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  • 1.
    Yuan, Xiaolong
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Wideband Sigma-Delta Modulators2010Licentiate thesis, monograph (Other academic)
    Abstract [en]

    Sigma-delta modulators (SDM) have come up as an attractive candidatefor analog-to-digital conversion in single chip front ends thanks to the continuousimproving performance. The major disadvantage is the limited bandwidthdue to the need of oversampling. Therefore, extending these convertersto broadband applications requires lowering the oversampling ratio (OSR) inorder. The aim of this thesis is the investigation on the topology and structureof sigma-delta modulators suitable for wideband applications, e.g. wireline orwireless communication system applications having a digital baseband aboutone to ten MHz.It has recently become very popular to feedforward the input signal inwideband sigma-delta modulators, so that the integrators only process quantizationerrors. The advantage being that the actual signal is not distorted byopamp and integrator nonlinearities. An improved feedforward 2-2 cascadedstructure is presented based on unity-gain signal transfer function (STF). Theimproved signal-to-noise-ratio (SNR) is obtained by optimizing zero placementof the noise transfer function (NTF) and adopting multi-bit quantizer.The proposed structure has low distortion across the entire input range.In high order single loop continuous-time (CT) sigma-delta modulator, excessloop delay may cause instability. Previous techniques in compensation ofinternal quantizer and feedback DAC delay are studied especially for the feedforwardstructure. Two alternative low power feedforward continuous-timesigma-delta modulators with excess loop delay compensation are proposed.Simulation based CT modulator synthesis from discrete time topologies isadopted to obtain the loop filter coefficients. Design examples are given toillustrate the proposed structure and synthesis methodology.Continuous time quadrature bandpass sigma-delta modulators (QBSDM)efficiently realize asymmetric noise-shaping due to its complex filtering embeddedin the loops. The effect of different feedback waveforms inside themodulator on the NTF of quadrature sigma-delta modulators is presented.An observation is made that a complex NTF can be realized by implementingthe loop as a cascade of complex integrators with a SCR feedback digital-toanalogconverter (DAC), which is desirable for its lower sensitivity to loopmismatch. The QBSDM design for different bandpass center frequencies relativeto the sampling frequency is illustrated.The last part of the thesis is devoted to the design of a wideband reconfigurablesigma-delta pipelined modulator, which consists of a 2-1-1 cascadedmodulator and a pipelined analog-to-digital convertor (ADC) as a multi-bitquantizer in the last stage. It is scalable for different bandwidth/resolutionapplication. The detail design is presented from system to circuit level. Theprototype chip is fabricated in TSMC 0.25um process and measured on thetest bench. The measurement results show that a SNR over 60dB is obtainedwith a sampling frequency of 70 MHz and an OSR of ten.

  • 2.
    Yuan, Xiaolong
    et al.
    KTH, School of Information and Communication Technology (ICT), Electronic, Computer and Software Systems, ECS. Zhejiang University, China .
    Signell, S.
    Wu, Xiaobo
    KTH, School of Information and Communication Technology (ICT), Electronic, Computer and Software Systems, ECS.
    Single amplifier sigma delta modulator with input feedforward2006Conference paper (Refereed)
    Abstract [en]

    It has recently become very popular to feedforward the input signal in wideband delta-sigma modulators, so that the integrators only process quantization errors. The advantage is that the actual signal is not distorted by op-amp and integrator nonlinearities. The paper applies this technique to single amplifier based sigma delta modulator in both analog and digital way. Their characteristic has been analyzed and simulation results are given which show the improved performance.

  • 3.
    Yuan, Xiaolong
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication: Services and Infrastucture, Communication Systems, CoS.
    Tan, Nianxiong
    Signell, Svante
    KTH, School of Information and Communication Technology (ICT), Communication: Services and Infrastucture, Communication Systems, CoS.
    On low power design of feedforward continuous-time sigma delta modulators with excess loop delay2008In: PROCEEDINGS OF 2008 IEEE INTERNATIONAL SYMPOSIUM ON CIRCUITS AND SYSTEMS, 2008, p. 1882-1885Conference paper (Refereed)
    Abstract [en]

    In high order single loop continuous-time (CT) sigma delta modulators, excess loop delay may cause instability. In this paper, previous techniques in compensation of internal quantizer and feedback DAC delays are studied especially for the feedforward structure. Two alternative low power feedforward continuous-time sigma delta modulators with excess loop delay compensation are proposed. Simulation based CT synthesis from discrete time topologies is adopted to obtain the loop filter coefficients. Design examples are given to illustrate the proposed structure and the synthesis methodology.

  • 4.
    Yuan, Xiaolong
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Wu, X.
    Signell, S.
    Continuous-time quadrature bandpass sigma delta modulators with different feedback DAC2008In: IEEE Int. Conf. Circuits Syst. Commun., ICCSC, 2008, p. 580-583Conference paper (Refereed)
    Abstract [en]

    Continuous time (CT) quadrature bandpass sigma delta modulators (QBSDM) are widely used in wireless receivers recently due to its inherent anti-aliasing filtering and complex filtering embedded in the loops to efficiently realize asymmetric noise-shaping. The complex noise-transfer-function (NTF) can be realized with complex coefficients or by implementing the loop as a cascade of complex integrators with only real coefficients, which is desirable due to its lower sensitivity to loop mismatch. In this paper, we will study the effect of different feedback waveforms inside the modulator on the NTF of quadrature sigma delta modulators. Complex NTF design is discussed for different bandpass center frequencies relative to the sampling frequency.

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