Polyphase Decomposition for Tunable Band-Pass Sigma-Delta A/D Converters

The use of the polyphase decomposition technique applied to the noise transfer function (NTF) of band-pass sigma-delta (BPΣΔ) modulators is introduced and theoretically analyzed. Schemes for a second-order and fourth-order band-pass noise shaping are discussed in detail. The method is usable for any order but the analog inaccuracy limits its application. It is shown that an extension to MASH configurations is possible. The method allows tunability of the centre frequency over a wide frequency range. Moreover, MASH schemes allow a rough-fine tuning with the rough tuning in the analog section followed by a fine digital adjustment. Simulation results verify the benefits and outline possible limits.