A 1.1-3.1-GHz tunable quadruplexer with compact size and bandwidth control

This paper proposes a varactor-tuned quadruplexer that allows simultaneous control of the center frequency and bandwidth of each channel and independent control on different channels. To achieve compactness, each channel consists of only a dual-mode resonator where the even-and odd-mode resonances contribute to a two-pole response. The fabricated prototype shows that, for the input return loss greater than 10 dB, the available tuning ranges of the channel frequency and the 3-dB bandwidth are 1.1-1.4 GHz and 100-140 MHz, 1.55-1.9 GHz and 120-150 MHz, 2.05-2.4 GHz and 190-240 MHz, and 2.63-3.1 GHz and 240-270 MHz, respectively, for channels 1, 2, 3, and 4. The channel insertion loss is less than 5 dB and isolation is greater than 25 dB for all tuning states. The quadruplexer has a footprint of 0.45λ_g× 0.25λ_g, where λ_gis the guided wavelength at 1.1 GHz.