Space vehicles for deep space exploration rely on microwave and millimeter wave links for communication with earth stations. As the mission of space probes expands, the data rate to be transmitted increases, and effective, compact, methods of modulating the RF carrier with high-speed data become a critical necessity. Passive MMIC modulation approaches introduce loss into the RF path and analog phase shifters in particular can exhibit significant shortcomings such as insertion loss variations and phase-shift range limitations. For more complex digital or hybrid modulation schemes, passive modulator components may also need elaborate calibration look-up tables to achieve desired phase and amplitude modulation accuracy with respect to process and temperature variations. Hittite has recently developed high-performance, active I/Q vector modulator MMICs for communication systems operating up to C-band, based on SiGe and GaAs HBT process technologies. Our Phase I study has shown the feasability of implementing a high-performance X-band modulator on the same 40-50 GHz Ft process. We propose to fabricate and evaluate this modulator as part of this Phase II program. We also propose to use this modulator in conjunction with frequency multipliers recently developed by Hittite to implement a high-speed,linear phase modulator at Ka band.