Common-mode excitation caused by an imperfect ground plane on a printed circuit board (PCB) has been conventionally explained with the 'current driven' scheme, in which the common-mode current is driven by the ground voltage across the unintentional inductance of the ground plane. We have developed an alternative method for estimating common-mode excitation that is driven by the difference of the common-mode voltages for two connected transmission lines. A parameter called current division factor (CDF) that represents the degree of imbalance of a transmission line explains the common-mode voltage. In this paper, we calculate the CDF with two-dimensional (2-D) static electric field analysis by using the boundary element method (BEM) for asymmetric transmission lines with an arbitrary cross-section. The proposed 2-D method requires less time than three-dimensional simulations. The EMI increase due to a signal line being close to the edge of the ground pattern was evaluated through CDF calculation. The estimated increase agreed well--within 2 dB--with the measured one.

Concerned is a spectral profile of electromagnetic (EM) emission from a signal line on a high-speed digital circuit. The authors have proposed and examined an a priori method to predict the peak frequencies on spectral profile of EM emission from printed circuit boards (PCBs). Profile of an EM spectrum is determined by the resonance of digital circuits. It is the purpose of this paper to investigate the parameters that determine the spectral profile of EM emission from a signal line on a PCS. In this paper, measurements and calculations of EM spectra were carried out for different load capacitances. EM emissions were measured with a small loop antenna at a 50mm from the surface of the PCB. Measured EM spectra had two peaks. Calculated EM spectra, which was based on transient current given by the analog simulator SPICE, had two peaks too. Results of calculations of EM spectra for different internal capacitances of an IC tell that lower peak frequency is determined by the resonance frequency of the resonant loop which is composed of an IC package and a decoupling capacitor. Comparison with measured EM spectra and calculated EM spectra for different load resistances tell that sharpness of the other peak depends on Q factor of a resonant loop which includes a signal line. Therefore the peak frequencies of EM emission spectrum can be predicted as two resonance frequencies of two resonant circuits.

A de-embedding technique for the measurement of very small parasitic capacitances of package or small module interconnects is presented. At high frequencies small parasitic capacitances become important, and measurement probes can strongly affect measurement results. The present technique is based on additional measurements with only one tip of the probe touching one conductor, while the second tip is kept floating on the substrate. A necessary condition for its application is that the measured capacitance does not depend on the position of the floating probe tip. Measurements with inverted probe tip polarities are also used. In this way, the capacitances between probe tips and DUT can be estimated together with the parasitic capacitances of interest. Depending on the required accuracy, de-embedding of different orders have been introduced, which consider capacitance configurations of increasing complexity. The technique requires the solution of one or more systems of non-linear equations. In the present example the minimization of the norm of the residual of the system has been treated as a least squares problem, and has been solved numerically with MATLAB. The accuracy of the measurement can be also approximately estimated with the residual. As application example, a small module with power and ground planes has been considered. Two different probes have been used. Even though the stray capacitances of the probes are very different, the values of the extracted parasitic capacitances are in agreement with each other. The accuracy has been verified also with simulation results. To this purpose, a combination of known formulas from the literature, a 2D Finite Element Method (FEM) tool and a 3D Boundary Element Method (BEM) tool have been used. A high accuracy can be obtained, even when a strong capacitive coupling between probe ground and DUT is present. The technique can be applied also when only a subset of measurement results are available.