Numerical Model of an Injection-Locked Wideband Frequency Modulator for Polar Transmitters

We present a numerical model of a wideband injection-locked frequency modulator used in a polar transmitter for 3G cellular radio application. At the heart of the system is a self-injection-locked oscillator with a programmable linear tuning range of up to 200 MHz at 4-GHz oscillation frequency. The oscillator is injection locked to a time-delayed version of its resonating voltage, and its frequency is modulated by manipulating the phase and amplitude of the injected current. The model is used to study the feasibility of the proposed system by analyzing the impact of various impairments in the auxiliary injection loop on the system performance. The model is written in MATLAB/SIMULINK, and the simulation output is analyzed by a vector signal analyzer in terms of 3GPP specifications. Based on the simulation results, key specifications for individual blocks in the system are determined. The key benefits of the presented modeling methodology are simplicity, efficiency, and portability.