A multibit reference feedback sigma-delta modulator for radio receivers

A reference feedback multibit sigma-delta modulator (SDM) for A/D conversion in a radio receiver is studied in this thesis. The study shows that the reference feedback SDM is naturally signal-matched over process and temperature variations, which offers considerable advantage to reference circuit design and allows intentional dynamic range & AGC control, an advantage that is not available to a conventional feedback SDM loop and is not practical for a Nyquist-rate A/D converter. A 4th-order 3-bit bandpass continuous-time reference feedback SDM is designed, built and experimentally evaluated.; A multibit SDM has a wider bandwidth than its 1-bit counterpart. Quantitative comparison of multibit vs 1-bit SDMs is conducted in terms of dynamic range. It shows that with a moderate level of D/A element mismatch, the multibit SDM is better than a 1 bit SDM that inherently has no element mismatch, for bandwidths above 500 kHz. It suggests that a multibit SDM without using dynamic element matching (DEM) can still be a viable solution for broadband applications.; A novel biquad circuit is also developed for a continuous-time SDM loop filter. By cascading the novel biquad with regular biquad circuits, any higher order loop filter can be easily designed and implemented, considerably easing higher order continuous time SDM design and prototyping. The novel biquad is implemented in the 4th-order loop filter design and experimentally demonstrated.