Distortion mechanisms in supply modulation of polar transmitters

The distortion mechanisms in supply modulation introduced by the PWM generator, the frequency response of DC regulator and the limited bandwidth of the high frequency recovery unit are analyzed for EDGE polar transmitters. It is demonstrated that there is an optimum regulator bandwidth that minimizes the total residual energy originated from the PWM generator and the low pass filter characteristics of the buck converter. The output spectrum was analyzed for a 5MHz PWM clock frequency buck converter. The results show that in order to meet EDGE transmission mask requirements, the bandwidth requirements of the high frequency recovery unit should be at least 10MHz for 300KHz converter bandwidth while a 95KHz converter bandwidth demands for no compensation at high frequencies.