Analysis And Design Of Highly Selective Planar Microstrip Filters

Microwave filters are essential components in RF circuit of the modem communication and radar systems.With further development of communication technology,the planner microwave filters with compact sizes are critically required.Due to the limited frequency spectrum,high frequency selectivity and out-of-band suppression of microstrip filters are also demanded.The multiband filters have greatly stimulated the multiband communication systems,while the balanced filters can provide better performance in suppressing common-mode interference,and they are widely used in the field of communications or radar.Based on the analysis of transmission zeros and some resonators,this thesis focuses on analysis and design of the bandpass filter,multiband and balanced filter for wireless communication application.The main contents of this work are listed as follows:1.The design of high selectivity bandpass filter and diplexer:first,two transversal high selectivity narrow-band bandpass filters with improved upper stopband using open/shorted dual-behavior resonator?DBR?are proposed.Three and six transmission zeros are realized respectively in the upper stopband and they can be adjusted freely at both side of passband by only changing the electrical length of the open/shorted stubs,while the passband responses are nearly unchanged.The proposed bandpass filters have advantages of compact effective circuit size,simpler design theory and wideband harmonic suppression.Based on the above discussion,a diplexer with high isolation using T-junction is thus proposed.2.The design of high selectivity dual-band bandpass/bandstop filter:first,the characteristics of ?/4 stepped-impedance resonator?SIR?with unequal electrical length are analyzed.Two novel dual-band microstrip bandpass filter?BPF?with four and six transmission zeros respectively are presented using only a pair of ?/4 SIRs.Four/six transmission zeros of the filters can be adjusted by changing the degree of the coupling of the two resonators.Then,the dual-band bandstop filter using open/shorted coupled lines is designed.The multiple out-of-band transmission poles are analyzed based on odd/even-mode method.A tunable dual-band bandstop filter using center-loaded coupled structure is introduced,in which the lower stopband can be electrically tuned with the fixed absolute bandwidth,whereas the performance of the upper one is kept constant.3.The design of high selectivity wideband balanced filter:Two novel balanced filters based on open/shorted coupled line with four/six transmission zeros are proposed and high common mode suppression up to 3f₀ can be easily realized.The passband response and common mode suppression are achieved using stub-loaded open coupled lines.Transmission zeros close to the differential mode passband are generated by using the transversal signal-interaction concepts.