Design Of Electrically Tunable Filter Based On Multimode Resonator

RF/microwave filter is an indispensable device for ground microwave communication system,satellite communication system,wireless transceiver system and military electronic reconnaissance,interference and defense equipment.It can reduce the impact of interference signals on the system in the process of data transmission and reception.The traditional filter can realize the tuning of fixed center frequency or bandwidth.In multi-channel communication,the traditional filter can be combined as a filter unit to form a filter module,but it will increase the size and complexity of the whole multi-channel communication circuit.The appearance of adjustable filter makes it possible to process multiple signals on one filter circuit,which improves the shortcomings of traditional filter.The adjustable filter can not only increase the integration of the filter module of the multi-channel communication system,but also reduce the size of the multi-channel communication equipment.Based on the basic knowledge of microwave,the basic theory of tunable filter and transmission line theory,and combining the microstrip transmission line structure with pin diode,this paper has successfully developed a filter with tunable bandwidth or center frequency,a filter with separately tunable bandwidth and center frequency,and a dual band fully tunable filter.The specific contents are as follows:1.The T-shaped dual-mode resonator with dual resonant mode is theoretically analyzed,and the external coupling and central frequency adjustment of the resonator are realized by loading switching diodes and varactor diodes at the tap and branch ends.The center frequency of the tunable bandpass filter can be continuously tuned between0.87 GHz and 1.94 GHz,and the relative tuning rate of the center frequency is 76%.2.Based on the cross-shaped step impedance three-mode resonator,the switching diode is loaded at the end of the branch that controls the even-mode resonance point to connect to different transmission stubs to change the circuit bandwidth,and the end of the transmission stub is loaded with a variable voltage.Capacitor diodes are used to tune the even-mode resonance frequency of the filter.The center frequency of the finally realized wide-range bandwidth-tunable tunable filter is at 1.55 GHz,and the absolute bandwidth can be continuously tuned between 238MHz-1028 MHz.3.Based on the cross-shaped step-impedance three-mode resonator,and adding varactor diodes to the main channel and the even-mode branch at the same time,a filter whose bandwidth and center frequency can be individually tuned can be realized.When the absolute bandwidth is fixed at 340 MHz,the center frequency can be continuously tuned between 2.2GHz-2.75 GHz,and the relative tuning rate of the center frequency is22.2%;when the center frequency is fixed at 2.4GHz,the absolute bandwidth can be continuously tuned between 100MHz-617 MHz tuning.4.Based on the parallel coupling line indirect coupling feed structure,combined with the circuit models of T-shaped and cross-shaped resonators,a T-shaped dual-mode dual-band tunable filter and a cross-shaped three-mode dual-band tunable filter were developed.These two dual-band tunable filters with different structures both realize independent tuning of bandwidth and center frequency,and can ensure the stability of another passband when tuning one passband.