Simulation and modeling of intermodulation distortion in communication circuits

Distortion in circuits along the signal path of a transceiver plays a key role in determining the overall performance of digital communication systems. This paper describes how recent improvements in the mixed frequency/time algorithm (MFT) expand our ability to predict the distortion of these circuits. The MFT algorithm extends traditional shooting methods to directly compute the quasiperiodic steady-state response of circuits driven by two or more periodic signals, each at independent frequencies. With these improvements, MFT can be directly applied to accurately and efficiently compute the intermodulation distortion of large circuits driven by a small number of discrete tones. This is representative of the signals found in receivers. However in transmitters, concern focuses on the affect of intermodulation distortion from complex digitally modulated signals. Such signals cannot be handled directly by MFT, but we demonstrate how a macromodel can be constructed that is used to rapidly predict the spectral regrowth caused by the power amplifier and associated circuitry. This approach allows much more of the transmitter to be simulated than with envelope-transient methods.