In order to accurately design microwave, millimeter-wave components and circuits, a simple and accurate method for extracting extremely wideband characteristics of the printed circuit board (PCB), including relative dielectric constant, dielectric loss tangent, and equivalent copper conductivity, is proposed and validated in this paper. By properly selecting wideband distribution of multiple TMmn0 modes in the designed circular substrate integrated waveguide (CSIW) cavities, extremely broadband properties of PCB can theoretically be extracted with only one pair of CSIW cavities. The proposed method overcomes the limitations of conventional methods that only support narrowband applications, require a large number of cavities, and have relatively large errors. The experiments show that the proposed method is able to extract, accurately and efficiently, 8-110 GHz characteristics of a TLY-5Z substrate-based PCB and the extracted results are verified by designing and measuring the fourth-order Chebyshev filter at X-band.