands or channels. Most bandpass filters include a plurality of reactively coupled resonators which are coupled at a center frequency. Generally, the resonators are coupled to each other by reactive components, such as capacitors, inductors, or a combination thereof. Usually, a band pass filter on an RF stage is constituted by using many resonators. In the band pass filter constituted by many resonators, types of filter characteristics to be realized are determined by a value given to each coupling between the resonators. Bandpass filters have been used in transmission circuits or signal processing circuits in high frequency bands, such as microwave and millimetric wave bands. This type of bandpass filter includes, for example, surface acoustic wave (SAW) filters, dielectric filters, waveguide filters, microstrip line filters, and filters including lumped constant components having reactance. Other that the appliaction in broadcast communications, a band-pass filter has been adopted in a measuring equipment and a signal processing device so as to extract, from a signal from a signal source or a signal input device, only components lying within a specified frequency band. Adoption of such a filter enables production of highly reliable equipments and devices by removing unnecessary frequency component noises.
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