Research On Design And Application Of Broadband Bandpass Filter

With its many advantages and good commercial prospects,ultra wideband communication system has gradually become a research hotspot,and gradually from military to civil.UWB filter is an important part of RF transceiver link,and its performance is closely related to the performance of the whole transceiver link.This paper mainly studies the design of planar microstrip ultra wideband bandpass filter and the notch technique in practical application.Firstly,based on the detailed analysis of the in band resonant frequency distribution of the transmission line structure with short-circuit branches,a 13 order Chebyshev broadband filter with linear arrangement is designed.Then,the filter is miniaturized by the method of ring structure and common ground in the middle.In order to further improve the performance of out of band suppression,a 9-order low-pass filter based on stepped impedance structure is cascaded.Finally,a miniaturized ultra wideband filter with high out of band suppression performance is designed and fabricated.Secondly,the layout of the odd and even mode resonant frequency and the cause of the two transmission zeros of the passband of the stepped impedance(SIR)resonator are analyzed in detail,and the rudiment of the T-type five mode ultra wideband bandpass filter is designed.Then the circuit is miniaturized.The stopband suppression performance of the filter is optimized by adding open notch branch.Finally,the influence of the stepped impedance structure at the tap on the in band reflection is explored,and the in band reflection of the filter is further optimized by adjusting the impedance ratio of the stepped impedance structure at the tap.Thirdly,considering that there are many other civil frequency bands that interfere with UWB communication in practical application,starting from the above designed T-type multimode UWB filter,the notch unit with stepped impedance structure is added to realize the anti-interference notch in fixed frequency band.Firstly,the pre designed notch unit is loaded on the main resonator of T-shaped multimode structure,and a fixed single notch UWB bandpass filter is designed.The center frequency of the notch is set at 8 GHz of X-band,which is commonly used in satellite communication.Furthermore,by adding a notch cell side by side,the center frequency of the notch is set at 5.8GHz in the high frequency band of WLAN and 8GHz in the X-band of satellite communication,and a fixed dual notch UWB filter is successfully designed.In order to improve the notch depth and anti-jamming performance,an improved fixed double notch ultra wideband filter is designed.By analyzing the current density distribution of the notch filter in detail,the generation mechanism of each band notch is clarified.Fourthly,an adjustable notch cell is designed by adding varactor diode to the fixed notch cell.The tunable notch elements with the same size are coupled at different positions of the main transmission line of the UWB filter,and the UWB filter with tunable notch function is successfully designed.