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A compact ultra-wideband differential bandpass filter (BPF) based on a self-coupled ring resonator is proposed. The self-coupled line contributes to not only three poles in differential-mode (DM) but also three zeros in common-mode (CM) offering both a wide differential passband and good CM suppression. A fabricated filter with a 3 dB fractional bandwidth of 118% for the DM and insertion loss greater than 18 dB for the CM has been designed and measured. This filter is only 30.4 × 1.0 mm (1.0λ0 × 0.03λ0) and it is only 15% of the size of the most reported differential BPFs. The simulated and measured results are in good agreement.