Direct Sigma Delta GMSK modulator modeling and design for 2.5 G TX applications

This paper proposes a new Simulink modeling technique for an SD kind of modulator. In fact the system analysis for this kind of design cannot use the standard tools used to design classical analog transmitters. In this solution the digital section is predominant on the analog one, but the analog behavior of the TX path has to be evaluated as a function of different analog and digital parameters. In particular the model has been used to study the impact of the analog transfer function of the PLL on a GMSK modulation. The model allows us to find the right pre emphasis filter and implementation. Different kinds of Sigma Delta have been analyzed and a 1-1-1 Mash structure has been selected. This model has been used to study and define a direct Sigma Delta modulator for 2.5 G applications and the measurements on the implemented TX section show very good agreement with the model results.