Design of Compact Quad-Channel Diplexer Using Quad-Mode Stub-Loaded Resonators
In this paper, a compact quad-channel diplexer utilizing quad-mode stub-loaded resonators (QMSLRs) is proposed. The proposed quad-channel diplexer is composed of two dual-band bandpass filters (BPFs) based on QMSLRs and source-load coupling lines. Due to the symmetry of the proposed quad-mode resonator, its resonance characteristics are analyzed by using the even-odd-mode method twice. All four modes equivalent circuits of the resonator are quarter wavelength resonators, so the circuit size can be very compact, and the first resonance frequencies is three times of the fundamental ones. Stub-to-stub coupling is introduced to split two identical resonance modes, which is in favor of implementing dual-band BPF by using a single quad-mode resonator. By tuning the corresponding physical dimensions of the stubs, the resonant modes can be individually adjusted. The source-load coupling lines are properly designed to provide appropriate external coupling for the passbands and high isolation level between channels. For demonstration, a quad-channel diplexer (0.9/1.2 GHz at Load 1 and 1.5/1.8 GHz at Load 2) using the quad-mode resonator is designed, fabricated and measured. The simulated and measured results with good agreement are presented.