A 28 GHz HBT Vector Modulator and its Application to an LMCS Feedforward Power Amplifier

A vector modulator employing four amplitude modulated orthogonal channels is presented. Operating at 28 GHz, this device consists of a four-way 6 dB power divider, four GaAs HBT MMIC variable gain amplifiers, and a 4-to-1 power combiner All of the passive functions were realised on alumina ceramic while the amplifier chips were fabricated at Nortel Technologies using a 3.5 ¿m × 6 ¿m GaInP-GaAs HBT process. By testing a vector modulator prototype, we were able to evaluate the architecture and be the first to assess its suitability as a gain/phase control element for a Ka-band MCS adaptive feedforward lineariser.