Compact wideband lumped balun with out-of-band suppression using tail inductor

A novel compact 2.45 GHz wideband lumped balun with out-of-band suppression using a tail inductor has been proposed. By adding an extra transmission zero in even-mode around operation frequency, the tail inductor can improve phase balanced performance and enlarge operational bandwidth of the balun. Besides, the tail inductor together with a band-stop resonator can realise out-of-band transmission zero in even-mode. The measured results illustrate a relative 41% bandwidth, namely from 2.35 to 3.35 GHz. Over the operation frequency, a less than 0.5 dB amplitude imbalance and a less than +/- 7 degrees phase error have been demonstrated. The out-of-band suppressions of two output ports at 5.3 GHz are 24.5 and 27.5 dB, respectively.