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This paper describes a sigma-delta (/spl Sigma/-/spl Delta/) synthesizer for Gaussian frequency and minimum shift keying (GFSK/GMSK) modulation. The key innovation is an in-service automatic calibration circuit which tunes the phase locked loop (PLL) to compensate for process tolerance and temperature variation. The PLL, including 1.8 GHz voltage controlled oscillator (VCO), /spl Sigma/-/spl Delta/ modulator, and calibration circuit has been implemented in a 0.6 micron BiCMOS integrated circuit. The test chip achieves 2.5 Mbit/second using GFSK and 5.0 Mbit/second using 4-FSK.