An 800-MHz monolithic GaAs HBT serrodyne modulator

An 800-MHz monolithic mixed-signal serrodyne modulator IC has been developed in a GaAs/AlGaAs HBT HI₂L process optimized for digital applications. This 3×2.8 mm, 2000+ transistor chip consists of a 7-b phase accumulator driving a vector modulator, implemented as a pair of balanced mixers, 5-b switched attenuators, buffer amplifiers, and central circuits. The balanced mixer´s LO leakage and 3-1 products are typically 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 and phase errors of 1.5 dB and 1.5°, 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 dBc. Post-fabrication modeling indicates that a 5°C thermal gradient across the IC may be responsible for this undesired dc-level variation. This first generation chip consumes 2.5 W of dc power and clocks to speeds in excess of 925 MHz