Extended Noise-shaping in Cascaded N-tone Σ∆ Converters

An N-tone Σ∆ converter is obtained by introducing N zeros in the noise transfer function (NTF) while maintaining a flat signal transfer function (STF). N-tone Σ∆ converters are particularly well suited for multi-carrier systems. While oversampling is a widely used technique to improve the resolution of the converter, different methods will have to be explored for systems with low over-sampling. Even though, the achievable resolution is limited by the number of bits per Nyquist sample, using higher-order loop-filters and multi-bit loop quantizers can be used to achieve high resolution. But, the former is prone to instability and the latter is sensitive to DAC linearity. However, use of cascaded converters has proven to overcome these inherent disadvantages of the before-mentioned methods. In this paper, we extend this existing low-pass technique to the N-Tone Σ∆ converters.