A novel C-band IQ vector modulator is presented in this paper. The circuit uses two GaAs MMIC FET quads operating in resistive mode. The balanced configuration of each resistive FET bi-phase modulator provides high linearity and isolation. The vector modulator substrate, incorporating the multilayer baluns, 90 degree coupler and in-phase power combiner, measures 25.4mm /spl times/ 25.4mm and is fabricated using photoimageable thick-film processing technique. This modified thick-film process is capable of implementing 3D Multi-Chip Modules (MCM-C), a promising technology for low-cost solutions for microwave components.