RF communication systems using advanced modulation formats rely on the linearity of the signal chain components to meet throughput performance requirements. Nonlinearities result in mixing products which include harmonics and intermodulation (IM) products. Though passive components such as cables, connectors, switches, and antennas are highly linear, they can become significant sources of passive intermodulation (PIM) when transmitting. This article describes the theory, challenges, and methods for testing highly linear passive components for PIM.
A linear device or system produces an output signal that is proportional to its input. The resulting transfer function relating output to input can be plotted as a straight line with slope corresponding to the system gain. Component nonlinearity typically results in the output signal compressing at large input levels. Such a nonlinear transfer function may be approximated by a Taylor series expansion with exponential terms. Frequency mixing occurs in higher order terms (for example 2nd order squaring, 3rd order cubing, etc..) producing both integer harmonics and intermodulation products.