A triple-mode X-band microstrip ring resonator filter

This paper presents the design, fabrication and measurements of a low-loss and compact triple-mode ring resonator filter operating at X-band. This filter is fabricated on a 635-µm thick duroid substrate (εr = 10.2, tan δ = 0.0023). A classical ring resonator topology with a perturbation allows the first mode to split into two degenerate modes while an additional serpentine line in the middle of the ring which also includes a perturbation and a coupling line creates a third mode whose resonance frequency can be tuned towards the two first mode resonances. Therefore, a third-order response filter is achieved. This filter exhibits insertion loss of 1.67–2.07 dB in the [9.5–10.5] GHz frequency range while return loss is better than 14.7 dB. The 1-dB fractional bandwidth is 13.2%. The presented topology allows a footprint size reduction compared to standard filter topologies.