A high capacity fully digitalized modulator using parallel phase process techniques is developed for digital radio systems. Through the use of 1.2 p m BiCMOS technology, a modulator-LSI (MOD LSI) which guarantees 112 MHz operation is fabricated. Experimental results obtained for a 16 QAM MODEM with the MOD LSI show that this MODEM can be applied directly to a sub STM-l(51.84 Mbps) of an SDH system, to increase its cost-effectiveness, reliability and performance.