Distortion arising from polar to PWM/PPM conversion in an all digital upconverter for switching rf power amplifier

Existing 3G base stations require high efficiency linear radio frequency (RF) power amplifiers (PAs). Switch Mode PAs can have improved efficiency but require a pulse train as a drive signal. This paper studies the unwanted spectral components that arise when baseband polar signals are upconverted to RF using pulse width modulation and pulse position modulation (PPM) in sigma delta based modulators. The dominant distortions are shown to be third order and image components generated in the PPM circuit. In a single carrier environment, an increase in offset frequency increases the unwanted spectral components. Offsets less than 10 MHz are required to keep unwanted components 40 dB below the signal level Also, the effect of distortions on adjacent channel powers is quantified using an OFDM input signal.