A technique for the passband and stopband control of wideband band-pass substrate integrated waveguide (SIW) filters designed with elliptic stepped impedances

Abstract

In this paper, a design technique based on a stepped impedance synthesis is proposed for the first time for the passband and stopband control of band-pass filters in substrate integrated waveguide (SIW) technology. The structure of the band-pass filters is implemented by means of elliptic patterns etched on the top side of the SIW, which work as impedance inverters. The design method consists of previously mapping the height of the elliptic patterns and the phase shift (cut-off angle) of the SIW lines, connected to the input and output ports of the ellipses, as a function of the passband and stopband characteristics of various stepped impedance SIW filters, to then select the design parameters of the band-pass SIW filter that achieve the desired frequency response. This new approach is applied to the design of two fourth-order Chebyshev stepped impedance band-pass SIW filters with different passband and stopband characteristics. One of the band-pass SIW filters is characterized by a fractional bandwidth FBW1 = 14.5% and a spurious-free range SFR1 = 2.8 GHz, while the other filter has a FBW2 = 22.3% and a SFR2 = 1.55 GHz. Both band-pass filters were designed with a SIW TE10 mode cut-off frequency and a higher passband cut-off frequency fixed at fcTE10 = 2.85 GHz and fc,H = 3.9 GHz, respectively. Electromagnetic (EM) simulations showed that the lower passband cut-off frequency fc,L can vary with respect to fc(TE10) by 13.6% and that the first spurious frequency can appear higher than fc,H by 69.3%. The measurement results agreed well with the EM simulations, thus confirming that this method can be useful to design high-performance wideband Chebyshev stepped impedance band-pass SIW filters with a high degree of control on the passband and stopband characteristics.