Continuous-time bandpass SigmaDelta modulators employing on-chip Q-enhanced LC filters and undersampling of RF signals for wireless communications

The rapid development of digital wireless systems has led to a need for high-resolution and high-speed bandpass analog-to-digital converters. Continuous-time bandpass ΣΔ modulators are very suitable for such high-frequency applications. In this thesis, design, analysis, simulation and implementation of a continuous-time bandpass ΣΔ modulator for use in modern cellular/PCS receivers is given. The design employs undersampling relative to a radio receiver's RF or IF center frequency, while oversampling the signal bandwidth. This technique enables clocking at a frequency much lower than the RF/IF frequency, allowing use of standard CMOS technology and reducing the complexity and power consumption in the subsequent digital signal processing stages. Also, because it downconverts the signal inherently, it shows a possibility of developing a receiver offering the performance of a super heterodyne scheme without requiring extra mixers to downconvert the IF signal to baseband.; A complete analysis of the proposed continuous-time bandpass ΣΔ modulator with the mixer and local oscillator in the feedback is given. Transfer functions are derived for the resulting mixed signal system. The analysis shows that the loop transfer function can be made equivalent to the standard discrete-time bandpass ΣΔ output power spectrum of the modulator, noise shaping and the down conversion of the IF signal. The signal-to-noise ratios for different sampling rates and input levels modulator. The modified z-transform is used to study the effect of delay on the stability of the system. Simulation results have been used to verify the predicted output power spectrum of the modulator, noise shaping and the down conversion of the IF signal. The signal-to-noise ratios for different sampling rates and input levels are also given.; The modulator was implemented in a 1.2 μm CMOS process. The bandpass filter is implemented using an on-chip LC resonator with Q-enhanced circuit. A Q-enhanced LC filter with both Q-tuning and frequency tuning is used to allow for variation in manufacturing and temperature drift. The modulator operates at a 195 MHz IF with a bandwidth of 300 kHz. A sampling frequency of 20 MHz is used which centers the noise shaping and converted signal at 5 MHz. The filter measured response and the effect of the frequency and Q tuning oil this response are presented. The measured modulator output spectrum is also given. Simulated and measured output spectra agree well and the desired band-reject noise shaping is achieved.