Tom,

  The best place to start is

http://www.designers-guide.org/Analysis/delta-sigma.pdf

The issue is that while there many tools for simulating switched-capacitor
circuits in SpectreRF, the overall simulation of the sigma-delta modulator
is difficult. A Sigma-Delta modulator does not have a periodic steady-state
so you will need to rely on transient analysis for verification.

                                                           Best Regards,

                                                              Sheldon