Intermodulation

Intermodulation is a form of distortion that occurs when two or more signals pass through a nonlinear device or channel. The nonlinearity causes mixing of the input signals, producing new frequency components that are sums and differences of the originals. Unlike harmonic distortion, which stems from multiples of a single tone, intermodulation products arise from interactions between different tones.

In practical systems, intermodulation occurs in RF amplifiers, mixers, filters, and other nonlinear components. When multiple signals share a path, the nonlinearity generates intermodulation products at frequencies such as f1 ± f2, 2f1 ± f2, 2f2 ± f1, as well as higher-order combinations (for example 3f1, 3f2, etc.). The third-order products, notably close to the original tones, are often the most problematic because they can fall inside the passbands of interest or adjacent channels, causing interference and desensitization of receivers.

Measurement of intermodulation often uses a two-tone test, where two signals of equal or specified amplitudes are applied to a device. The resulting IMD products are analyzed to quantify linearity, commonly summarized by the third-order intercept point (IP3). A higher IP3 indicates better linearity and fewer troublesome IMD products.

Mitigation focuses on improving linearity and suppressing unwanted products. Approaches include using higher-linearity components, operating with adequate back-off from saturation, applying feedback or predistortion techniques, implementing RF filtering, and ensuring good impedance matching and isolation. Proper channel planning and power budgeting also help minimize the impact of intermodulation in complex systems.