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A 1.9 GHz GaAs direct-conversion quadrature modulator MMIC adopting a new technique to generate 90°; phase shift for W-CDMA applications is presented. This 90° phase shifter achieves a phase error less than 1.5° from 1.8 GHz to 2 GHz with extremely low implementation loss. The package and PCB interconnects are also analyzed using the 3-D EM simulation tool and transformed into the equivalent-circuit elements for co-simulation with the designed modulator MMIC. The dominant package and PCB effect is to degrade the sideband suppression, which is disadvantageous to the modulation quality.