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We have derived the transfer functions for two types of six-port quadrature phase-shift keying (QPSK) modulators and investigated their performances in terms of carrier leakage, Gray mapping, and conversion efficiencies. Our result shows that the carrier leakage is minimized when GammaON = -GammaOFF. In addition, our analysis reveals here that only the parallel six-port QPSK modulator demonstrates a Gray mapping feature. Our analysis further proves that the serial and parallel modulator give a maximum conversion efficiency of 100% and 50%, respectively, but the efficiency of the serial modulator deteriorates faster than the parallel modulator when the terminations are not ideal. We have also shown that the serial modulator requires tighter design tolerances due to its cascaded topology. Our theoretical and measured results show good agreements with the six-port modulators developed for 4.2 GHz.