Research And Design Of Reflectionless Microstrip Filtering Circuits

As a key component for signal selection in RF frontend system,the filtering circuit remains one of the hot spots in microwave technology research.With the rapid development of wireless communication technology,higher performance requirements have been paid on the filtering circuits.However,typical filters suffer from multiple reflected out-of-band signals,which constantly inject into adjacent active circuits and cause unstability issue,increased spurious signals,reduced linearity.To this end,a reflectionless filter with absorbed reflected signal has been paid more and more attention.In addition,planar microstrip circuits have been widely used in RF system,with obvious advantages of easy,simple and cost-efficient planar integration.To this end,based on planar microstrip structure,multiband fixed-frequency reflectionless filters and independently tunable dual-band reflectionless filters with standard terminal impedances,interstage reflectionless matching network with complex input impedance,and wideband reflectionless filtering phase shifter have been studied in this work,from circuit theory,structure and design method.Some new ideas and solutions for reflectionless microstrip filtering circuits have been provided.This work focuses on the following aspects:（1）In order to extend the absorbed frequency range on reflected signals,the multiband reflectionless filter architecture by reusing even-mode resonances of multimode resonantor has been proposed firstly.By reusing the even-mode resonant poles in passband branch as the resonant zeros in stopband branch,the corresponding transmission response between two branches can be guaranteed.Then,a wide absorptive range from DC upto harmonic frequency can be obtained.Menawhile,the required half number of the resonances are saved for the stopband branch.Secondly,based on the proposed architecture,three compact multiband reflectionless filters are designed and implemented,including the high selectivity dual-band and triple-band reflectionless filter,the dual-wideband reflectionless filter with capacitive coupling structure.The measured absorptive bandwidth covered DC～6 GHz or DC～10 GHz,with sizes of less than 0.22λ_g².Compared to the reported duplexer and directional architectures,the absorptive bandwidth limitation is broken in the proposed architecture with reduced size by more than 50%.（2）To be compatible with more communication standards and applied in the reconfigurable system,tunable dual-band reflectionless filter has been further studied.To address the correlation influence issue between passbands,single multi-mode resonantor utilizing parallel resonant paths for independent resonant poles in the bandpass branch,and the absorptive networks utilizing series resonant paths for independent resonant zeros in the bandstop branch have been proposed.Based on these independent resonant poles and zeros,the independently tunable reflectionless bands can be achieved.Secondly,the attenuated transmission amplitude and absorbed reflection signals of harmonic passbands can be achieved by reusing the absorptive network to establish additional transimission path for harmonics.Based on the propsoed design method,a dual-band reflectionless filter has been designed and implemented with independently tunable passbands at 1.45～1.95 GHz and2.45～3 GHz,continuous absorptivity on reflections,and absorbed harmonics at 7.4 f₀.（3）Aiming at the complex output impedance of the prestage active circuit,a reflectionless interstage matching network design with complex impedance has been further proposed,based on the above mentioned reflectionless filter with standard terminal impedances.Matched complex impedance at full band is realized by reusing capacitor for in-band matching as the impedance inverter,and combining with the resistive element.Furthermore,based on the proposed reflectionless network,the adjacent active circuits achieve an absolute stability state at full band,without unexpected oscillations.Compared with the typical matching structure,the stopband stability can be improved by 11 times.Finally,based on Si Ge Bi CMOS technology,the proposed reflectionless complex matching network has been designed and implemented with matched impedance from DC upto 110 GHz,and basically absorbed reflection signals at output port.（4）In order to further simplify the system architecture,reflectionless filtering phase shifter has been researched further.To address the narrow operational bandwidth,multiple stopband reflection signals and large phase deviation in the typical tunable passive filtering phase shifter,a wideband reflectionless filtering architecture is proposed firstly.Utlizing the capacitor-loaded coupling structure and inductive absorptive network to directly establish corresponding transmission response,the wide operational bandwidth（128%）,full band absorptivity（DC～12.5 GHz）have been achieved.Then,based on the proposed wideband reflectionless filtering architecture,a design method of reflectionless filtering phase shifter with wideband and low phase deviation is proposed.By utilizing capacitive impedance to compensate impedance variation and reusing inductive absorptive network for additional phase,the in-band phase deviation is effectively reduced to less than±6.5°.Meanwhile,the operational bandwidth of 120%and absorptivity range from DC to 12.5 GHz have been realized.Finally,in the optoelectronic oscillator system,the continuous fine tunabililty on the oscillation frequency of the poposed phase shifter has been verified in wide frequency range.At the same time,based on the automatic feedback control of the phase lock loop module on the phase shifter’s voltage,the oscillation frequency is accurately locked at the expected frequency.Taking 4.74 GHz as an example,the long-term stability of the output oscillation frequency in optoelectronic oscillator system has been achieved.