In this case, the field decays exponentially along the waveguide axis and the wave is thus evanescent.- Google Patents US8867879B2 - Single-mode optical fiber The wave equations are also valid below the cutoff frequency, where the longitudinal wave number is imaginary. As a voltage ratio this is a fall to 1 / 2 ≈ 0.707

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. 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.

1.1 Single-pole transfer function example.

In the case of a waveguide or an antenna, the cutoff frequencies correspond to the lower and upper cutoff wavelengths. 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. 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 point (a frequency for which the output of the circuit is −3 dB of the nominal passband value).

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. 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.

(The slope −20 dB per decade also equals −6 dB per octave.) A Bode plot of the Butterworth filter's frequency response, with corner frequency labeled.

Cable cut off wavelength