High performance LPF Structure with sharp roll-off and low VSWR

A symmetrical compact microstrip lowpass filter (LPF) with ultra-wide rejection band and low voltage standing wave ratio (VSWR) using a novel structure of stepped impedance and butterfly-shaped resonators is presented and fabricated. The LPF has a 3 dB cut-off frequency at 1.513 GHz. The series structure of the butterfly-shaped resonators result in a high roll-off rate of 187.8 dB/GHz. By generating multiple transmission zeros, a wide stopband (with 35 dB rejection degree) is achieved. To reach a small circuit, the bended transmission line is properly utilised. Moreover, 171% relative stopband bandwidth and dimensions of 0.087λ_g × 0.157λ_g (λ_g is the guided wavelength at 1.513 GHz) are other specifications of this LPF. In addition, an acceptable match is obtained between the simulated and measured responses. In the end, the LPF achieves a high figure-of-merit of 82288.