Bandpass filters are two-port devices that provide transmission at frequencies within the passband of the filter and attenuation of other frequencies outside of the band. A band pass filter is used to prevent interference of signals and effectively utilize a frequency. 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. The role which a bandpass filter is to fulfill is transmitting to a few loss signals which lie in a desired frequency band while intercepting all the frequencies outside the desired band. A bandpass filter freely passes frequencies within specified range, while rejecting frequencies outside the specified limits, and can be designed to provide symmetric or asymmetric characteristics. In microwave communications, the microwave frequency spectrum has become severely crowded and has been subdivided into a vast number of different frequency bands. Microwave bandpass filters provide an output signal only at a precise (narrow) frequency band. Also, the filter can be tuned to a precise frequency band with there being a separate filter for each precise frequency band. In the field of communications, performance of a filter determines an effective use of a frequency which is an important resource. A bandpass filter allows a narrow range of frequencies to pass through it unattenuated, and blocks out all other frequencies. This narrow range of frequencies is the filter's passband. In the satellite communications technology, band pass filters in the microwave range play an important role in the preselection of individual communication channels. In optical communications, a number of different light signals of different wavelengths may be mixed together (multiplexed) into a single light beam in a technique known as wavelength division multiplexing (WDM). A single multiplexed light beam may carry many times the information that may be transmitted by a non-multiplexed light beam. The multiplexing and/or demultiplexing may be accomplished using a series of light bandpass filters. Each filter is formed as a substrate and a multilayer dielectric light-transmissive optical stack deposited upon the substrate. The bandpass filter transmits only light of a specific wavelength. Optical bandpass filers transmit light over a predetermined band of wavelengths while rejecting, by absorption, radiation or scattering, all other wavelengths. In optical communications systems, they can be used at the input of an optical receiver to separate unwanted light such as spontaneous emission noise outside the wavelength band of the signal.

A band pass filter can be implemented in any one of several technologies. Passive analog filters utilize resistors, capacitors, and inductors to achieve the desired frequency response. Active filters add one or more operational amplifiers to prevent a signal from becoming too attenuated by the passive components and to exaggerate or to minimize a particular response by controlled feedback. A bandpass filter has symmetric characteristics when transmission zeros are symmetrically disposed about a center frequency of a filter's usable bandwidth. In contrast, a bandpass filter has asymmetric characteristics when transmission zeros are placed asymmetrically about the filter's passband. The later is useful for satisfying desired out-of-band amplitude and/or in-band group delay asymmetric specifications. Bandpass filters are often second-order or biquadratic. The bandwidth of such filters is directly the coefficient of the first-order term in the denominator polynomial of the second-order filter transfer function. Bandpass filters operating at microwave frequencies generally use coupled resonant cavities, made of waveguide sections provided with appropriate coupling irises. The interior volume of the cavities depends on the operating wavelength and it increases as the desired resonance frequency decreases. 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. The bandwidth of a filter can be described by its quality factor (Q) and its center frequency .omega..sub.0. Q is a measure of the sharpness of the peak of the bandwidth response function. Another measure of the filter performance is the noise figure, which is the signal to noise ratio of the applied input to the signal to noise ratio at the output. It is desirable that the noise figure be low to preserve dynamic range. Changing the characteristics of a bandpass filter involves altering the electrical and/or magnetic characteristics thereof. Both bandpass Q and insertion loss depend to some extent on the electrical length L of a resonant cavity, and bandpass Q depends on the characteristics of dielectric substances present in the cavity. In microwave bandpass filters, the frequency band of the signal of the filter is a function of the resonant frequency of resonators that are incorporated within the filter and respective coupling coefficients between each of these resonators. In order to achieve a specific precise bandwidth, the resonators are longitudinally spaced in a sequential manner.

Bandpass filters come in a variety of structural and topological arrangements. 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. Bandpass filters used in transmission circuits or signal processing circuits in high frequency bands, such as microwave and millimetric wave bands, may include, for example, surface acoustic wave (SAW) filters, dielectric filters, waveguide filters, microstrip line filters, and filters including lumped constant components having reactance (capacitors, chip inductors, air-core coils). The bandwidth is a function of the coupling between the resonators and the frequency of the resonance of the resonators. Varying of the spacing between the resonators results in variations in the bandwidth. The design of bandpass filters for Accordingly, overall filter dimensions, such as the filter length, typically must be varied in order to tune a filter to a precise bandwidth. Dual-mode filters allow for a reduction in filter size and mass, and have realized more complex filter functions by their ability to easily couple non-adjacent resonators. Radio frequency equipment uses a variety of approaches and structures for receiving and transmitting radio waves in selected frequency bands. Filtering structures are used to maintain proper communication in frequency bands assigned to a particular band. The type of filtering structure used often depends upon the intended use and the specifications for the radio equipment. RF receivers/transceivers require numerous tradeoffs with respect to circuit topology, bandwidth, clement realization, insertion loss, rejection level, etc. RF transceivers require bandpass filters that provide minimum insertion loss over the desired frequency band. For example, dielectric and coaxial cavity resonator filters are often used for filtering electromagnetic energy in certain frequency bands, such as those used for cellular communications.

Many different types of band pass filters are known for the processing of electrical signals. Multilayer band pass filters may be in the form of multiple cavity or multiple half-wave band pass filters, which include a combination of alternating high and low refractive index dielectric layers. Multilayer band pass filters may also be in the form of combination of long and short wavelength pass filters, often termed edge filters. Front-end filters may have fixed bandwidths or they may have tunable bandwidths. Filters having fixed bandwidths are typically used in narrow band receivers where the operational frequency is limited to a narrow range. Dielectric filters can be broadly classified as devices using coaxial resonators and devices using multilayer resonators. Hgh dielectric constant ceramic filters, such as coaxial or mono-block types, have become very popular due to their high performance and small size. Microstrip filters are filters constructed with coupled microstrip resonators. Microstrip coupled line bandpass filters comprise a cascade of parallel half-wavelength-long printed resonators open-circuited at both ends. The resonators are positioned parallel to each other, in such a way that adjacent resonators are coupled along the length equal to the guided quarter-wavelength of the center frequency of the filter. In a microstrip filter, microstrip resonators are arranged on the surface of a dielectric substrate, the substrate having a conductive ground plane beneath it. Optical bandpass filters generally include an optical resonant cavity comprising an optical fiber having a specific length and mirrors formed at each end of the optical fiber. Apertures in each mirror allow an RF modulated light signal to be injected into and resonate at the RF modulation frequency within the fiber. An interdigital bandpass filter is structured such that a plurality of resonator elements are arranged at selected intervals. Each of the resonator elements comprises a quarter wavelength line which has one end open-circuited and the other end short-circuited. Finite impulse response (FIR) filters are completely digital, using a shift register with a plurality of taps. An FIR filter generally has a linear phase versus frequency response and a constant group delay.

Bandpass filters are used in many different types of consumer and industrial electronic product to select or reject electrical signals in a range of frequencies. Bandpass filters are used extensively in radio communication equipment for many purposes such as the rejection of spurious response frequencies in superhet receivers, suppression of unwanted radiation from oscillators, impedance transformation, etc. Bandpass filters are used in signal generating systems to remove spurious signals outside a desired frequency band and in signal detecting systems to prevent overloading by signals outside the desired band and to remove other undesired signals. Tunable bandpass filters are used in RF (radio frequency) and IF (intermediate frequency) stages of radio equipment such as transmitters and receivers. Ceramic bandpass filters are used in electrical or electronic devices designed to process signals in the microwave frequency band. Applications for such bandpass filters include global positioning system (GPS) receivers, cellular phones, wireless modems and other remote communication/signal receivers. Resonant cavity bandpass filters are used in satellite communication systems operating at microwave frequencies. FIR filters find widespread use in digital communication systems, speech processing, image processing, spectral analysis, and other areas where non-linear phase response is unacceptable.