| Summary: | 碩士 === 淡江大學 === 電機工程學系 === 92 === Analog to digital converters have been widely used in digital signal processing, and over-sampling and sigma-delta modulation techniques are used in analog to digital conversion interface of modern very large scaled integrated circuits. Due to over-sampling characteristics, the sigma-delta modulators usually are limited on the application of voice band signals. As the integrated circuit process improvement, it makes many researches transfer to applications of wide-bandwidth gradually, such as xDSL, Bluetooth, GSM ,and WCDMA.
Since cellular telephony made the transition from the first-generation analog systems to the second—generation (2G) digital systems less than a decade ago, mobile telecommunications have experienced tremendous growth. Today, several different 2G standards are used worldwide, of which the global system for mobile communication (GSM) has highest penetration. Driven by market forces, the number of second-generation subscribers is expected to increase further in the next few years. On the other hand, the demand for higher system capacity and data rates has led to the development of third-generation (3G) systems, which are in their final stage of standardization now and will be deployed in many parts of the world in the next couple of years. Dictated by the continuing popularity of exisited 2G services and the time needed to build up equivalent coverage, quality, and variety of service, a 3G cellular network requires multi-standard mobile terminals, to take advantage of both 2G and 3G services during the transition period.
In this thesis, we want to design a band-pass sigma-delta modulator suitable for WCDMA application. The center frequency is 100MHz, and the bandwidth is 3.84MHz.Because of the limitation on sampling frequency of the traditional switched-capacitor circuit (usually below 100MHz), even though double sampling or pseudo two-path techniques are used, the band-pass sigma-delta modulator’s center frequency is still below 50MHz.Therefore we use another approach continuous time circuit, to design the sigma-delta modulator. The impulse time invariant transformation is used to transfer the discrete time filter transfer function to the continuous-time filter transfer function. The continuous-time filter uses transconductance-capacitance circuits to fulfill the implementation. Its advantage is that it can work at a higher frequency band. In this manner, it can compensate the switched-capacitor circuit which cannot work at high the frequency band. Our circuit architecture is with multiple-feedback, its structure has two different one-bit digital-to-analog converters to control the feedback current.
Our proposed band-pass sigma-delta modulators for wireless receiver application whose operating voltage is 1.8V；the center frequency is 138.24MHz；the sampling frequency is 553MHz, and the oversampling ratio is 72. The simulation result show that the bandwidth is 3.84MHz；the sampling frequency is 553MHz；the dynamic input range is 60dB and the maximum signal to noise ratio is 56 dB. Its input signal is 0.2V；the effective resolution is 9.5bits, and the total power dissipation is 75mW.
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