Design of Wideband Substrate Integrated Waveguide Filters With Tunable Lowpass Characteristics

In this paper, the application of the stepped impedance synthesis technique in the design of tunable wideband substrate integrated waveguide (SIW) Chebyshev filters is explored. By employing Chebyshev transfer functions and stepped impedances, achieved through impedance inverters, a Chebyshev bandpass response can be attained by combining the lowpass characteristics of this stepped impedance technique with the highpass behavior of the SIW (quasi-TE10 mode). The novel contribution of this work lies in extending this approach to designing tunable wideband SIW filters using varactor diode tuning elements. Unlike existing design methods of tunable SIW bandpass filters, the proposed technique fixes the lower cut-off frequency of the bandpass response around the cut-off frequency of the SIW TE10 mode, while essentially allowing adjustment of the higher cut-off frequency. This adjustment is facilitated by varying the impedance inverters and phase shifts of the filter sections. This is achieved by inserting varactor diodes between elliptic patterns and the SIW structure, and between SIW line sections and floating metallic posts. This innovative method has been applied to a third-order tunable wideband SIW filter. The measurements of the manufactured prototype have offered a significant tuning range (15.3%) with high-performance frequency responses (return loss higher than RL =13 dB and insertion loss lower than IL =1 dB), and a completely different tuning mechanism as compared to previous approaches.