A Wide Stopband Low-Pass Filter With High Roll-Off Using Stepped Impedance Resonators

In this letter, a microstrip low-pass filter (LPF) is presented with outstanding merits such as sharp roll-off, low insertion loss, and wide stopband bandwidth. The structure of the proposed filter comprises three coupled stepped impedance resonators which offer a very high selectivity. To extend the stopband rejection bandwidth, two circular slots are etched out in the ground plane. The lumped equivalent model of the proposed distributed LPF is extracted and validated by simulation and theory. The fabricated LPF demonstrates a small transition bandwidth of 50 MHz (-3 and -25 dB at 1.95 and 2 GHz, respectively) and high roll-off rate of 440 dB/GHz. The stopband bandwidth extends up to 16 GHz (8.2f
_c
harmonic suppression) with an attenuation level better than -12 dB. Insertion loss is 0.4 dB at 1 GHz, and group delay is <;1.73 ns in the entire passband. The size of the proposed filter is 0.03357λ
_g
²
.