The importance of sub-harmonic frequency terminations in modeling spectral regrowth from CW am-am and am-pm derived non-linearities

This paper examines several issues in applying the quadrature modeling technique to predict gain, efficiency, and distortion characteristics of GaAs MESFET rf power amplifiers. This technique relies on representing the power amplifier as a bandpass nonlinearity described by a series expansion of in-phase and quadrature-phase gain transfer functions. It is shown that the order of the series expansion, as well as the selected series basis functions, have a direct influence on model accuracy. Additionally, this method is extended to predict amplifier drain current and efficiency when driven by time varying non-constant (envelope) amplitude signals, e.g., two tone sinusoids and digitally modulated signals. The effects of sub-harmonic frequency terminations occurring near the zeroth spectral zone, such as those due to bias decoupling on the device's drain terminal, are also examined. Generally, they are not considered in deriving the nonlinear transfer functions, and under certain circumstances, exhibit deleterious effects not predicted by this analysis technique.