||With the rapidly progress in the microwave communication systems, miniaturization and performance enhancement have become two main requirements of the microwave devices. Microwave dielectric substrates would be the best choice for these requirements, because high dielectric constant of the substrates would reduce the size of the devices, high quality factor of the substrates would improve the microwave characteristics of the devices, and low temperature coefficient of resonant frequency would reduce the shift of the operating frequencies due to the variation of temperature. As mentioned above, the main research of this dissertation is divided into two parts: microwave dielectric materials and microwave filters.|
1. Microwave dielectric materials
AB2O6 (A=Mg, Zn; B=Ta, Nb) microwave dielectric ceramics have been developed as the microwave dielectric resonators (DRs) in the past, because the dielectric resonators fabricated by AB2O6 ceramics reveal the good microwave dielectric characteristics. However, the temperature coefficients of resonant frequency of MgTa2O6, MgNb2O6, ZnTa2O6, and ZnNb2O6 ceramics are still not good enough for the applications at the microwave frequency. In addition, MgTa2O6 and ZnTa2O6 ceramics reveal positive temperature coefficients of resonant frequency but the MgNb2O6 and ZnNb2O6 ceramics reveal negative temperature coefficients of resonant frequency. In this study, combining of MgNb2O6 ceramics (with negative temperature coefficients of resonant frequency) and MgTa2O6 ceramics (with positive temperature coefficients of resonant frequency) to form Mg(Ta1-xNbx)2O6 ceramics and combining of ZnNb2O6 ceramics (with negative temperature coefficients of resonant frequency) and ZnTa2O6 ceramics (with positive temperature coefficients of resonant frequency) to form Zn(Ta1-xNbx)2O6 ceramics, which all reveal near-zero temperature coefficients of resonant frequencyand are suitable for the applications of microwave communication devices. The sintering and microwave dielectric characteristics of the Mg(Ta1-xNbx)2O6 and Zn(Ta1-xNbx)2O6 dielectric ceramics are also investigated.
2. Wide-band, dual-band, tri-band, and tetra-band bandpass filters
Microwave filters have been widely used in the communication systems. The optimal microwave dielectric characteristics of AB2O6 ceramics developed in this thesis were adopted as the substrates of the filters. The performance of the filters was improved obviously due to the high dielectric constant and high quality factor of the microwave dielectric ceramic substrates. At first, a wide-band and a dual-band (2.45/5.2 GHz) bandpass filters are developed by the combination technique of modified end-coupled microstrip lines and half-wavelength ombination technique will generate three transmission zeros easily in the stop-band to improve the characteristics of the filters. And the next, the tri-band (1.57/2.45/5.2 GHz) bandpass filters are developed by the combination of modified end-coupled microstrip lines, outer-frame structures and half-wavelength U-shaped hairpin resonators. The Defected Grounded Structures (DGS) are add into the ground planes of the tri-band bandpass filters to generate the fourth frequency (3.5 GHz), hence, the tetra-band (1.57/2.45/3.5/5.2 GHz) bandpass filters are accomplished. In addition, due to the uses of the high dielectric constant ceramic substrates and the combination techniques, the size of this tetra-band bandpass filter is only 26.3 mm*9.9 mm. Besides, six deeply transmission zeros are generated in the stop-band to improve the characteristics of the filters (1~7 GHz), all the characteristics of this tetra-band filters (frequency, bandwidth, insertion loss, and stop-band rejection) are suitable for the applications of modern communication systems.