Investigation And Design Of Microwave Balanced Bandpass Filters

With rapid development of modern wireless communication systems, the requirements such as high sensitivity and efficiency for those systems are ever increased. Thus, more and more scholars pay close attention to the balanced systems with great anti-noise performance. As an important part of the balanced system, the balanced filters with small size, harmonic suppression, tunable frequency research, have been drawn much attention around the world. This thesis focuses on the size reduction and harmonic suppression of the balanced filters and tunable balanced filters. All these designed filters are based on the distributed-element half-wavelength resonator on printed circuit board(PCB) or low temperature co-fired ceramic(LTCC).The importance and development of the balanced filters are illustrated, respectively. Then the mixed-mode S-parameters theory for analyzing the balanced filters is discussed.To solve the common problem of large size of the developed planar balanced filters, the mechanism of the half-wavelength resonator under differential-mode excitation is analyzed. Then a balanced bandpass filter is designed by making full use of the three-dimensional LTCC technology and the High Frequency Structure Simulator(HFSS) from Ansoft. This filter has compact size, good performance, but high-order harmonics can’t be well suppressed. Actually, the topic on the harmonics suppression in the balanced filter has been rarely mentioned before. To address this problem, a simple method for suppressing the third-order harmonic is proposed to widen the stopband significantly, i.e. the coupling point of the feed line is chosen to be the voltage-null point of the third-order harmonic on the centrally-loaded(resistor and capacitor in series) half-wavelength resonator. Based on this method, a wide-stopband balanced bandpss filter with good performance is designed. To meet the requirement of modern reconfigurable system, the tunable balanced filter using the varactors is proposed. By analyzing the properties of the varactor-loaded half-wavelength resonator under differential- and common-mode excitations, respectively, the deterioration of common-mode suppression in the tunable differential-mode passband is reduced significantly.