Bandpass Filters

Band-pass filters only transmit a certain wavelength band and block others. The width of such a filter is expressed in the wavelength range it lets through and can be anything from much less than an Ångström to a few hundred nanometers. Such a filter can be made by combining an LP- and an SP filter.

Band-pass filters are often used in astronomy when one wants to observe a certain process with specific associated spectral lines.

Bandpass filters transmit in a band of wavelength while rejecting or attenuating spectral energy on both the long and short wavelength sides of the transmission band. These optical filters are defined by cut-on and cut-off slopes establishing the transition between high rejection and high transmission.

See the following curve for bandpass filters.

Typical Applications

•    Paint-color formulation
•    Blood chemistry analysis
•    Traffic control

COE Optics Bandpass filters are widely used in wireless transmitters and receivers. The main function of such a filter in a transmitter is to limit the bandwidth of the output signal to the band allocated for the transmission. This prevents the transmitter from interfering with other stations. In a receiver, a bandpass filter allows signals within a selected range of frequencies to be heard or decoded, while preventing signals at unwanted frequencies from getting through. Signals at frequencies outside the band which the receiver is tuned at, can either saturate or damage the receiver. Additionally, they can create unwanted mixing products that fall in-band and interfere with the signal of interest. Wideband receivers are particularly susceptible to such interference[3]. A bandpass filter also optimizes the signal-to-noise ratio and sensitivity of a receiver.

In both transmitting and receiving applications, well-designed bandpass filters, having the optimum bandwidth for the mode and speed of communication being used, maximize the number of signal transmitters that can exist in a system, while minimizing the interference or competition among signals.

Outside of electronics and signal processing, one example of the use of band-pass filters is in the atmospheric sciences. It is common to band-pass filter recent meteorological data with a period range of, for example, 3 to 10 days so that only cyclones remain as fluctuations in the data fields.