Design and fabrication of a continuous tunable microstrip lowpass filter with wide stopband and sharp response

Tunable microstrip lowpass filters (LPFs) with a good performance are used in most telecommunication systems. In this article, a third-order circuit with stepped impedance resonator is used to design LPF, and suppression cells are used to improve the performance of the filter in stopbands up to 20 GHz. The −3 dB cut-off frequency can be controlled in the range of 0.35 to 1 GHz with the tuning range of 96%. Rejection stopbands of 30 dB (type-I) and 20 dB (type-II) can be extended to more than 20 GHz. For the passband, the insertion loss variations are in the range of 0.35 to 2.12 dB. The proposed tunable LPF has a sharp roll-off rate (42-96 dB/GHz). The varactor-tuned microstrip LPF is designed, simulated, fabricated, and measured. The results of simulation and measurement for the proposed LPF are in good agreement. The size of the proposed tunable LPF is small and the filter dimensions are equal to 0.267λg × 0.13λg.