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Title: Coplanar waveguide components and their applications in microwave circuits
Other Titles: Gong mian bo dao yuan jian ji qi zai wei bo dian lu zhong de ying yong
Authors: Mo, Tingting (莫亭亭)
Department: Dept. of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2006
Publisher: City University of Hong Kong
Subjects: Microwave integrated circuits
Microwave transmission lines
Wave guides
Notes: 123 leaves : ill. ; 30 cm.
CityU Call Number: TK7871.65.M6 2006
Includes bibliographical references.
Thesis (Ph.D.)--City University of Hong Kong, 2006
Type: Thesis
Abstract: This thesis covers the microwave components and circuits employing coplanar waveguide (CPW), which is one of the most important transmission line media commonly used in current wireless communications. The research focuses on five different applications of CPW to be discussed in the following chapters. Chapter 1 is a historical review of CPW and its characteristics. Compared with microstrip line (MSL), CPW has merits of easy access to the ground plane, easy shunt connection, wide impedance range, substrate thickness insensitivity, low dispersion and phase inversion capability. These merits have been fully utilized in the following applications of CPW components and circuits. Chapter 2 presents a basic structure etched on the centre line of CPW. It works as photonic bandgap (PBG) structure with the characteristics of low pass, slow wave and band stop and is named as compact CPW resonant cell (CCRC). Two other forms of CCRC, the finite ground CCRC and the defected ground CCRC, have been developed. All of them share the features of PBG structures. Parameters studies manifest ways to control the bandwidth of the stop-band, return loss of the passband etc. Direct applications of CCRC in passive and active circuits are presented respectively in the following two chapters. In Chapter3 is an edge coupling type bandpass filter incorporating CCRC. Study has revealed that an asymmetrical CCRC is capable of introducing two transmission zeros to shape the passband. Because CPW and MSL can co-exist on the opposite side of one substrate, a CPW feed bandpass filter incorporating a microstrip resonant cell is designed. Due to the difference of edge coupling and broadside coupling, the latter one has a wider bandwidth. Moreover, a bandpass filter based on the grounded form of CCRC is proposed. Similar to the previous two items, it shapes the passband with extra transmission zero which is movable by tuning the parameters of the CCRC. Chapter 4 presents a class A power amplifier employing CCRC for linearization. The proposed architecture is one of the easiest ways to offer the second harmonic a return path and injected to the transistor to suppress the intermodulation distortion at the end. The effectiveness of the injection of the second harmonic is proven not only by the output expressions of BJT nonlinear model, but also by the simulations and experiments. Chapter 5 begins with the distinct feature of CPW. It is the 180° instant phase reversal realized by a swap between center line and the ground of CPW, which is not only simple in its form, but also ideally independent of frequency. As a result, it fits best the applications where a large operation bandwidth is required. This chapter presents a new microstrip rat-race circuit adopting this CPW phase inverter to enhance its bandwidth and to reduce its bulk size. The effect is magnificent as the bandwidth is enlarged from 20% of the conventional one to 80% and the size is 50% smaller. To realize the circuit, the first step is to develop a compact wideband transition between CPW and microstrip, then to arrange the CPW swap by vias and strips instead of wire bonds for easy fabrication and at last the simplification of the transitions-swap combination and the optimization of its phase and insertion loss. The experiment has been conducted and proved that this proposed circuit is a compact rat-race with attracting bandwidth and size. Continuing the discussion of CPW phase inverter, Chapter 6 extends to a switched-line typed 180° phase shifter. By substituting the conventional extra half wavelength section with a CPW swap, this phase shifter achieved 90% bandwidth at 2GHz and much more compact size. However, the bandwidth cannot be as wide as ideal due to the confines of the diode and the biasing circuit. To optimize the whole performance of the proposed phase shifter, three switching configurations have been discussed and the series-shunt type is adopted. Experiment has proved the effectiveness of the proposed phase shifter. Finally, in Chapter 7, we give a conclusion of our research work and a vision for future work on CPW components and circuits.
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