Tunable SIW Bandpass Filters with Improved Upper Stopband Performance

This brief propose the design and measurement of tunable, highly selective and wide-stopband microwave filters using substrate integrated waveguide (SIW) technology. A two pole filter is designed using optimized cavity parameters. Two higher order (4-pole) filters are designed by connecting two cavities vertically and horizontally through coupling slots. In this way, four poles are achieved in two cavities, in which the proposed filter produces $2\times \text{N}$ poles in N cavities. A varactor diode is employed in both configurations to make the filters tunable. The proposed filters have wide upper stopband performance [20 dB up to $4.8f_{1}$ (Filter 1) and 20 dB up to $4.2f_{1}$ (Filter 2)], lower insertion losses [0.2-1.1 dB (Filter 1) and 0.2-1.5 dB (Filter 2)] and good tunability ranges [2.7–3.4 GHz (Filter 1) and 3.1–3.9 GHz (Filter 2)]. In order to validate the proposed design, a prototype is fabricated on RT/duroid 5870 substrate and measured. The simulated results accord closely with the measured results.