Contents I. Introduction
In modern wireless communication applications, the support of multiple operation modes represents usually a trend to obtain the success of a product. The adoption of multi-standards implies different types of modulation and channel bandwidths leading to highly flexible implementations. If signal digitization occurs in the baseband the analog circuitry complexity is higher with an important impact in the final cost. However, if the signal could be digitized closer to the antenna, then the multi-standard functionality including demodulation and channel-selection would be performed in the digital domain [1]–[6]. An optimum solution to fulfill this objective, at an intermediate-frequency (IF), implies digitizing the analog signal through a bandpass sigma-delta modulator, which will exhibit higher performance when compared with wideband Nyquist-rate ADCs. Since traditionally the bandwidth of IF signals is much smaller than the carrier frequency the bandpass sigma-delta modulator is an efficient architecture for such type of applications which benefits from its oversampling characteristic to achieve a high signal-to-noise-ratio [7]. But when the signal bandwidth increases, and for a fixed carrier frequency, the OSR value must be reduced which turns oversampling less attractive. Thus for wider bandwidth applications, high-order multi-bit sigma-delta modulators are naturally the best choice for an optimum system solution. MASH-type sigma-delta modulators such as 4–4-L mb and 4–2L mb are usually utilized for the implementation of high-order noise-shaping modulators. Recent trends of sigma-delta modulator research include the maximization of the SNQR through the spreading of NTF zero while achieving expandable architecture and minimizing the overloading point [8]. By exploring different trade-offs between these two topologies a semi-MASH sigma-delta modulator, recently proposed [9], can achieve a high-order expandable and extendable noise-shaping characteristic. The main idea behind this new architecture is the creation of a feedback path between the cascade modulator stages to obtain an aggressive noise-shaping with a minimum overloading region. In this paper, the semi-MASH technique is further enhanced and applied to bandpass sigma-delta modulation. In section II, the proposed bandpass semi-MASH architecture is thoroughly studied and a design example is presented to illustrate its behavior and compare it with different existing topologies. In section III a novel double-sampling mismatch-free technique will be proposed and explored for future application in the bandpass sigma-delta modulator. Finally, the conclusions will be drawn in section IV.
