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Microwave Bandpass Filters for Wideband Communications by Rui Li, Sheng Sun, Lei Zhu

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CHAPTER 4

PLANAR TRANSMISSION LINE RESONATORS

4.1 INTRODUCTION

Microwave resonators have been widely used in many applications, such as filters, oscillators, amplifiers, and microwave measurement. Among different types of resonators, the planar transmission line resonator has been considered as the most commonly used resonator for application in radio frequency (RF) and microwave integrated circuits. Unlike waveguide or dielectric resonators, which can offer very high quality factor (Q-factor), planar transmission line resonators, constructed with microstrip line, coplanar waveguide (CPW), and coplanar-stripline (CPS) topology, usually have ordinary Q values due to their intrinsic loss factors of metallization, dielectric substrate, and radiation. However, because of their attractive features, such as low-cost, compact size, lightweight, easy massive fabrication, and so on, a large number of integrated filters have been developed using planar resonators for various communication systems [1].

Figure 4.1 shows the cross-sectional view of a few commonly used planar transmission lines, including stripline, finline, microstrip line, CPW, slotline, and CPS. By terminating a finite portion of a uniform transmission line with either open-circuits or short-circuits at two ends, a transmission line resonator can be made up. Figure 4.2 shows a half-wavelength microstrip open-circuited resonator whose two terminals are open ended. Over the past decades, this resonator has been widely used ...

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