Utilizing a High-Speed Marx Supply Modulator in an Amplitude-Modulated Mixerless Transmitter for 64 QAM

Abstract

This article reports on the theory and application of a high-speed Marx generator (HSMG) circuit to realize supply modulation (SM). In order to achieve high-modulation bandwidth, the supply modulator complexity is kept low, enabling multilevel SM with four discrete voltage levels. A prototype is designed and fabricated on a four-layer FR4 board. The efficiency of the proposed modulator is measured with passive loads, achieving 89% at the 5-MHz switching frequency. Applied to a power amplifier (PA), the modulator demonstrates an error vector magnitude (EVM) of 2.64% at 10-MHz switching frequency with a four-ASK signal. Finally, the HSMG is utilized in an innovative mixerless transmitter architecture. Its capabilities are validated at the center frequency of 3.5 GHz for different modulation bandwidths from 2 up to 70 MHz and different constellation schemes-16 QAM, 32 QAM, and 64 QAM. EVM of 5.88% at 10-MHz bandwidth with 64 QAM is obtained without digital predistortion (DPD).