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A DCS1800 offset-phase-locked-loop upconversion modulation loop integrated circuit (IC) fabricated in a 0.18-μm CMOS technology is presented in this paper. This IC operates at 2.8-V supply voltage with a current consumption of 36 mA. The measured root-mean-square and peak phase errors of the Gaussian minimum shift keying (GMSK) transmission signal are 1.6° and 4°, respectively. It is shown that such circuits can be implemented in CMOS process with current dissipation and performance comparable to BiCMOS chips. Advantages of upconversion modulation loop and design issues of I/Q modulators are also described.