Design Of Multi-parameter Tunable Filter

Due to the rapid development of modern wireless communication technology,spectrum resources are becoming increasingly scarce.In order to use the limited spectrum resources more effectively and reduce the size of the communication systems,a reconfigurable wireless communication system was brought out.As a key component of the microwave reconfigurable system,the performance of the tunable filters determines the communicate quality of the entire system.Therefore,the tunable filter has attracted more and more attention.This article studies the tuning methods of multiple parameters such as the resonant frequency,passband bandwidth and transmission zero of the tunable filter.The details are as follows:First of all,the method of improving the out-of-band suppression of tunable filters is studied in this article.Through introducing a tunable parallel resonant circuit in the input and output structures,the continuous tune of the transmission zeros can be introduced near the passband frequency which can achieve the improvement of the outof-band suppression of tunable filters.In order to verify the effectiveness of the proposed method,a third-order tunable filter based on a U-shaped split ring resonator was designed in this paper.A tunable parallel resonant circuit is introduced at its input and output ends.The tunable filter is fabricated and measured.The measured results show that this method can improve the out-of-band suppression performance of the tunable filter effectively,and has little effect on the passband characteristics.Secondly,the study of the tuning method of the resonant frequency and bandwidth of the tunable filter was carried out.Adopting the principle of impedance tuning,a multiparameter tunable filter with tunable bandwidth and frequency is designed.This filter is based on a quarter-wavelength stub bandpass filter.By loading a tunable device at a specific position of the classical bandpass filter,the tunable input impedance of the quarter-wavelength transmission line is realized,and then the bandwidth tuning of the band-pass filter is realized.At the same time,a tunable resonator is realized by loading a tunable reactance on the open end of the quarter-wavelength stub which can realize the tuning of the center frequency.The measured results are agreed well with the simulation ones,which indicating that the designed multi-parameter tunable filter can realize the tunable bandwidth and center frequency.The transmission zero tunable technology proposed in this paper and the realization technology of multi-parameter tunable filter based on tunable input impedance can be easily extended to the design of tunable filters with other performance.