Asymmetry and memory effects in quadrature modulators

In this study, an experimental nonlinear characterization of a quadrature modulator for communications following a standard approach based on a two-tone test is investigated. A simple nonlinear model is adopted according to experimental observations. Measurements are presented showing the behavior of the intermodulation (IM) products versus power, and suggesting that distortion is dependent on the baseband rates of the complex envelope input signals. An asymmetry observed at the second-order intermodulation (IM2) products takes especial importance and is theoretically explained. Simulations of the asymmetrical behavior of IM2 based on the proposed model are presented showing good agreement with measurements. The analysis is focused on the 1dB/dB slope characteristic versus the carrier power observed at IM2, a matter that suggests that nonlinear effects can be attributed to baseband nonlinear blocks.