A subsampling delta-sigma modulator for global navigation satellite systems
Next-generation Global Navigation Satellite Systems (GNSS) receivers should\ud be capable of processing multi-frequency signals in order to provide better\ud positioning accuracy and signal availability to end-user. Nevertheless, the\ud realization of such receivers with conventional receiver architectures leads to\ud power-hungry devices and hinders monolithic integration of the receiver.\ud Multi–frequency receivers can be realized with lower power and cost by\ud performing sub-Nyquist sampling (subsampling) at Radio Frequency (RF). In\ud practice, the use of subsampling receivers has been limited due to their poor\ud noise performance. This is particularly a concern in applications of Code\ud Division Multiple Access (CDMA) such as GNSS since subsampling may\ud saturate the Analog-to-Digital Converter (ADC) as the thermal noise floor is\ud above the signals of interest. Continuous-Time (CT) Delta-Sigma (??)\ud modulation is an attractive candidate for subsampling Analog-to-Digital (A/D)\ud conversion as it provides noise-shaping and inherent Anti-Alias (AA) filtering.\ud However, the attenuation of the RF alias in the feedback path of the\ud modulator and the reduction of the effective Quality (Q-factor) of the loop\ud filter prevent conventional CT-?? modulators to be utilized in subsampling\ud receivers. This thesis proposes a novel CT-?? modulator at both the system\ud and the circuit level that is capable of compensating for the effects of\ud subsampling. These are achieved by modifying the feedback path of the\ud conventional modulator architecture to accommodate for the RF alias and by\ud enhancing the Q-factor of the loop filter. The proposed CT-?? has significant\ud improvements over previously published subsampling modulators as it\ud provides jitter and alias suppression and excess loop delay compensation to\ud improve the dynamic range, thus enabling the modulator to be utilized in\ud subsampling receivers when a relatively low sampling rate is desired. Based on\ud the novel CT-?? modulator, this thesis also proposes a subsampling receiver\ud architecture for multi-constellation GNSS applications. Simulations results\ud indicate that the proposed receiver architecture can successfully acquire and\ud track the civilian radionavigation signals with a high performance.
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