Compact, Broad-Stopband Lowpass Filters Using Sirs-Loaded Circular Hairpin Resonators

A novel compact resonator for LPF is proposed in this paper. It is composed of a circular hairpin resonator and a pair of coupled parallel stepped impedance resonators (SIRs) inside. With the loaded SIRs, additional two transmission zeros can be introduced and adjusted easily to cancel the spurious responses for stopband extending, while do not change the filter size. Filters using one and two of the new cells were designed and measured. The two-cell LPF has an insertion loss less than 0.6 dB from DC to 1.6 GHz, including attenuation of double SMA transitions at both sides of the circuit which is about 0.3 dB, and a wide -10 dB stopband from 2.5 to 13 GHz (corresponding to 146% normalized 10 dB stopband), but has a size of only 0.129 × 0.073 .