Baseband Model for Uniformly Sampled RF-PWM

A baseband model for a polar RF modulator based on uniform sampling (US) pulse width modulation (PWM) is presented. RF-PWM allows to encode a complex baseband signal (amplitude and phase) into the pulse width and pulse position of a binary signal towards an efficient and fully digital RF transmitter implementation. We extend the well-known derivation of the naturally sampled (NS) RF-PWM to obtain a detailed model of the US RF-PWM which includes the intrinsic inband and side-bands distortion. In RF-PWM the carrier frequency is equal to the PWM frequency (first PWM harmonic). We develop the model such that the first or any other higher harmonic of the PWM can be used as the carrier signal. The distortion signal is analyzed in detail and a discrete-time implementation of the model which relates to previous approaches is presented. This model is key to the analysis, characterization and simulation of a fully digital transmitter.