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Digital Compensation of Supply Voltage Variations for Switched-Capacitor Power Amplifiers (SCPA)

Influence of voltage supply variations (e.g. DC-DC switching ripple) on the output signal of the SCPA.

 

Project staff
DI Stefan Trampitsch

Project start
01.04.2013

Project description
Mobile transmitters have highly constrained output characteristics in terms of transmit power and linearity. The spectrum of the output signal must conform to specified spectral masks to omit leakage to adjacent channels. The nonlinear behavior of transmitter components such as the power amplifier (PA) degrades the linearity of the system and increases the adjacent channel leakage ratio (ACLR). While digital pre-distortion of internal nonlinearities are well studied in literature, there are, to our best knowledge, no investigations to compensate effects of external sources by using digital algorithms in literature. This project focuses on digital compensation techniques of power supply distortions for a Switched-Capacitor Power Amplifier (SCPA). The SCPA is a configurable capacitive voltage divider, which combines the functionality of a digital-to-analog converter (DAC), a mixer, and a power amplifier (PA). The supply voltage of the SCPA can be seen as its reference voltage. Hence, variations on the supply cause additional mixing products with the input signal of the SCPA and create unwanted distortions in its output, degrading the linearity of the SCPA. Currently, additional circuitry (LDO) is used to suppress the supply variations, decreasing the efficiency of the system and increasing the bill of materials. This work investigates digital pre-distortion techniques to compensate the introduced distortions by the supply voltage variations to omit additional circuitry and thus increase the efficiency of the system while simultaneously maintaining the linearity of the SCPA.

Block diagram

Publications

3) Trampitsch S., Markovic J., Oßmann P., Fritzin J., Zaleski J., Mayer C., Fulde M., Pretl H., Springer A., Huemer M.: "A Nonlinear Switched State-Space Model for Capacitive RF DACs," in: IEEE Transactions on Circuits and Systems I: Regular Papers, Volume 64, Number 6, Page(s) 1342-1353, 2017.

2) Trampitsch S., Gruber D., Lunglmayr M., Thaller E., Huemer M.: "Digital Compensation of DC-DC Converter Voltage Ripple for Switched-Capacitor Power Amplifiers," in: IEEE (Eds.): Proceedings of the 14th IEEE International New Circuits and Systems Conference (NEWCAS 2016), 2016.

1) Trampitsch S., Knoblinger G., Huemer M.: "Switched State-Space Model for a Switched-Capacitor Power Amplifier," in: Proceedings of the International Symposium on Circuits and Systems (ISCAS 2015), Page(s) 1478-1481, IEEE, 2015.