General Dual-Band Impedance Transformer With a Selectable Transmission Zero

This paper presents general design formulas for a dual-band impedance transformer with an inherent and selectable transmission zero. The inherent transmission zero can block unwanted signal or interference and improve the overall circuit performance by suppressing spurious response and enhancing selectivity. The considered load has unequal complex impedances at two uncorrelated frequencies. The proposed transformer consists of three sections of transmission lines and two shunt stubs. The parameters of each transmission line and stub are obtained using the derived analytical formulas without any time-consuming try-and-error procedure or numerical solution. The proposed design formulas reduce the need to use a transmission line with extremely high or low impedance. Two impedance transformers are designed and fabricated. Good agreement between numerical and measured results validates the proposed design formulas.