Compact Differential Wideband Bandpass Filters Based on Half-Wavelength Lines

A compact differential wideband bandpass filter is proposed based on three half-wavelength transmission lines. The main advantages of small size and simple configuration are achieved. The improved design with two cascaded quarter-wavelength coupled lines can further improve the bandwidth of common-mode (CM) suppression and frequency selectivity. Two prototypes centered at 1.8 GHz with the size of $0.07\lambda _{g}^{2}(\lambda _{g}$ is the guided wavelength at the center frequency $f_{0}$ ) and $0.056\lambda _{g}^{2}$ are designed, fabricated, and measured. The measured results of Design I (Design II) exhibit the minimum insertion loss of 0.27 dB (0.6 dB), 3-dB fractional bandwidth of 57.8% (52%), and the bandwidth of 20-dB CM suppression of $0.67 f_{0}~(4.1 f_{0})$ .