An RF Pulse Width Modulator for Switch-Mode Power Amplification of Varying Envelope Signals

This paper presents the design of a RF pulse width modulator (RF-PWM) for switched-mode power amplification of varying envelope signals. The general idea is to modulate the varying envelope signal using squarewave signals with varying width and subsequently drive a class D-type power amplifier. The linearity in the modulator is ensured by a novel combination of low frequency feedback combined with predistortion. Measurements on a frequency wise down-scaled prototype show that the RF-PWM modulator modulates a UMTS signal with more than 17 dB margin to the modulation mask and EVM below 0.5% RMS. This leads to the conclusion that sufficient linearity and modulation accuracy can be obtained using RF pulse width modulation