Wideband balun using a transmission line transformer

초록

A wideband balun consisting of a transmission line transformer (TLT) and matching and phase conversion circuits is proposed and fabricated. A TLT using three 37.5-Omega coaxial cables converts the impedance from 50 to 25 Omega, and a matching circuit using four resistors from a singly terminated filter improves the matching and isolation characteristics over a four-octave range. A phase conversion circuit is implemented by cross-connecting two coaxial cables. The fabricated balun provides matching (S-11, S-22) and isolation (S-23) characteristics of about -15 dB, an output magnitude difference within 0.3 dB, and an output phase difference within 180 degrees +/- 1.4 degrees from 62 to 960 MHz.

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