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A differential-mode wideband bandpass filter on microstrip line is proposed with three transmission zeros under common-mode operation. A four-port branch-line differential-mode bandpass filter is implemented by linking two vertical arms using a stub-loaded connecting line. As such, its two-port symmetrical bisection behaves like a wide bandpass or bandstop filter topology under differential- or common-mode excitations. After its differential-mode bandpass filter is synthesized, three transmission zeros can be created under common-mode operation by adjusting the length and width of the connecting line and open-circuited stubs. It results to produce a widened stopband under common-mode operation. In final, a differential-mode wideband bandpass filter is designed and fabricated. The predicted and measured performances are found in good agreement with each other, showing good common-mode suppression with an insertion loss close to 10 dB over the differential-mode passband with a fractional bandwidth of 80%.