AN 800-MHZ MONOLITHIC GAAS HBT SERRODYNE MODULATOR

An 800-MHz monolithic mixed-signal serrodyne modulator IC has been dev eloped in a GaAs/AlGaAs HBT HI(2)L process optimized for digital appli cations, This 3 x 2.8 mm,, 2000+ transistor chip consists of a 7-b pha se accumulator driving a vector modulator, implemented as a pair of ba lanced mixers, 5-b switched-attenuators, buffer amplifiers, and contro l circuits, The balanced mixer's LO leakage and 3-1 products are typic ally 25 dB below the carrier at the nominal operating point, with all other products better than -50 dBc, Over a 32-dB control range, the 5- b switched attenuator typically achieves worst-case amplitude acid pha se errors of 1.5 dB and 1.5 degrees, respectively, from 50-250 MHz, DC -level variations versus attenuator-state limit the spurious response of an 800-MHz Composite DDS based on this serrodyne modulator to -20 d Bc, Post-fabrication modeling indicates that a 5 degrees C thermal gra dient across the IC may be responsible for this undesired dc-level var iation, This first generation chip consumes 2.5 W of dc power and cloc ks to speeds in excess of 925 MHz.