A Tunable low Pass Filter Based on Transmission Lines With Tunable Input/Output Impedance

A tunable low pass filter (TLPF) based on the tuning of input/output impedance was presented in this letter. The TLPF mainly consisted of improved quarter-wavelength stubs. The input/output impedance of the improved quarter-wavelength stubs can be tuned in a certain range. The design procedure of this TLPF was derived from the filters based on the quarter-wavelength transmission stubs. Through the tuning of the input/output impedance of the transmission lines, the cut-off frequency of the TLPF can be adjusted in a certain range. The TLPF has relatively good designability and can be easily changed to meet other performance levels. Finally, a TLPF was designed, fabricated, and measured. The measured results verify the effectiveness of the design method.