Adaptive digital predistortion linearization of frequency multipliers

A novel technique to linearize frequency multipliers for use in high-frequency transmission of digitally modulated signals is presented. Using this technique, a bandpass signal containing both amplitude and phase modulation can be translated without distortion to a higher frequency via nonlinear frequency multiplication. A theoretical analysis is performed to identify the bandpass transformation of the signal envelope in highly nonlinear devices in a manner such that its inverse transformation may be estimated. The theory was validated on a 2.46-GHz Schottky-diode frequency tripler constructed by the authors and on an 820-MHz commercially available frequency doubler. As predicted by the theory, the devices showed highly nonlinear characteristics in terms of their AM/AM, AM/PM, and PM/PM distortions. Adaptive lookup-table- and polynomial-based predistortion systems were designed and constructed to linearize the frequency multipliers. The predistortion results show a fair amount of improvements in adjacent-channel power ratio and error vector magnitude.