Curse of digital polar transmission: Precise delay alignment in amplitude and phase modulation paths

Abstract

Radio transmitters using the polar scheme utilize a CO-ORDInate Calculator (CORDIC) to transform the often interpolated baseband data in Cartesian format to its polar equivalent. This non-linear transmission typically results in an increased bandwidth for amplitude and phase modulated signals. In case of digital implementation, each of these amplitude and phase/frequency modulation paths are realized using segmented modulators, with fine resolution achieved by techniques such as sigma-delta modulation. This paper first describes the structure of a digital polar transmitter. This is followed by a study on the impact of the increased bandwidth in the polar domain on the timing alignment accuracy requirements between the amplitude and phase modulation paths. Furthermore, for each high-resolution modulation path the timing accuracy between the integer and the sigma-delta fractional path split is also analyzed.