Abstract: PLL frequency synthesizers and their calibration techniques are described. The PLL frequency synthesizers are used to generate digital modulation of a carrier signal. A digital frequency divider in the feedback path of the loop has its division ratio controlled by a digital &Dgr;-&Sgr; modulator. The modulation of the carrier is achieved by applying a modulation signal to the input of the &Dgr;-&Sgr; modulator and to the input of the voltage-controlled oscillator of the PLL. The high frequency path and low frequency path of the modulation signal must be adjusted with respect to one another in order to obtain a good modulation. As the low frequency path can be accurately set, the calibration is performed only on the high frequency path. Digital calibration techniques for the high frequency path are described.