Cutoff frequency

In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced (attenuated or reflected) rather than passing through.

Typically in electronic systems such as filters and communication channels, cutoff frequency applies to an edge in a lowpass, highpass, bandpass, or band-stop characteristic – a frequency characterizing a boundary between a passband and a stopband. It is sometimes taken to be the point in the filter response where a transition band and passband meet, for example, as defined by a half-power bandwidth (or half-power point), a frequency for which the output of the circuit is approximately −3.01 dB of the nominal passband value. Alternatively, a stopband corner frequency may be specified as a point where a transition band and a stopband meet: a frequency for which the attenuation is larger than the required stopband attenuation, which for example may be 30 dB or 100 dB.

In the case of a waveguide or an antenna, the cutoff frequencies correspond to the lower and upper cutoff wavelengths.

Electronics

In electronics, cutoff frequency or corner frequency is the frequency either above or below which the power output of a circuit, such as a line, amplifier, or electronic filter has fallen to a given proportion of the power in the passband. Most frequently this proportion is one half the passband power, also referred to as the 3 dB point since a fall of 3 dB corresponds approximately to half power. As a voltage ratio this is a fall to

1

/

2

≈

0.707

{\textstyle {\sqrt {1/2}}\ \approx \ 0.707}

of the passband voltage.

The half-power bandwidth is a commonly used definition for the cutoff frequency.

This occurs when the output voltage has dropped to

1

2

≈

0.707

{\displaystyle {\tfrac {1}{\sqrt {2}}}\approx {\text{0.707}}}

of the filter's nominal passband voltage and the power has dropped by half. A bandpass amplifier will have two half-power points, while a low-pass amplifier or a high-pass amplifier will have only one.