Novel RF resonators and bandpass filters for wireless communications : theory, design and application

Abstract

There has been a phenomenal growth in the wireless industry. With the advancement of wireless systems, the finite electromagnetic spectrum has to accommodate more and more systems and thus it is becoming increasingly crowded. As a result, RF signals should be strictly confined to the assigned spectrum ranges. As the key components to confine signals, bandpass filters are accordingly demanded with more stringent requirements. Hence, this dissertation concentrates on bandpass filters for wireless communications. Novel RF resonators and bandpass filters have been proposed. Harmonic suppression, dual-band operation and tunability of bandpass filters are highlighted as research focuses. Theory, design and applications are included in this dissertation. The work can be divided into five parts as follows. In the first part, centrally loaded resonators (CLRs) are introduced based on the investigation of half-wavelength transmission-line resonators. It is found that the coupling coefficient within a special coupling region can be zero at certain frequencies in certain situations. Furthermore, the voltage at some points is zero at certain frequencies. Hence, adding any element at the zero-voltage points will not affect the response at these frequencies. Benefiting from this property, some novel resonators, named as CLRs, are proposed basing on half-wavelength resonators. Oddand even-mode analysis is conducted to characterize the CLRs. Part two is the application of CLRs in harmonic-suppressed bandpass filters. To demonstrate the feasibility, several bandpass filters have been implemented based on CLRs. These resonators can be applied to both tunable and non-tunable bandpass filter designs. The experiments showed that the even-order harmonic responses can be effectively suppressed while passband performance remains nearly unaffected. Furthermore, a special coupling region is selected to eliminate the coupling between CLRs at second harmonic so as to achieve higher harmonic rejection levels. Subsequently, the application of CLRs is extended to dual-band bandpass filter designs in the third part. Dual-band operation can be obtained by using reactive loading elements. The experiments revealed that both capacitors and stubs can function as loading elements of CLRs to achieve dual-band bandpass responses. Aside from dual-band bandpass filters with fixed passband frequencies, the CLRs can also be applied to tunable dual-band bandpass filters. Benefiting from the resonant property of CLRs, the tunable dual-band bandpass filters can obtain a non-tunable passband and a tunable one. The tunable dual-band filters have been successfully implemented. Following the application of CLRs in dual-band filters is the designs of high-selectivity dual-band bandpass filters using two sets of resonators with common input and output ports. Several novel combination circuits have been proposed and experimentally verified. Dual-mode dual-band bandpass responses have been observed. On each side of each passband, a transmission zero was created, resulting in high skirt selectivity. Finally, an image-rejected up-converter has been constructed by incorporating a harmonic-suppressed tunable bandpass filter. The center frequency of the filter can be simultaneously altered with the frequency change of mixer output so as to select the modulated signals and reject the image as well as LO leakage. The feasibility of the proposed design has been verified by carrying out experiments.