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This paper describes a new approach to the automatic calibration of modulated phase-locked loop (PLL) frequency synthesizers. The new calibration approach tunes the response of the PLL while the synthesizer is in service and compensates for process and temperature variation. The availability of this new calibration method allows the use of precompensation techniques to achieve high data rate modulation of sigma-delta (Σ-Δ) frequency synthesizers. The calibration method can be applied to Gaussian frequency shift keying (GFSK), Gaussian minimum shift keying (GMSK) modulation, and also M-ary FSK modulation. A low-frequency board level prototype has been constructed to validate the calibration approach.