Tunable bandpass filters with constant absolute bandwidth

This paper presents the design of novel tunable bandpass filters with constant absolute bandwidth. A novel mixed electric and magnetic coupling scheme is utilized. Theoretical analysis indicates that it is able to achieve desired coupling coefficients between the proposed resonators at various frequencies so as to obtain constant absolute bandwidth. For demonstration, a frequency-tuning filter is implemented with 80-MHz constant absolute bandwidth. Experimental and simulated results are presented to verify the theoretical predications.