Design and fabrication of a compact microstrip low-pass filter with ultra-wide stopband and sharp roll-off-rate

Microstrip low-pass filters (LPFs) with good performance are used in most telecommunication systems. In this study, a modified T-shaped hairpin resonator is used to design LPF, and suppression cells are used to improve the performance of the filter in the stopband up to 22 GHz. The proposed LPF had a -3 dB cut-off frequency of 1.89 GHz. The proposed LPF is designed, fabricated and measured. The simulated and measured results for the proposed LPF are in good agreement with each other. Attenuation in the stopband is 34 dB, which is an extremely large number. Roll-off-rate (ROF or ζ) parameter is sharp and equal to 96.3 dB/GHz. The group delay in the passband is almost constant and in the range of 0.3-0.42 ns. In the following, an SMV-1247-079LF SMD varactor diode is used to adjust the proposed LPF. The tuning range for the proposed LPF is 0.46-3.2 GHz.