Compact ultra‐wideband differential bandpass filter using self‐coupled ring resonator

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 (1.0 λ 0 × 0.03 λ 0 ) and it is only 15% of the size of the most reported differential BPFs. The simulated and measured results are in good agreement.