Design of a wideband passive planar duplexer

A wideband passive planar duplexer is introduced in this paper. Its low frequency band range is 10MHz ∼ 3 GHz and high frequency band range is 3GHz ∼ 20 GHz. This duplexer is realized by high-pass and low-pass filters in the form of microstrip structure. Filter circuit components is realized by gold wire inductor and thin-film capacitor which have minimal parasitic parameter. This type of structure can realize wideband filter characteristic for high frequency channel and low frequency channel. In order to reduce the interference between duplexer's low frequency band and high frequency band, low and high frequency band circuits are realized by complementary filter structure and output port admittance (or impedance) is nearly a constant real number in the whole frequency band. There are no abnormal resonance points in two frequency bands. Test results show that this duplexer's insertion loss is less than 3.7 dB at 3 GHz and less than 3.6 dB at 20 GHz. Because of its good performance, this passive planar duplexer has been applied to wideband frequency-agility signal source port to realize combined output. This wideband passive duplexer can effectively solve the problem of leaking of the control voltage to RF channel during signal switching.