Using a forced nonlinear oscillator to implement a QPSK modulator without a 90 degree phase shifter

A new quadriphase-shift keying (QPSK) modulator is presented. It comprises two nonlinear oscillators, one RC differentiator and two voltage control current sources (VCCSs); it is named the 'direct QPSK modulator'. Experimental results are also reported. Such a modulator is suitable for generating direct sequence spread spectrum signal in a wireless communication system, and can be packaged into an IC.