A Common-Mode Current Measurement Technique for EMI Performance Evaluation of PCB Structures

An experimental technique that measures the common-mode current on a cable attached to a DUT for assessing EMI performance is introduced herein. The technique was applied to evaluate the EMI performance of a module-on-backplane configuration with different connectors and different connector pin-outs

External Parasitic Inductance in Microstrip and Stripline Geometries of Finite Size

An external parasitic ground (return) plane inductance, or a mutual inductance associated with fringing magnetic fields in planar transmission line structures, is the culprit of common-mode voltage (ground plane noise) that leads to parasitic radiation of the corresponding unintentional antennas in high-speed electronic equipment. Mutual inductance of this sort in microstrip and stripline structures is studied here using an analytical quasi- magnetostatic approach and FDTD modeling. Closed-form expressions for mutual inductance in symmetrical and asymmetrical microstrip and stripline structures are presented

High-Performance Inter-PCB Connectors: Analysis of EMI Characteristics

Electromagnetic interference (EMI) coupling associated with inter-board connection is investigated. Two experimental techniques, based on |S21| measurements, including both common-mode current and near-field measurements, are reported. Both methods, as well as finite difference time domain (FDTD) modeling, were used as experimental and numerical tools for inter-printed-circuit-board (inter-PCB) connector evaluation. The EMI performance of a lab-constructed stacked-card connector, and a commercially available module-on-backplane connector were studied. EMI characteristics of the connectors are demonstrated by investigating a few aspects of the design: type of shield/ground blade for signal return, number and length of ground pins, signal pin designation, etc. Good agreement is achieved between the measurements and the FDTD modeled results

Investigation of Split Groundplanes at the Connector for EMI Control

EMI can often be reduced by selectively filtering various parts of a given system. One common method employed by designers is to split the groundplane near the chassis and route I/O lines over the split. The rationale is based on providing a large series impedance to common-mode currents on the I/O lines. In this manner, PCB designers hope to lower the level of noise currents contributing to radiation. This work studies the efficacy of the groundplane split as a deterrent for EMI associated with I/O lines being driven against other extended reference structures. A test-board was developed to analyze the impedance of the groundplane split with various configurations

FDTD Modeling of Thin Wires for Simulating Common-Mode Radiation from Structures with Attached Cables

The analysis of shielding enclosures is complicated by the existence of apertures and cables. The finite-difference time-domain (FDTD) method can model shielding enclosures with complex geometries, but has difficulty modeling wires and cables of arbitrary radii. Modeling the wire by setting the axial component of the electric field to zero in the FDTD results in a wire with a radius determined by the mesh discretisation. Neglecting wire radius in applications such as electromagnetic interference (EMI) or printed circuit board modeling may result in gross errors because near field quantities are typically sensitive to wire thickness. Taflove (1990) developed a wire modeling algorithm for FDTD analysis which models wires well for far-field calculations such as the radar cross section. The method uses a quasi-static field approximation to model wires with a user-specified radius. The wire model is reviewed and investigated for near-field accuracy via input impedance computations, since FCC class A and B regulations are tested in the near field. The input impedance for a center-fed dipole antenna is computed with FDTD methods and compared to the input impedance results from moment methods. A simulation of a shielding enclosure with an attached cable demonstrates the utility of FDTD analysis in EMC applications

Power Bus Decoupling on Multilayer Printed Circuit Boards

Guidelines for the selection and placement of decoupling capacitors that work well for one-sided or two-sided printed circuit boards are not appropriate for multilayer boards with power and ground planes. Boards without internal planes take advantage of the power bus inductance to help decouple components at the higher frequencies. An effective decoupling strategy for multilayer boards must account for the low inductance and relatively high capacitance of the power bus

Mutual External Inductance in Stripline Structures

The Method of Edge Currents (MEC) proposed in our previous paper [1] is applied herein for calculating the mutual external inductance associated with fringing magnetic fields that wrap ground planes of a stripline structure. This method employs a quasi-static approach, image theory, and direct magnetic field integration. The resultant mutual external inductance is frequency-independent. The approach has been applied to estimating mutual inductance for both symmetrical and asymmetrical stripline structures. Offset of the signal trace from the centered position both in horizontal and vertical directions is taken into account in asymmetrical structures. The results are compared with numerical simulations using the CST Microwave Studio Software

Method of Edge Currents for Calculating Mutual External Inductance in a Microstrip Structure

Mutual external inductance (MEI) associated with fringing magnetic fields in planar transmission lines is a cause of socalled ground plane noise , which leads to radiation from printed circuit boards in high-speed electronic equipment. Herein, a Method of Edge Currents (MEC) is proposed for calculating the MEI associated with fringing magnetic fields that wrap the ground plane of a microstrip line. This method employs a quasi-magnetostatic approach and direct magnetic field integration, so the resultant MEI is frequency independent. It is shown that when infinitely wide ground planes are cut to form ground planes of finite width, the residual surface currents on the tails that are cut off may be redistributed on the edges of the ground planes of finite thickness, forming edge currents. These edge currents shrink to filament currents when the thickness of the ground plane becomes negligible. It is shown that the mutual external inductance is determined by the magnetic flux produced by these edge currents, while the contributions to the magnetic flux by the currents from the signal trace and the finite-size ground plane completely compensate each other. This approach has been applied to estimating the mutual inductance for symmetrical and asymmetrical microstrip lines

Stop That Noise

The authors discuss electromagnetic compatibility (EMC) and electromagnetic interference (EMI). After a brief look at the causes of EMI, they describe conductive coupling and electromagnetic radiative coupling. Career opportunities in EMC problem solving are looked at

Considerations for Magnetic-Field Coupling Resulting in Radiated EMI

Parasitic inductance in printed circuit board geometries can worsen the EMI performance and signal integrity of high-speed digital designs. Partial-inductance theory is a powerful tool for analyzing inductance issues in signal integrity. However, partial inductances may not adequately model magnetic flux coupling to EMI antennas because the EMI antennas are typically open loops. Therefore, partial inductances may not always accurately predict radiated EMI from noise sources, unless used in a full-wave analysis such as PEEC. Partial inductances can be used, however, to estimate branch inductances, which can be used to predict EMI. This paper presents a method for decomposing loop or self inductances into branch inductances. Experimental as well as analytical investigations are used to compare branch- and partial-inductances