Balanced MMIC QPSK modulator with low amplitude and phase errors

Abstract

A broadband monolithic microwave integrated circuit (MMIC) quadrature phase shift keying (QPSK) modulator with low amplitude and phase errors is proposed. The presented modulator adopts two wide-band reflection-type binary BPSK modulators, each consisting of a modified Lange coupler with additional lambda/4 transmission lines, varactor diodes, inductors and resistors. The proposed QPSK modulator is implemented in 0.15 mu m GaAs low-noise pHEMT process. The measurements show an error vector magnitude and a demodulated I/Q offset <5.8% root-mean-square and -16.1 dB, respectively, from 18 to 26.5 GHz.