Fractional-N Phase Locked Loops and It’s Application in the GSM System

This paper presents an overview of the fractional-N frequency synthesis technique. After a short excursion into the history of DeltaSigma modulation, different fractional-N techniques are shown. Following this, the DeltaSigma fractional-N PLL is explored in detail, and important results relating to fractional-N PLLs are presented. In the next section the blocks of the analogue part of the PLL are listed, and relevant design criterions are discussed. Finally the specifications imposed by the GSM system are mentioned, and two techniques for using a fractional-N PLL as a GMSK modulator are shown.