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Nonlinear distortion is a term used to describe the phenomenon that the relation between the "input" and "output" signals of, for example, a device is not exactly linear. The term is used in fields such as electronics, audio and telecommunications.
For many devices, a linear model is accurate only for small signal levels. For example, at 2 volts input, a typical audio amplifier might put out 20 V, meaning the linear gain is 10 V/V. For 3 V input, it might then output 30 V. However, the model implies that at 50 V input it would produce 500 V, which is not possible with most amplifiers.
For larger values of u, the higher order coefficients such as a2 and a3 come into play.
Effects of nonlinearity
Nonlinearity can have several effects, which are unwanted in typical situations. The a3 term for example would, when the input is a sine wave with frequency ω, result in an extra sine wave at 3ω, as shown below.
In certain situations, this spurious signal can be filtered away because the "harmonic" 3ω lies far outside the frequency range used, but in cable television, for example, third order distortion could cause a 200 MHz signal to interfere with the regular channel at 600 MHz. Similarly, "mixing" of two or more frequencies could occur.
Causes of nonlinearity
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