Investigation on via-coupling mechanism and its application to complementary compact microstrip resonant cell

Abstract

With the development of wireless communication systems, microwave bandpass filters (BPFs) become more and more important for two kinds of applications. The first is the in-system application. Since the electromagnetic spectrum is becoming increasingly crowded, BPF is the key component to confine the operation bandwidths of the RF/microwave signals. The second is the in-component application. BPFs can be utilized in other microwave components, such as power amplifiers, oscillators, and mixers. In these components, BPFs are used to manipulate the phases and magnitudes of different frequencies and/or harmonics so that the performance can be improved. For both applications, BPFs are generally required to own characteristics of high selectivity, compact size, wide upper stopband and excellent fabrication tolerance. Hence, this dissertation concentrated on BPFs satisfying these requirements. According to the coupled resonator theory, resonator and coupling are the two key factors in coupled-resonator BPFs. Past research on microwave BPFs, especially on the microstrip ones, focused on proposing and utilizing various new kinds of resonators to improve filter performance or achieve special functions. For coupling, end- and edge-couplings, both of which belong to gap-coupling, dominate the coupling mechanism in BPFs. In this dissertation, a new coupling mechanism is presented based on metallic vias, namely via-coupling. It owns merits of better fabrication tolerance and more design flexibility as compared with the traditional gap-coupling, which are verified by two Chebyshev response BPFs using the via- and gap-coupling respectively. Then it finds many applications in BPF designs. Firstly, an end-coupled quasi-elliptic response BPF is realized with the via-couplings being main couplings and the gap-coupling being cross-coupling. Since the via-coupling is magnetic coupling and the gap-coupling is electric coupling, the signals of the main path and cross path will eliminate each other so that transmission zeros can be generated close to the passband. Secondly, the via-coupling is applied between stub-loaded resonators to implement an end-coupled dual-band BPF. Because the via-couplings are used, this filter features excellent fabrication tolerance, which is verified by experiment. Then, the via-coupling finds two kinds of applications in edge-coupled BPFs though it is proposed based on end-coupled structure. One application is to enhance the couplings between edge-coupled resonators. As an example, a combline BPF with enhanced couplings is presented, whose bandwidth is about three times as wide as that of the traditional combline filter with the same coupling gaps. The other application is to generate relatively weak cross-couplings for edge-coupled BPFs. Therefore, we propose the complementary compact microstrip resonant cell (CCMRC), which uses the via-coupling as cross-coupling. Subsequently, the CCMRC is studied and designed according to the coupled resonator theory. Since the CCMRC is a complementary filter of the compact microstrip resonant cell (CMRC), it inherits the compactness from the CMRC. In addition, the cross-coupling introduces two transmission zeros aside the passband, which guarantee high selectivity. And there are quarter-wavelength step-impedance resonators in the CCMRC, resulting in a wide upper stopband. Due to its symmetry, the upper or lower part of the CCMRC can be removed, resulting in a half-CCMRC. The half-CCMRC owns similar bandpass performance, but its size is only half of the CCMRC. After the CCMRC and the half-CCMRC are proposed, they are utilized to design other passive devices for in-component application due to their compactness and excellent performance. Two different half-CCMRCs are combined together to realize an asymmetric CCMRC. Each half-CCMRC contributes to one passbands, resulting in dual passbands. Another dual-band filter is proposed by making the first and last resonators of the CCMRC become stub-loaded resonators. The loaded stubs offer a signal path for the second passband. Similar to the asymmetric CCMRC, a diplexer is also implemented by combining two different half-CCMRCs. Besides, the half-CCMRC is designed into a chip filter by using substrate with high permittivity. The chip filter can be used in microwave circuits like the surface acoustic wave (SAW) filters and the chip inductors/capacitors, yet with higher working frequency and lower insertion loss. Finally, a quasi-elliptic CCMRC with symmetric responses is designed by getting rid of the unwanted cross-couplings of the original CCMRC. Two transmission zeros are symmetrically generated just aside the passband and the selectivity is dramatically improved. Most of the designed BPFs are fabricated by the simple printed circuit board (PCB) process and measured by the vector network analyzer (VNA). Good agreements have been observed between the measured and simulated results. Therefore, metallic via becomes a good choice in microwave BPF designs due to its advantages not only in resonator but also in coupling.

Date of Award	15 Jul 2013
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Quan XUE (Supervisor)