Design of Time-to-Digital Converter Circuits for 3D Time-of-Flight Measurements
Sammanfattning: The three-dimensional view of the world is something that we often take for granted, a phenomenon that has proven challenging for machine vision applications in which description of the environment requires the relative position and motion of different objects in the scene. Currently, range imaging (RIM) measurements are based on digital imaging technology and are merged with the ability to measure the distance to the corresponding object point in each pixel. The distance measurement is based on either the direct or indirect time-of-flight principle. The distance to the corresponding object point in each pixel is directly correlated with time-resolved imaging concerning the measurement of the photon arrival time. This time-resolved image generates a distance image by using the direct time-of-flight technique. This distance image is much more useful than an ordinary picture in regard to measuring and controlling anything, including in the process industry, obstacle detection for automotive safety, navigation, and path planning. The time resolved imaging system consists of two essential building blocks: 1) a photon detector capable of sensing single photons and 2) a fast time resolver or time-to-digital converter that can measure the time of light to picosecond resolution. To address emerging applications, a miniaturized time resolver with acceptable performance and a low cost must be designed that could be integrated with an array of single photon detectors. The goal of this thesis is therefore to investigate, design, and layout a time-to-digital converter to achieve an acceptable cm-level resolution for a range of 10-15 meters. In this thesis, two ideas have been selected for investigation based on their appealing attributes, including their improved resolution, area, and power consumption: 1) an on-pixel time stretcher based on analog time expansion and 2) the combination of the on-pixel time stretcher with a global gated ring oscillator-based time-to-digital converter. Both ideas have been used in conjunction to demonstrate a new architecture for a time-to-digital converter for 3-D time-of-flight measurements. The time stretcher uses analogue time expansion, where the time interval to be measured is stretched by a factor k. This is achieved by charging a capacitor with a constant current I, followed by discharging the capacitor by a current I/k . To achieve an acceptable linearity and constant current generation, wide swing cascode current source/sinks have been used. An idea to build precisely matched current mirror as a time stretcher has also been addressed. The final time-to-digital conversion is performed by the gated ring oscillator-based time-to-digital converter. The multiphase gated ring oscillator, which is the heart of time-to-digital converter, is capable of measuring the stretched time interval by counting the full clock cycles and determining the timing positions within the clock cycle. The work has been discussed in light of the previous research by designing circuits, performing layouts, simulating and conducting parasitic extractions in a 0.35 μm CMOS process. Based on simulations and results, a prototype of an integrated time-to-digital can be built to achieve a cm-level distance error.
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