Microstrip Tunable Diplexer With Separately-Designable Channels

Abstract

A microstrip tunable diplexer with two separately- designable channels and multiple transmission zeroes (TZs) using common element-loaded uniform impedance resonator (UIR) is proposed. The common UIR operating in third harmonic ( ${f} _{3}$ ) and fifth harmonic ( ${f} _{5}$ ) is used for designing the two diplexer channels. The stub is loaded at the voltage-null point of ${f} _{3}$ on the common UIR and the varactor is loaded at the open end of the stub. The varactor-loaded stub can separately tune ${f} _{5}$ but has no effect on ${f} _{3}$ so that the two channels can be separately designed. Meanwhile, the input port is tap-connected to the common UIR, which necessarily produces a common TZ between ${f} _{3}$ and ${f} _{5}$ , existing in both channel filtering responses. In the tunable high channel design (at ${f} _{5}$ ), the major design concept relies on choosing the properly values of three varactor diodes, the desired external quality factor and coupling coefficient can be easily obtained for realizing the constant absolute bandwidth (ABW). By properly choosing coupling schemes of the two channels, more TZs are realized at the desired locations. For demonstration, a diplexer with a fixed low channel and a tunable high channel with the constant ABW is designed, fabricated and measured.