Lumped isolation circuits for improvement of matching and isolation in three-port balun band-pass filter

A method of improving the output impedance and isolation characteristics of the balun filter is presented in the paper. By introducing the so-called isolation network to the outputs of the balun filter, both the isolation and matching at the output ports of the balun filter, which is derived from a symmetrical four-port network, are improved to a large degree. Two isolation networks including the microstrip-line based and high-pass lumped-element based networks are proposed and implemented on the given balun filter. Compared to the standalone balun filter, the measured output isolation and return loss improvements of the balun filter with the isolation networks are more than 25 dB and 18 dB, respectively, at the center frequency of the output S-parameters. Furthermore, the proposed high-pass lumped-element based network is considerably miniaturized compared to the first microstrip-line based network. Only one-tenth of the length of the corresponding microstrip line is required.