Research On Wideband Bandpass Filters Based On Novel Multi-mode Resonators

The rapid development and wide application of modern wireless communication and radar technologies put forward higher requirements on specification of information capacity and transmission rate. The traditional narrowband system has been unable to meet the need of practical application. Wideband or even ultra-wideband wireless technology with high capacity, high speed and low power consumption has become a hot direction of academic research. Microwave filters play an important role in selection of the frequency, and the good frequency selectivity ensures effective suppression of the external interference signal. In this thesis, some microstrip multi-mode resonators (MMR) are studied, and several wideband bandpass filters with high performance are designed. Measurement verifies the good performance of the fabricated filters. The main contents of this thesis are as follows:1. A ?-type resonator is studied,and its resonant characteristics are analyzed by using transfer matrix theory. The study shows that the resonator has two main resonant modes. Based on this ?-type resonator, a wideband bandpass filter with 3dB passband frequency range of 2-4GHz is designed by setting its two transmission poles at the appropriate frequency. On this basis, by adding the short-circuited stub at the top of the?-type resonator, a transmission zero is generated in the upper stopband and the ability to suppress the stopband is improved.2. Based on the step-impedance resonator (SIR), two multi-mode resonators are designed to load the ring resonator and open-circuited stub respectively at the center position. The resonant characteristics are analyzed by odd- and even-mode analysis method. The resonant characteristics show that it has three main resonant modes. The resonator generates a transmission zero at low frequency side of the passband. At the same time, a transmission zero is introduced at high frequency side of the passband by feedlines loading open-circuited stub, and then design two wideband filters. The measured results show that the 3dB passband range of the two filters is 2-4GHz, and the roll rate on the left and right sides of the passband is higher than 200dB/GHz and 122dB/GHz respectively.3. A stubs-loaded half-wavelength resonator is studied, and its transmission characteristics are analyzed by odd- and even-mode analysis method. The resonant characteristics show that it has three main resonant modes. An ultra-wideband bandpass filter is constructed by using the triple-mode resonator. The measured results show that the 3dB passband range of the filters is 3.1?10.6GHz,and the roll rate on the left and right sides of the passband is higher than 26.4dB/GHz and 50dB/GHz respectively. On the basis of this ultra-wideband bandpass filter, a line in the coupling feedlines is extended to generate a notched band at 6.2GHz, which might be used to suppress the possible interference at that frequency point.