Research On Miniaturized Planar Tunable Radio Frequency Filters

With the development of modern wireless communication, various wireless communication services are available in different bands. Future multi-band/software defined radio systems require building multi-band and reconfigurable radio frequency (RF) front end. RF filters are the key to constitute the channel and reduce the interferences of radio systems. Tunable RF filters with reconfigurable frequency responses will be the essential components for future wireless communication systems.In chapter Ⅰ, recent developments on tunable filters are reviewed. At present, many tunable filters based on distributed resonators are researched. With the development of modern wireless communication systems, compact and integrated devices are required. In this dissertation, novel bandstop and bandpass filters based on sub-wavelength resonators, i.e. SRR (Split ring resonator), CSRR (Complementary split ring resonator) and microstrip LC resonators are presented, and the tunable mechanics are studied.Based on SRR and CSRR the bandstop filters are proposed, and the stopband tunable mechanics of the filters are studied. VLSRR (Varactor loaded split ring resonator) loaded CPW (Coplanar Waveguide) tunable bandstop filter is designed. DMS (Defected microstrip structure) with CSRR etched on the microstrip surface is studied, and the half circuit of the structure is proposed to build a more compact bandstop filter and a1-bit reconfigurable bandstop filter. The main contributions of the work on bandstop filters are listed as follows:(1) A novel tunable bandstop filter based on CPW loaded with SRR is presented. The unit cell of the bandstop filter is formed by etching single SRR on the CPW back substrate side, and the SRR can be excited by the CPW slot’s magnetic field. Lumped equivalent circuit models are developed to analyze the frequency responses, and it shows that by loading the common cathode varactor diodes at the split region of the SRR outer ring, the resonant frequency of the bandstop filter can be electronically adjusted by tuning the reverse voltage of the varactor diodes.(2) Novel bandstop filters based on the defected microstrip structure with CSRR/half-CSRR etched on the microstrip surface are presented. By changing the configurations of the CSRR, varied stop-band and pass-band characteristics are observed, and half-CSRR is proposed to build a novel bandstop filter. A detailed explanation for the generation and variations of the stop-band and pass-band has been given. Lumped equivalent circuit analysis model is developed as well, and the frequency tuning capability of the bandstop filter is studied. It shows that the filter stopband can be reconfigured by the loading capacitance. A1-bit2.5GHz-3GHz reconfigurable bandstop filter with wide band harmonic suppression ability is designed based on the proposed novel bandstop filter loaded with floating capacitors.Novel bandpass filter is designed based on SIW embedded with CSRR, and the HMSIW (Half-mode substrate integrated waveguide) structure of the filter is proposed. The passband tunable mechanics of the HMSIW filter is studied, and tunable HMSIW filters are designed. Novel mixed coupled microstrip LC bandpass filters are presented, and the tunable mechanics of the structures are studied. Based on electric coupling coefficient compensation, a modified tunable circuit model with constant fractional/absolute bandwidth is presented. The main contributions of the work on bandpass filters are listed as follows:(1) CSRR loaded substrate integrated waveguide (SIW) and half mode substrate integrated waveguide (HMSIW) filters are proposed and their applications to mechanical and electrical tunable/switchable bandpass filters (BPF) are studied. Novel SIW and HMSIW loaded with coupled CSRR achieve forward electromagnetic wave transmission below the waveguide cutoff frequency. The half mode configuration of the proposed SIW bandpass filter is an opening structure, and the proposed HMSIW BPF is suitable to load with the control elements. The electric/magnetic coupling effects and frequency responses reconfigurable mechanics of the proposed HMSIW structure were studied based on lumped equivalent circuit model. By using floating capacitors with mechanical switches, PIN diodes and semiconductor varactor diodes loaded on the HMSIW resonators, a1-bit HMSIW mechanical tunable BPF,1-bit electrical tunable BPF, and an electrical continuous tunable BPF are demonstrated with good spurious suppression characteristics and excellent out of band rejection.(2) We propose and develop novel tunable BPF based on mixed coupled microstrip LC resonators. The equivalent circuit model of the proposed filter is presented. The filter can be reconfigured by tuning the capacitance of the LC resonators. A tunable BPF based on semiconductor varactor diode loaded microstrip LC resonators is demonstrated. Secondly, we propose and develop a novel microstrip LC filter and tunable BPFs with constant fractional/absolute bandwidth. The equivalent circuit model of the proposed microstrip LC filter is presented, and the tunable transmission coefficient of the microstrip LC filter is analyzed. It shows that the filter can be reconfigured by changing the capacitance of the LC resonators. Based on electric coupling coefficient compensation, a modified tunable circuit model with constant fractional/absolute bandwidth is presented. Finally, we propose and develop a novel3rd-order microstirp LC BPF, and three transmission zeros are obtained. The equivalent circuit model of the proposed filter is developed to study the filter’s structure. It shows that the filters can be reconfigured by changing the capacitance of the microstrip LC resonators. For demonstration, semiconductor varactor diode loaded microstrip LC resonator has been adopted in our work to design the tunable filter.