Nonlinear Quantization in Low Oversampling Ratio Sigma-Delta Noise Shapers for RF Applications

Abstract

Baseband signal processing for current base stations or 3rd generation mobile systems will impose high bandwidth and high VLSI integration demand. Many of the desired integration aspects can be satisfied with sigma-delta converter front-ends. However, under the technology constraints there are simultaneous requirements for high sample rate and low oversampling ratio in order to achieve the desired baseband width. In this paper, we present system architecture results for the 4th-order cascaded noise shaper architectures to be used in baseband front-ends. We show that the cascaded structures with proper scaling will satisfy simultaneous demand on linearity (spurious free dynamic range) and high SQNR with low oversampling ratio based on usage of multibit quantizers outside the actual signal noise shaping path. We also present results for nonlinear quantization effects in low oversampling ratio cascaded noise shaper architectures. We analyse the effect of the non-linearity in both the A/D and D/A-block in quantization error quantizer path for the 4th-order cascaded topology and the design constraints associated to the performance of the used A/D and D/A structures. The performance requirement for the multi-bit quantizer for high SQNR is shown for the case of low oversampling ratios. The results show that non-uniform quantization around zero input are far more crucial to the SQNR than nonlinear quantization deviating from the ideal transfer function. As the key difference to standard multibit quantizers, no special error correction or error distribution schemes are required; the linearity requirements are satisfied with 0.2 LSB accuracy of the few bit quantizer. Finally, the performance of non-linear quantization using multitone test signals are also shown.