D.M. Hockanson

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. >

An investigation of PCB radiated emissions from simultaneous switching noise

Processors are currently operating with fundamental clock frequencies that are at or above the resonant frequencies of typical processor boards and modules. Adequately decoupling printed-circuit boards (PCBs) at high frequencies has become an increasingly urgent task in the light of increasing clock frequencies with decreasing rise times. Providing sufficient charge at frequencies near and above 1 GHz is extremely difficult with lumped-element capacitors. To further complicate the issue, modern PCB power buses may be analogous to microstrip-patch antennas. Exciting a power bus at board harmonics may result in significant radiated EMI from the bus. Much has been done to improve high-frequency decoupling from a signal-integrity perspective. However, the benefit to EMI is somewhat unclear, because the mechanism by which power-bus noise results in radiated EMI is not well understood. Input impedance of a power bus, transfer impedance across a power bus, and radiated emissions from a PCB are presented herein. The results are discussed to provide characterization of radiated EMI directly from a PCB power bus.

Numerical and experimental corroboration of an FDTD thin-slot model for slots near corners of shielding enclosures

Simple design maxims to restrict slot dimensions in enclosure designs below a half-wave length are not always adequate for minimizing electromagnetic interference (EMI). Complex interactions between cavity modes, sources, and slots can result in appreciable radiation through nonresonant length slots. The finite-difference time domain (FDTD) method can be employed to pursue these issues with adequate modeling of thin slots. Subcellular FDTD algorithms for modeling thin slots in conductors have previously been developed. One algorithm based on a quasistatic approximation has been shown to agree well with experimental results for thin slots in planes. This FDTD thin-slot algorithm is compared herein with two-dimensional (2-D) moment method results for thin slots near corners and plane wave excitation. FDTD simulations are also compared with measurements for slots near an edge of a cavity with an internal source.

RF isolation using power islands in DC power bus design

Power island structures are often employed for minimizing the propagation of high-frequency noise on DC power buses. The rationale is based on introducing a series impedance in the power plane to provide isolation of a noise source from the rest of the PCB design. The power island concept is investigated herein experimentally, to determine its noise mitigation attributes and limitations. A modeling approach that is suitable for arbitrary PCB island geometries including lumped SMT decoupling capacitors is also presented. The modeling and measurements indicate that island structures can achieve some degree of isolation under certain conditions.

Mutual External Inductance in Stripline Structures

Abstract—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.

The EMI benefits of ground plane stitching in multi-layer power bus stacks

The effect on EMI of stitching multiple ground planes together along the periphery of multi-layer PCB stacks is studied. Power bus noise induced EMI and radiation from the board edges is the major concern herein. The EMI at 3 meters for different via stitch spacing and layer thickness is modeled with FDTD modeling. It is shown that the ground plane stitching effectively reduces the radiated EMI that results from fringing fields at the power bus edges. Two families of curves are generated to demonstrate the variation of the radiated EMI as a function of layer thickness and stitch spacing. Further studies show that the reduction of the EMI from ground plane stitching may be compromised by other radiation mechanisms.

Application of the finite-difference time-domain method to radiation from shielded enclosures

The finite-difference time-domain method is applied to the analysis of radiation from shielding enclosures with internal sources. Results from the three-dimensional code which has been developed are compared with analytical results from waveguide problems and the Lawrence Livermore TSAR code. Two enclosure examples are given to demonstrate the utility of the FDTD method for this application. One example is for radiation from slots, and the other is coupling of energy from a nonresonant aperture to an attached shielded cable that results in enhanced radiation.<<ETX>>

Reducing radiated emissions from CPUs core power interconnect design

Radiated emissions associated with the second harmonic of the CPU core operating frequency often prove problematic throughout the design of computer systems. One of the primary radiating mechanisms for the second-harmonic emissions is high-frequency harmonic current injected from the CPU directly into the computer power distribution system (PDS). Prom the PDS, the current can couple to and radiate from other devices and/or radiate directly from the PCB. To mitigate the emissions from such current injection, as much current as possible must be kept from reaching the PCB. In this work, the design of the core-power interconnect for a CPU is modified to limit the level of high-frequency current injected into the PCB PDS. The design is driven by analyzing the parasitics associated with the PDS. A reduction of more than 12 dB in the radiated fields is achieved through a geometric redesign of the CPU socket interconnect.

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.

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. >