A Flat-Passband Predistorted Bandpass Filter With Folded Substrate Integrated Waveguide

A predistorted folded substrate integrated waveguide (FSIW) filter is developed for a flat passband. The unloaded ${Q}$ -factor of FSIW cavity is predicted numerically and then applied to the coupling matrix synthesis. A filter prototype is fabricated with the low temperature cofired ceramic technology (LTCC), where the direct and cross couplings are realized in a 3D configuration. The synthesized, simulated and measured results agree well with each other. The insertion loss is 3.48 dB at 5 GHz and the 0.2-dB bandwidth is 338 MHz which is equivalent to a conventional filter with the uniform unloaded ${Q}$ -factor of 870, much higher than the one of 180 provided by the applied FSIW resonator. Two transmission zeros locate at 4.50 and 5.46 GHz to improve the frequency selectivity. The filter size is only $18.04\times 9.2\times 1.4$ mm ³ and the occupied area is $0.72\times 0.37\,\,\lambda _{d}^{2}$ .