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What Is Nonlinear Distortion?

What Causes Nonlinear Distortion?

Preview: Learn more about nonlinear distortion and how it affects the performance of communication systems.

Nonlinear distortion occurs when the output of an electronic device is no longer directly proportional to its input. In an ideal communication system, doubling the input signal would produce exactly twice the output signal. Practical electronic devices, however, operate linearly only over a limited range. When driven beyond this range, the relationship between input and output becomes nonlinear, causing the signal waveform to be distorted and generating unwanted frequency components.

Many electronic devices exhibit nonlinear behaviour when operating close to their maximum output capability. Power amplifiers, mixers, travelling-wave tube amplifiers (TWTAs), low-noise amplifiers (LNAs), and analogue-to-digital converters can all introduce nonlinear distortion if they are overloaded or operated near saturation. As communication systems increasingly demand higher power and greater spectral efficiency, controlling nonlinear distortion has become an important aspect of system design.

One consequence of nonlinear distortion is the generation of harmonics. If a pure sinusoidal signal passes through a nonlinear device, additional frequency components appear at integer multiples of the original frequency. When multiple signals are present simultaneously, nonlinear devices also generate intermodulation products, producing new frequencies equal to sums and differences of the original frequencies. These unwanted signals can interfere with adjacent communication channels and reduce overall system performance.

A useful analogy is stretching a spring. Within its elastic range, doubling the applied force doubles the extension. If the spring is stretched too far, however, it no longer behaves proportionally and becomes permanently deformed. Electronic amplifiers behave similarly: once driven beyond their linear operating region, they distort the signal rather than simply amplifying it.

In digital communication systems, nonlinear distortion produces several undesirable effects. It causes constellation points to shift from their ideal positions, increasing the bit error rate (BER). It also broadens the transmitted spectrum, producing spectral regrowth that may interfere with neighbouring channels. For high-order modulation schemes such as 64-QAM or 256-QAM, even relatively small nonlinearities can significantly reduce communication quality.

Communication engineers employ several techniques to minimise nonlinear distortion. Power amplifiers are often operated with output back-off, allowing them to remain within their linear operating region. Digital predistortion (DPD) intentionally distorts the signal before amplification so that the amplifier's nonlinear characteristics restore the desired waveform. Careful amplifier design, linearisation techniques, and adaptive signal processing also help reduce distortion.

It is important to distinguish nonlinear distortion from linear distortion. Linear distortion changes the amplitude or phase of different frequency components without creating new frequencies and can often be corrected using equalization. Nonlinear distortion, by contrast, generates entirely new frequency components, making it much more difficult to remove once it has occurred.

Today, controlling nonlinear distortion is essential in satellite communications, cellular networks, digital broadcasting, radar, microwave links, and optical communication systems. As communication systems continue to employ higher-order modulation schemes and increasingly crowded radio spectrum, maintaining amplifier linearity has become one of the key requirements for achieving high spectral efficiency and reliable communication.