Cutoff frequency

In communication engineering, the cutoff frequency, transition frequency or corner frequency (English: cutoff frequency = "Maximum frequency " ) is the value of the frequency, the signal amplitude (voltage) or the modulation amplitude at the output of a component decreases beyond which below a certain value.

Electrical Engineering


The cutoff frequency of an amplifier is in a usual convention the frequency at which the voltage or current gain is decreased to the - times the value of the maximum gain ( approximately 70.7 %). The to a purely resistive load resistor (consumer ) power output is exactly half the value of the maximum power.

Is expressed in dB voltage gain is in this cut-off frequency at -3 dB (exactly: ) is smaller than the maximum gain. The application range of amplifier circuits is limited by physical effects in the active components and their external circuitry (eg coupling capacitors ) on a certain frequency range, this is called the transmission range. The cut-off frequencies delimit this area.

High - and low-pass 1st order

In simple RC or RL- high - and low-pass filters the voltage sensitivity has its maximum value 1 At the cutoff frequency, the transmitted amplitude decreases to the times the value. At the cut-off frequency occurs between input and output signal to a phase shift of 45 °.

In a low-pass first order, the following relationship to the cutoff frequency for the rise and fall time of:

The relationship to the time constant is:


In physics, the cutoff angular frequency is selected in place of the cut-off frequency much. In some technical applications, such as in the emphasis, it is common to provide the cutoff frequency rather than the time constant. Wherein a band-pass between the upper and lower cut-off frequency as the geometric mean center frequency.

Quantum physics

In quantum physics, the cutoff frequency is related to the photoelectric effect. Light quanta, whose frequency is below this cutoff frequency, no longer have enough energy to remove electrons from the atomic shell. The necessary minimum energy is equal to the work function of the material.

Cut-off frequency in the waveguide

Signals propagate only above a certain frequency in the waveguide from (). It is of the dimensions of the waveguide, especially the longer side, depending ( in a waveguide having a rectangular cross section). The geometric configuration and dimensions of a waveguide are thus standardized, and divided into frequency ranges ( bands). Propagation conditions exist when the wavelength is smaller than the cutoff wavelength so-called. The propagation can be done in different vibration modes.

The cutoff wavelength for the first capable of propagation mode ( fundamental mode ) rectangular waveguide is obtained from the equation:

For the cut-off frequency follows:

For example, rectangular waveguide having the longer side length of the waveguide ( cutoff wavelength ).