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Encyclopedia > Bandpass filter
The frequency axis of this symbolic diagram would be logarithmically scaled.

A band_pass filter is (usually) an electronic circuit that lets through frequencies between two other given frequencies. For example, an ideal bandpass filter would let through all signals above 30 hertz but below 100 Hz. All of the signal outside this range is attenuated or damped. See RLC circuit for basic theory regarding the frequencies passed.

It can be created by a combination of a low-pass filter and a high-pass filter. In practice, no bandpass filters are ideal and do not attenuate frequencies just outside the desired frequency range completely. There is generally a smooth and quick decrease in transmitted frequency outside the band. This is known as the roll-off, and is usually expressed in dB per octave.

In the atmospheric sciences, for example, it is common to band-pass filter the data with a period range of, say 3 to 10 days, so that only cyclones remain as fluctuations in the data fields.

Between the lower cutoff frequency f1 and the upper cutoff frequency f2 of a frequency band there is the center frequency f0.
It is calculated as the geometric mean:

Often a mistake is made in calculating the arithmetic mean as the passed band:

If, for instance, we are looking for the center frequency of the telphone audio band from 300 Hz to 3300 Hz, we get (3300 + 300) / 2 = "1800 Hz" for the short arithmetic mean calculation, but the root of 300 x 3300 = "995" Hz with the correct geometric mean formula.

The bandwidth of the filter is simply the difference between f2 and f1.

• Calculations and comparisons between the geometric mean and the arithmetic mean (http://www.sengpielaudio.com/calculator-geommean.htm)

Results from FactBites:

 bandpass filters at Filtronetics, Inc. (735 words) A bandpass filter transfers energy from the source to its load in a select band of frequencies between the lower cutoff frequency and the upper cutoff frequency. The LC or lumped element bandpass filter is comprised of inductors and capacitors. Narrowband crystal filters with low insertion loss are achievable for bandwidths from 0.005% to 2% of the center frequency.
 Bandpass Filter - Band Reject Filter - Bandpass Filter Design (967 words) The BPF 2/3 is a 3-helical resonator bandpass filter with aperture-coupling between the resonators. The filters are delivered in two basic models: The BPBR-types have the reject range on the high side of the pass frequency and the BRBP-types have the reject range situated on the low side of the pass frequency. The filter has very small physical dimensions owing to the use of high-Q, temperature compensated helical resonators The housing is made of extruded aluminium, the chassis of passivated steel, and teflon insulation has been applied in the rigid coaxial cables and in the connectors.
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