Supply Modulator Ripple in Envelope Tracking Systems - Effects and Countermeasures

Envelope tracking power amplifiers (ET PAs) are one of the promising architectures to provide high efficiency amplification for future wireless communication systems. This is due to their ability to offer high efficiency over a wide output power range, by modulating the supply voltage applied to the PA. Generating this dynamic supply voltage with switching-mode modulators is highly efficient, but filtering the output remains a challenge, resulting in a residual ‘ripple’ on the supply voltage. This paper presents an experimental investigation into the interaction between a PA and a supply modulator in the presence of this ripple. By adding a varying ripple magnitude to the modulated drain voltage of a 2.14 GHz GaN ET PA with a 10 MHz LTE signal, the effects on the linearity of the RFPA can be observed and analysed to allow the system designer an insight into the amount of ripple that is tolerable, while still being able to achieve linearity and efficiency targets. The mixing products of the ripple and RF signal are shown to be a potential issue in ET PAs. This paper shows, for the first time, the full impact of the ripple voltage magnitude on the output spectrum. Further more, the ripple sensitivity of two different ET PA linearisation approaches are explored; firstly applying generic memory polynomial digital pre-distortion (DPD) and secondly optimising the ET tracking signal shaping function to improve linearity. Measurements show that for the case where the ripple and RF are not synchronized, neither approach is able to significantly mitigate the ripple effect on the PA linearity.