Symmetrical Four-Port Reflectionless Network And Its Application For Wideband Filter Design

In modern wireless communication systems,bandpass and band-stop filters are increasingly needed to choose the wanted signal or reject interference.In many cases,passive filter is cascaded with active circuit.Traditional filter will reflect the signal out of the passband to the active circuit,thus introduce interference to the active circuit.In order to avoid such interference,reflectionless filter is attractive.In the past years,various types of reflectionless passive circuits have been proposed,including reflectionless filter,reflectionless coupler and so on.However,most of these circuits are reflectionless at input ports,which can not achieve reflectionless of all ports.On the other hand,most of these passive circuits are implemented with lumped elements.Due to the parasitic effect,the insertion loss is great in relatively high frequency band,and it is difficult to achieve broadband.In this dissertation,a four-port reflectionless network is introduced and an all-port reflectionless bandpass/bandstop filtering circuit based on this network is proposed.Firstly,a four-port filter network,which consisting of two pairs of band-pass filter and band-stop filter alternately connected into a ring,is introduced.Based on the ABCD matrix theory,the S parameter relationship between the two-port bandpass filter and bandstop filter is calculated when the four-port network have no reflection signal of all ports.Secondly,the structure of two-port bandpass filter and bandstop filter are introduced.Both of them adopt microstrip and defected ground structure(DGS).The bandpass filter adopts T-junction and DGS to strengthen the coupling strength.The bandstop filter introduces resonance points by microstrip and DGS.Finally,the four-port circuit is adjusted to reduce the return loss.The measured results show that the center frequency of the filtering circuit is 4.34 GHz,the fractional bandwidth(FBW)of 3d B is 58%,and the minimum passband insertion loss is 0.34 d B.Meanwhile,in the range of 0.01-9 GHz,the return loss of all ports is lower than-10 d B.The in-band isolation between ports 1 and 4 is better than-13 d B.It can be seen that the reflectionless filter has the advantages of low return loss,wide band and multi-function,which has strong application value in the wireless communication system.