General methods for reducing printed circuit board (PCB) emissions over a broad band of high frequencies are necessary to meet EMI requirements, as processors become faster and more powerful. One mechanism by which EMI can be coupled off a PCB or multichip module (MCM) structure is from high-frequency fringing electric fields on the DC power and reference planes at the substrate periphery. An approach for EMI mitigation by stitching multiple ground planes together along the periphery of multilayer PCB power-bus stacks with closely spaced vias is reported and quantified in this paper. Power-bus noise induced EMI and coupling from the board edges is the major concern herein. The EMI at 3 m for different via stitch spacing and layer thickness is modeled with the finite-difference time domain (FDTD) method. Design curves and an empirical equation are extracted from a parametric study to summarize the variation of the radiated EMI as a function of layer thickness and stitch spacing.

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EMI mitigation with multilayer power-bus stacks and via stitching of reference planes

Local decoupling, i.e., placing decoupling capacitors sufficiently close to device power/ground pins in order to decrease the impedance of power bus at frequencies higher than the series resonant frequency, has been studied using a modeling approach, a hybrid lumped/distributed circuit model established and an expression to quantify the benefits of power bits noise mitigation due to local decoupling developed. In this work, a test board with a local decoupling capacitor was studied and the noise mitigation effect due to the capacitor placed adjacent to an input test port was measured. Closed-form expressions for self and mutual inductances of vias are developed, so that the noise mitigation effect can then be estimated using the previously developed expression. The difference between the estimates and measurements is approximately 1 dB, which demonstrates the application of these closed-form expressions in the PCB power bus designs. Shared-via decoupling, capacitors sharing vias with device power/ground pins, is also modeled as an extreme case of local decoupling.

/pdf/estimating-the-noise-mitigation-effect-of-local-decoupling-4eu5oh977b.pdf

Estimating the noise mitigation effect of local decoupling in printed circuit boards

A fast time-domain integral equation (TE)-based scheme for analyzing transient electromagnetic compatibility and interference (EMC/EMI) problems involving printed circuit boards and cables/connectors that reside in shielding enclosures, is presented. The proposed algorithm hybridizes a classical marching-on-in-time (MOT) solver for analyzing radiation from perfect electrically conducting (PEC) surface/wire geometries with the previously introduced two-level plane wave time-domain (PWTD) algorithm. The accuracy and efficacy of the resulting MOT-PWTD algorithm are validated via analysis of the radiation characteristics of a number of structures including a loaded motherboard that resides in a chassis. For a problem with N/sub s/ spatial and N/sub T/ temporal unknowns, the computational complexity of the two-level MOT-PWTD algorithm scales as O(N/sub T/N/sub S//sup 1.5/ log N/sub S/) as opposed to O(N/sub T/N/sub S//sup 2/) for a classical MOT scheme.

A two-level plane wave time-domain algorithm for fast analysis of EMC/EMI problems

A systematic approach for inductance extraction for via arrays between two parallel planes is presented. Both self and mutual inductance values are obtained based on a cavity model. The physics associated with the via inductances is analyzed, and a rigorous method is developed to derive an equivalent total inductance for multiple via arrays. Analytical equations for the equivalent total inductance are derived in closed forms for simple cases. The proposed method is corroborated with measurements, and application of the method for power distribution network designs is demonstrated.

Physics-Based Inductance Extraction for Via Arrays in Parallel Planes for Power Distribution Network Design

Two approaches, one based on the circuit concept and the other based on field theory, are used to model the crosstalk between two microstrip lines of finite length and arbitrary orientation. In the circuit-concept modeling, a set of equations for the line voltages and currents has been derived from a modified telegrapher's equation. A four-port network expression is resultantly obtained by solving the equations, thus the crosstalk can be predicted by applying terminal conditions to the network expression. On the other hand, the extended finite-difference time-domain (FDTD) method has been used to model the terminal resistors and the feeding resistive voltage source in the crosstalk analysis. Several physical models have been fabricated and experiments performed. The calculated results are compared to measurements. In our experiment, for microstrip lines of finite length and arbitrary orientation, there are short line-sections or vias at each of the four ports, which should be incorporated into the crosstalk analysis. This effect has been investigated numerically and experimentally.

Analysis of crosstalk between finite-length microstrip lines: FDTD approach and circuit-concept modeling

The electromagnetic radiation from a modules-on-backplane configuration found in computers is analyzed by means of the finite-difference time-domain (FD-TD) technique. The issues pertaining to the effects of the modules in influencing the overall radiating capacity of the configuration are addressed. Analyses are facilitated by using simplified modules-on-backplane models and by using a voltage source as the source of electromagnetic energy at a connector between a module and the backplane. The potential for enhancement of spurious emissions by the module-on-backplane configuration is demonstrated. For modules of typical dimensions, resonance is observed at the hundreds of MHz frequency range. Comparisons between numerical predictions and measurements are conducted to validate the numerical method. >

M. Tsuk

Papers

FD-TD analysis of electromagnetic radiation from modules-on-backplane configurations