Digital predistortion linearizes RF PAs

RAPID enhancements in digital signal processors (DSPs) have made it possibl e for designers to use digital predistortion to perform real-time correctio n of distortion in RF power amplifiers (PAs). Unfortunately, unlike purely analog-based solutions, this technique puts stringent requirements on sever al components in the transmission chain, including predistortion circuits, data converters, reconstruction filters, and the RF transmitter (Tx) compon ents (Fig. 1). These constraints arise because there must be a perfect tran smission path between the predistorter and the nonlinear PA, since any erro rs in this path will result in increased adjacent-channel power (ACP). Alth ough an adaptive digital predistortion ssystem can compensate for some of t he nonlinearities in the forward path not attributable to the PAs, many oth er imperfections cannot not be overcome. Therefore, a successful implementa tion of digital predistortion requires an understanding of potential system -level limitations and the means of overcoming them. This article presents an empirical overview of circuit errors in non-adaptive systems. Since many of the components in the correction loop exhibit characteristics similar t o those in the forward path, the information presented is also applicable t o a fully adaptive, digital predistortion system.