Behavioural modelling and impact analysis of physical impairments in quadrature modulators

Non-linear distortion and linear static errors (DC offset and gain/phase imbalance) are the major analogue impairments in microwave I/Q quadrature modulators. They can adversely impact the performance of the systems using them, especially for direct-conversion transceiver systems. The impacts of these impairments were investigated for three different modulator applications: double sideband modulation, zero intermediate frequency (IF) single sideband (SSB) orthogonal frequency-division multiplexing (OFDM) modulation and low-IF SSB OFDM modulation. A behavioural model for characterising the non-linear response and the linear static errors was developed. The non-linear characteristic of each input was modelled by a complex power series based on amplitude-to-amplitude modulation and amplitude-to-phase modulation measurements. The linear static errors were modelled as three static error terms added into the complex power series based on a four-DC-point vector network analyser measurement. The non-linear model was validated by measurements with three types of signals representing the three modulator applications.