l\inable Narrow Bandpass Sigma-Delta Analog-To-Digital Conversion for Mobile Communication Terminals

A whole-of-band receiver structure is proposed. In this structure, the whole mobile band (125 channels for G S M ) is digitized and channel selection is performed using D S P filters. The A D C is a critical component in this structure, requiring 13bits resolution at sampling rate of 5 0 M H z . A modified Z A A D C converter is proposed for this application. The Z A A D C must be able to null the quantization noise at the frequency of the desired channel. The research considers the feasibility of varying this frequency for tuning different channels. This is a novel concept for Z A systems and so the simplest form of Z A structure was chosen for evaluation to reduce the degrees of freedom in the many variables involved. A second order noise tunable bandpass ZA ADC is studied using pole zero placement techniques in the z-plane. Tuning is performed by moving the zero positions around the unit circle. The optimized pole position is a compromise between noise performance and stability. The signal noise ratio is equivalent to Sbits, indicating a higher order Z A A D C would be required for a practical radio receiver. Sensitivity analysis showed that the quantization noise is sensitive to errors in the system's main tuning coefficient adjustment. Accuracy for this coefficient has to be better than 1%, indicating that some external precision components will be required for any VLSI implementation. The effect of multichannel signals on the performance of the ADC is studied. It is shown that saturation of intermediate nodes dominated the intermodulation performance of the A D C . However, the loss of performance is not great (3dB to 6dB) and can be offset by over designing the A D C to give an appropriate safety margin. The successful implementation of noise4 tunable Z A converter requires accurate setting of the adjustable coefficients. This is seen as the major design obstacle.