Z-domain formulations of equal-length coupled-serial-shunted lines and their applications to filters

A new configuration consisting of parallel coupled lines (PCLs) and nonuniform transmission lines (NTLs) is employed to study lowpass and bandstop filters. In particular, each section of both PCLs and NTLs has the same electrical length. As a result, the transfer functions of such networks are formulated in the Z-domain. An optimisation algorithm is used to tune the characteristic impedances of coupled-serial-shunted lines so that the transfer functions of signal lines are close to the system functions of ideal prototype filters. A Butterworth bandstop filter and two equal-ripple lowpass filters are realised in the form of microstrip lines and their frequency responses are measured to validate this novel method.