## 4 Results and Discussion
Figure 8: A test vehicle of the CMS-EBG structure fabricated by the multilayer printed circuit board (MPCB) manufacturing process. Figure 9: Measurement setup for the noise suppression characteristics. To compare the noise suppression characteristics of the CMS-EBG structure and the conventional PPW structure, the S\({}_{21}\) parameter of the conventional structure is additionally extracted using the FEM simulation. The conventional structure is a multilayer PPW (M-PPW), which is a stack of PPWs. The layer assignment and via locations are identical to those in the CMS-EBG structure. The difference is that the M-PPW contains no EBG pattern and the VCA branch. Two types of port location are employed for the M-PPW: for M-PPW 1, the port location is equivalent to the CMS-EBG structure; for M-PPW 2, Port 1 consists of two terminals connected to the power plane in L3 and the ground plane in L4, and Port 2 is formed in the same layers. The port positions of M-PPW 1 and M-PPW 2 are identical; however, the layers of port 2 of M-PPW 1 and M-PPW 2 are different. The reason why M-PPW and the corresponding port configurations are chosen as compared structures is because they are widely used in MPCB applications. The measured and simulated results of the noise suppression characteristics of the CMS-EBG structure are compared to the simulated results of M-PPW 1 and M-PPW 2 in Figure 10. It is seen that the electromagnetic noise in M-PPW 1 and M-PPW 2 can be significantly generated in the GHz frequency range considering the high level of resonant modes in the S\({}_{21}\) parameter values. In contrast, the CMS-EBG structure suppresses the resonant modes and achieves a low level of S\({}_{21}\) parameter in the GHz frequency range. In particular, the suppression level is below -40 dB in the frequency range from 2.7 GHz to 7.3 GHz. Thus, the CMS-EBG structure can substantially suppress the GHz electromagnetic noise in the wideband frequency range. The measured results of \(f_{L}\) and \(f_{H}\) show a good correlation with the predicted \(f_{L}\) and \(f_{H}\) by Floquet-Bloch analysis. In addition to the wideband noise suppression, the CMS-EBG structure achieves a notably short distance between the noise source and the victim, which is less than the unit cell length. To prove the distinguished characteristics of the CMS-EBG structure, the noise suppression characteristics of the CMS-EBG structure and the previous stepped impedance EBG (SI-EBG) structure are compared. The previous SI-EBG structure [14] consists of 4-by-4 EBG cells with planar branches, as shown in Figure 11a. Its total number of the EBG cells are the same as that of the CMS-EBG structure. For the port configuration of the SI-EBG structure, the port distance is 4 mm in the unit cell. The measured result of the CMS-EBG structure is compared to the FEM simulation result of the SI-EBG structure in Figure 11b. A stopband of the SI-EBG structure is not clearly observed because its stopband is severely degraded. Thus, the SI-EBG structure with a short source-to-victim distance substantially reduces the  
Figure 10: Comparison of the noise suppression characteristics between the CMS-EBG structure and the conventional multilayer parallel plate waveguide (M-PPW) structures. noise suppression bandwidth and level. On the contrary, the CMS-EBG structure with a short distance substantially suppresses the GHz noise in the wideband frequency range. In numerous MPCB applications, the planar area that the EBG structure occupies is more crucial than the number of layers that it uses. Moreover, a switching device (a noise source) is typically placed notably near the analog and wireless devices (noise victims) in the same MPCBs. The CMS-EBG structure achieves the wideband suppression of the GHz noise with a short source-to-victim distance and a compact area. In consequence, the CMS-EBG structure is the efficient solution for GHz noise suppression in such MPCB applications.