A Compact Sharp Roll-Off Microstrip Lowpass Filter With Ultra-Wide Stopband

This paper presents a compact microstrip lowpass filter (LPF) with sharp roll-off and ultra-wide stopband using novel stepped impedance hairpin resonators (SIHRs). The resonator consists of a conventional SIHR with an interdigital coupling introduced between adjacent stepped impedance stubs and an embedded square stub loaded coupled-line structure. The interdigital coupling is utilized to strength the roll-off sharpness, while the embedded coupled-line structure is applied to extend the stopband. A protype of LPF based on two cascaded asymmetrical SIHRs is implemented, featuring a sharp rolloff and an ultra-wide stopband from 1.72 GHz to 24 GHz with a suppression level of 20 dB.