Design of Compact Bandpass Filters and Multiplexers with Multi-Function Applications

This research is about the design of miniaturized filters, multispurious suppression filters, multi-passband filters, and miniaturized multiplexers. Several new layouts have been proposed and all of them are verified by experimental results. To begin with, stepped-impedance resonators (SIRs) that are the basic components of the proposed filters and multiplexers have been thoroughly studied. The SIR not only can be used to reduce the resonator size but also capable of controlling its all resonant frequencies. Complete design equations for SIRs have been provided, especially in the prediction of their fundamental and higher-order spurious frequencies. Additionally, novel compact net-type resonators have been presented. Actually, these net-type resonators are attributed to the variation of quarter-wavelength SIRs. The total electrical length and resonant frequencies of net-type resonators with different structural parameters have also been analyzed theoretically. Secondly, to miniaturize the filter size, the compact net-type resonator filters have been proposed. Three kinds of filters including Chebyshev, quasi-elliptic, and trisection bandpass filters are designed and fabricated to demonstrate the practicality. By adjusting the structural parameters of the net-type resonators, the spurious frequencies can be properly shifted to higher frequencies and the resonator size can be reduced, which is more than 67 % reduction in comparison with that of the traditional open-loop resonator. Moreover, a miniaturized low-temperature cofired ceramic (LTCC) multilayer quasi-elliptic bandpass filter has been proposed. The fourth-order filter is constructed by two open-loop resonators and two miniaturized hairpin resonators that can be coupled through the apertures on the common ground plane, and the skew-symmetrical feeding structure adds two extra transmission zeros in the stopband. All adjacent resonators of the filter are vertically stacked at different layers and, hence, it may dramatically reduce the circuit area. As a result, each of the designed filters occupies a very small circuit size and has a good stopband response. Thirdly, to reject the spurious passbands of filters, a simple and efficient mothod has been proposed. The basic concept is to choose the constitutive resonators with the same fundamental frequency but staggered the higher-order frequencies, which is demonstrated by a fourth-order Chebyshev filter and a fourth-order quasi-elliptic filter. Each filter is composed of four different SIRs. The results show that better than –30 dB rejection levels in the stopband up to 5.4f0 and 8.2f0 are achieved by the Chebyshev and quasi-elliptic filters, respectively. In addition, to design multiband filters, a novel design method has been proposed. Coupling structures with both Chebyshev and quasi-elliptic frequency responses are presented to achieve dual-band and triple-band characteristics without a significant increase in circuit size. The design concept is to add some extra coupled resonator sections in a single-circuit filter to increase the degrees of freedom in extracting coupling coefficients of multiband filter and, therefore, the filter is capable of realizing the specifications of coupling coefficients at all passbands. Four experimental examples of filters with dual-band Chebyshev, triple-band Chebyshev, dual-band quasi-elliptic, and triple-band quasi-elliptic performance have been successfully designed to verify the presented concept. Finally, to miniaturize the multiplexer size, high isolation and compact size multiplexers designed with common resonator sections have been proposed. By exploiting the variable frequency responses of SIR, resonators can be shared by all filter channels of a multiplexer if their fundamental and higher-order spurious resonant frequencies are properly assigned. Size reduction is, therefore, achieved by introducing a few common resonator sections in the circuit. This concept has been verified by the experimental results of three multiplexers. As a consequence, these multiplexers occupy extremely small areas while still keeping good isolations.Chapter 1 Introduction ------------------------------------------------------------------------------ 1 1.1 Research Motivation ----------------------------------------------------------------------- 2 1.2 Literature Survey --------------------------------------------------------------------------- 3 1.3 Contributions -------------------------------------------------------------------------------- 6 1.4 Organization of the Dissertation ---------------------------------------------------------- 7 Chapter 2 Basic Concepts and Theories of Filters ----------------------------------------- 9 2.1 Filter Characteristics ----------------------------------------------------------------------- 9 2.2 Chebyshev Function Filters ------------------------------------------------------------- 10 2.2.1 Lowpass Prototype Design ------------------------------------------------------- 10 2.2.2 Scaling and Conversion ----------------------------------------------------------- 15 2.2.3 Impedance and Admittance Inverters ------------------------------------------- 17 2.2.4 Practical Realization of Impedance and Admittance Inverters --------------- 23 2.2.5 External Quality Factors and Coupling Coefficients -------------------------- 24 2.3 Quasi-Elliptic Function Filters ---------------------------------------------------------- 27 2.3.1 Lowpass Prototype Design ------------------------------------------------------- 28 2.3.2 External Quality Factors and Coupling Coefficients ------------------------- 31 2.4 Basic Theory of Couplings -------------------------------------------------------------- 33 2.4.1 Electric Coupling ------------------------------------------------------------------ 33 2.4.2 Magnetic Coupling ---------------------------------------------------------------- 35 2.4.3 Mixed Coupling -------------------------------------------------------------------- 36 2.4.4 Numerical Example of Extracting Coupling Coefficient --------------------- 37 2.5 Basic Theory of External Quality Factor ---------------------------------------------- 39 2.5.1 General Formulation of External Quality Factor ------------------------------ 39 2.5.2 Numerical Example of Extracting External Quality Factor ------------------ 41 Chapter 3 Analysis of Stepped-Impedance Resonators ------------------------------- 43 3.1 Analysis of Half-Wavelength SIRs ----------------------------------------------------- 43 3.1.1 Resonant Conditions -------------------------------------------------------------- 43 3.1.2 Characteristics of SIRs ------------------------------------------------------------ 45 3.2 Analysis of Quarter-Wavelength SIRs ------------------------------------------------- 49 3.2.1 Optimal Design --------------------------------------------------------------------- 49 3.2.2 Net-Type Resonators -------------------------------------------------------------- 51 Chapter 4 Compact Filters and Multispurious Suppression Filters -------------- 57 4.1 Novel Compact Net-Type Resonator Filters ------------------------------------------ 58 4.1.1 Coupling Characteristics between Net-Type Resonators --------------------- 58 4.1.2 Third-Order Parallel-Coupled Net-type Resonator Filter -------------------- 59 4.1.3 Fourth-Order Cross-Coupled Net-type Resonator Filter ---------------------- 63 4.1.4 Third-Order Trisection Net-type Resonator Filter ----------------------------- 68 4.2 Miniaturized Multilayer Quasi-Elliptic Bandpass Filter ---------------------------- 72 4.2.1 Filter Design ------------------------------------------------------------------------ 72 4.2.2 Experimental Results -------------------------------------------------------------- 77 4.3 Multispurious Suppression Filters ------------------------------------------------------ 80 4.3.1 Four-Pole Parallel-Coupled Filter ----------------------------------------------- 81 4.3.2 Four-Pole Cross-Coupled Filter -------------------------------------------------- 85 Chapter 5 Multi-Passband Filters and Miniaturized Multiplexers --------------- 91 5.1 Multi-Passband Filters ------------------------------------------------------------------- 92 5.1.1 Coupling Structures of Dual- and Triple-Band Filters ------------------------ 92 5.1.2 Dual-Band Chebyshev Filter Design -------------------------------------------- 95 5.1.3 Dual-Band Quasi-Elliptic Filter Design --------------------------------------- 100 5.1.4 Triple-Band Quasi-Elliptical Filter Design ----------------------------------- 105 5.1.5 Triple-Band Chebyshev Filter Design ----------------------------------------- 109 5.2 Miniaturized Multiplexers -------------------------------------------------------------- 114 5.2.1 Diplexer Design with Chebyshev Response ---------------------------------- 115 5.2.2 Diplexer Design with Quasi-Elliptic Response ------------------------------- 122 5.2.3 Triplexer Design with Chebyshev Response ---------------------------------- 130 Chapter 6 Concluding Remarks --------------------------------------------------------------- 135 6.1 Summary of the Dissertation ---------------------------------------------------------- 135 6.2 Suggestions for Further Research ----------------------------------------------------- 139 References ---------------------------------------------------------------------------------------------- 141 Publication List -------------------------------------------------------------------------------------- 14