Compact Elliptic Resonator-Based Lowpass Filter Design with Ultrawide Stopband Performance

This paper presents the design of a compact low-pass filter based on third-order elliptic resonator for application in radio-frequency and microwave systems. The proposed filter operates at a frequency of 3.6 GHz, achieving an insertion loss of 0.04 dB and a return loss exceeding 30 dB across the passband. To enhance the stopband performance, two pair of attenuators were integrated, enabling a wide rejection range from 3.6 GHz to 20 GHz. Additionally, complementary split-ring resonators were implemented in the ground plane to further improve the rejection levels, achieving an attenuation of -20 dB. The proposed low-pass filter is very compact, with a total size of just 54 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2.</sup> The simulation results demonstrate the filter's effectiveness in suppressing unwanted harmonics across an ultrawide stopband, with a remarkable balance between size and performance.