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A novel tunable bandpass filter with constant absolute bandwidth is presented in this paper. Introducing a mixed electric and magnetic coupling structure and a new external coupling scheme, nearly constant absolute bandwidth is maintained across the tuning range. Theoretical analysis and design approach are demonstrated. The proposed filter is designed with a frequency coverage of 0.78-1.08 GHz and constant 3-dB bandwidth of 50 MHz over the tuning range. The experimental results show good performance of the filter and validate the design concept.

Abstract-In this paper, a compact tunable dual-stop-band filter is proposed. The proposed filter is based on the combination of double H-shaped defected ground structure (HDGS) and E-shaped defected microstrip structure (EDMS). The loaded HDGS/EDMS varactor diode is introduced to realize the tunable dual-stop-band filter. The equivalent-circuit models and theoretical analysis of the proposed structure are presented; also its performance evaluation is compared with traditional structure. The proposed filter has the characteristic of two independently adjustable stopbands and wide tuning range. EDMS also shows size reduction up to 38% compared with the Tshaped defected microstrip structure. The measured performance of the tunable dual-stop-band filter agrees well with the simulation results.

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