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 0.07λ
_g
²
(λ
_g
is the guided wavelength at the center frequency f
₀
) and 0.056λ
_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
).