An Investigation of Centrally Loaded Resonators for Bandpass Filter Designs

Microwave filters are indispensable components in wireless communications. They allow transmission of signals of certain frequency band while rejecting the others. An application of such a filter can be found in the mobile terminals of the Chinese Beidou Global Position System (GPS) which provides two-way communication through short messages between a client and headquarters. This system is ideal for a trucking company such as FedEx to monitor its trucks and trace past records of their routes in China. The filter isolates the mobile terminal outward microwave signals from the satellite incoming signals at a different frequency for avoiding interferences.Resonator is often a crucial building element in filter design. Supported by a completed CERG grant, the researchers have invented a class of resonators known as the compact microstrip resonant cells (CMRCs). CMRCs have been used in various filter designs and active microwave circuits. For this CMRC research, the researchers have received over a hundred citations and successfully implemented the technology in commercial power amplifier for Beidou GPS. A feature article on this work was published inResearch Frontier, Issue 8, May 2004 by the Research Grants Council. A lowpass filter was designed for the power amplifier to stop the higher-frequency receiving signals from being retransmitted. If a bandpass filter were used, the researchers would achieve better selectivity and thus overall system sensitivity.Recently, The researchers have proposed a new and yet simple resonator that consists of a transmission line (TL) and a loading element shunted at the center of the line. Upon theoretical investigation and their preliminary experimental verifications, it is found that the even-mode resonance can be easily controlled by incorporating different loading elements while the odd-mode characteristics are unaltered. Benefiting from this feature, the proposed resonator finds applications in various filter designs including single-band, dual-band and harmonic-suppressed bandpass filters. Different from previous tunable filters which have single passband, the proposed tunable ones can achieve two passbands. The center frequency of the upper passband can be electrically tuned whereas the performance of the lower one remains unchanged. All these different designs can be achieved by different loading elements varying from a resistor to a varactor diode.In recent years, stepped-impedance resonators (SIRs) that consist of sections of TL with different width have been investigated extensively for filter designs. In this project, the researchers make use of SIR based filters as benchmarks for our proposed resonators. Specifically, the researchers will investigate different central loadings for different bandpass filter applications. Some examples, in addition to the ones mentioned above, are proposed below. One is to use resistive loading to suppress higher-order harmonics, achieving spurious-free single-band tunable bandpass filters. The second task is to employ capacitive-coupled loading to individually control the bandwidth of dual-passband. The third task is to design tunable dual-band bandpass filters using a network of resistors, capacitors and varactor diodes. The fourth is to find ways to optimize filter performance and miniaturize the circuits. Based on their preliminary results, the researchers strongly believe that the proposed resonators will lead to research findings with both scientific impact and commercial potential.