All-digital encoders for RF switch-mode power amplifier applications

A new class of all-digital encoders for radio frequency switch-mode power amplifiers (SMPA) is introduced. The introduction of new signaling is motivated by practical limitations in SMPAs which restrict bandwidth. Harmonic frequency components or dither are synthesized at baseband and then upconverted to create a RF carrier. The resultant spectrum is band limited and the out-of-band spectrum is concentrated near the carrier frequency. The time domain waveform has quasi-constant envelope signal properties and power amplifier efficiency remains high as the signals do not create significant back-off in the PA. Because the architecture employs up-conversion, this significantly reduces the high frequency clock requirements that are typically required for other pulse encoding methods such as delta-sigma modulation. Two all-digital encoders and experimental results with WCDMA and LTE carriers at 990 MHz are shown to demonstrate the concepts. Results are also shown to evaluate efficiency enhancement assuming out-of-band energy is recycled in the SMPA.