Cutoff frequency

A Bode diagram of the frequency response of the Butterworth filter, with the marked cutting frequency.(The slope is −20 dB per decade that is equal to −6 dB per eighth).

In physics and electrical engineering, the cutoff frequency is a limit on the frequency response of a system at which the energy flowing through it begins to reduce (attenuate or reflect) instead of going through it.

Electronics

The frequency, either above or below which the output level of a circuit, such as a line, amplifier, or filter, is reduced to the value of -3 dB = 50% power relative to the input level. 0 dB reference = 100%. This is equivalent to saying that the output signal is reduced to 70.7% of the input signal, or when half the power is dissipated, or when there is a 45° phase shift from the input voltage.

A bandpass filter has two cutoff frequencies and one center frequency, while highpass and lowpass filters have only one cutoff frequency. The center frequency of a bandpass filter is the geometric mean of the upper and lower cutoff frequencies.

Radio Communications

The frequency below which a radio wave fails to penetrate a layer of the ionosphere with the angle of incidence required for radio transmission between two points by reflection from the layer.

Waveguide

The frequency below which a certain electromagnetic mode cannot be transmitted in a guided medium. The cut-off frequency of the TEM (transverse electromagnetic) modes is zero. The TE and TM modes that appear in waveguides (formed by linear and isotropic homogeneous dielectrics and perfect conductors) have non-zero cutoff frequencies that depend not only on the characteristics of the dielectric through which the energy propagates, but also on the geometry of the waveguide that supports the mode.