Compact microstrip LPF微带低通滤波器

Compact microstrip lowpass filterC.Jianxin,Y .Mengxia,X.Jun and X.QuanA novel compact microstrip lowpass filter using the slow-wave resonator is described.This filter has the advantages of low insertion loss in the passband,high and wide rejection in the stopband and compact size.A demonstration filter is designed,fabricated and tested.Theoretical and experimental results are presented.Introduction:Lowpass filters have been studied and exploited exten-sively as a key block in modern communication pact design,low insertion loss in the passband and high rejection in the stopband are necessary.To meet these requirements,there has been much effort to develop a variety of compact lowpass filters.Recently,a type of newly developed microstrip lowpass filter using the slow-wave resonator was realised,which is widely used because of its advantages of size reduction and spurious suppression in applications.A compact semi-lumped lowpass filter was designed [1],which has the capability of harmonics and spurious suppression.In [2],a lowpass filter using a single microstrip stepped impedance hairpin resonator with direct-connected feed lines was proposed.In [3],a lowpass filter using coupled lines was proposed,which has two attenuation poles in the stopband.In this Letter,we propose a novel compact microstrip lowpass filter with loading capacitance.The proposed lowpass filter has the advan-tages of compact size,low insertion loss,high rejection and wide stopband frequency response.Filter design:Fig.1shows the proposed lowpass filter with symmetric structure,which is composed of a high impedance micro-strip transmission line and two arms at the two ends of the high impendence line.Fig.2shows the equivalent circuit of the proposed lowpass filter.Through end coupling of two arms,loading capacitance and slow-wave effect are realised,the size of the proposed lowpass filter being greatly reduced compared with the conventional lowpass filter.Beyond the advantage of compact size,the effect of the loading capacitance can shift the spurious resonant frequency from the integer multiples of the fundamental frequency,and then the stopband between the first and second resonant frequencies can be extended [2].As can be seen from Figs.1and 2a ,the microstrip transmission line l a represents the 2Âl 1(the length of the high impedance line),the transmission line l b represents the length of microstrip line with width w 2.The capacitances C g 1and C g 1represent the two loading capaci-tances resulting from the two end couplings,respectively.C p repre-sents the capacitance between the microstrip and the ground.The transmission lines l a and l b can be treated as inductors.Therefore,a lumped-element equivalent circuit in Fig.2b is evolved,where the discontinuities of the microstrip are not considered.Observing the equivalent circuit,it has the characteristics of a typical elliptic-function lowpass filter.We simulate the proposed lowpass filter using Ensemble of ANSOFT.The low insertion loss in the passband,sharp and wide rejection in the stopband and compact size arerealised.Fig.1Proposed lowpassfilterFig.2Equivalent circuits of lowpass filtera Equivalent circuitb Lumped-element equivalent circuitMeasurement results:An experimental lowpass filter was designed,and fabricated and tested.The filter was fabricated on substrate with a relative dielectric constant e r ¼9.6and a thickness of 0.8mm.The dimensions of the proposed lowpass filter are as follows:g 1¼g 2¼0.4mm,w 1¼0.22mm,w 2¼0.3mm,w 3¼0.6mm,l 1¼4.4mm,l 2¼4.29mm,l 3¼0.99mm,l 1¼l 2þw 1=2,l 3þw 1=2¼l 1=4,and w ¼0.76mm and l ¼3mm are the width and length of feed line with 50O ,respectively.As can be seen from Fig.3,the insertion loss from DC to 2.5GHz is less than 1dB,the return loss is better than 10dB in the passband.Two attenuation poles are realised at 4and 5.4GHz.No spurious response occurs for the frequency up to 6GHz.Fig.3Experiment results of lowpass filter ——j S 21j ----j S 11jConclusion:A novel compact microstrip lowpass filter using the slow-wave resonator is proposed.The lowpass filter is not only of compact size,but has low insertion loss in the passband and high rejection in the stopband.The stopband between the first and second resonant frequen-cies was extended.Its low insertion loss,sharp and wide rejection and compact size are desirable in communication systems.ELECTRONICS LETTERS 27th May 2004Vol.40No.11#IEE200415February2004 Electronics Letters online no:20040481doi:10.1049/el:20040481C.Jianxin,Y.Mengxia and X.Jun(Institute of Applied Physics, University of Electronic Science and Technology of China,Chengdu, Si Chuan,People’s Republic of China)E-mail:jjxchen@X.Quan(Department of Electronic Engineering,City University of Hong Kong,Tat Chee83,Kowloon,Hong Kong)References1Sheen,J.-W.:‘A compact semi-lumped low-passfilter for harmonics and spurious suppression’,IEEE Microw.Guid.Wave Lett.,2000,10,(3), pp.92–932Hsieh,L.-H.,and Chang,K.:‘Compact lowpassfilter using stepped impedance hairpin resonator’,Electron.Lett.,2001,37,(14), pp.899–9003Lee,D.H.,Lee,Y.W.,Park,J.S.,Ahn,D.,Kim,H.S.,and Kang,K.Y.:‘A design of the novel coupled line low-passfilter with attenuation poles’, IEEE MTT-S Microw.Symp.Dig.,1999,3,pp.1127–1130ELECTRONICS LETTERS27th May2004Vol.40No.11。