Richard E. DuBroff

Quantifying SMT decoupling capacitor placement in dc power-bus design for multilayer PCBs

Noise on a dc power-bus that results from device switching, as well as other potential mechanisms, is a primary source of many signal integrity (SI) and electromagnetic interference (EMI) problems. Surface mount technology (SMT) decoupling capacitors are commonly used to mitigate this power-bus noise. A critical design issue associated with this common practice in high-speed digital designs is placement of the capacitors with respect to the integrated circuits (ICs). Local decoupling, namely, placing SMT capacitors in proximity to ICs, is investigated in this study. Multilayer PCB designs that employ entire layers or area fills for power and ground in a parallel plate structure are considered. The results demonstrate that local decoupling can provide high-frequency benefits for certain PCB geometries through mutual inductive coupling between closely spaced vias. The associated magnetic flux linkage is between the power and ground layers. Numerical modeling using an integral equation formulation with circuit extraction is used to quantify the local decoupling phenomenon. Local decoupling can effectively reduce high-frequency power-bus noise, though placing capacitors adjacent to ICs may limit routing flexibility, and tradeoffs need to be made based on design requirements. Design curves are generated as a function of power-bus layer thickness and SMT capacitor/IC spacing using the modeling approach to quantify the power-bus noise reduction for decoupling capacitors located adjacent to devices. Measurement data is provided to corroborate the modeling approach.

Power-bus decoupling with embedded capacitance in printed circuit board design

This paper experimentally investigates the effectiveness of embedded capacitance for reducing power-bus noise in high-speed printed circuit board designs. Boards with embedded capacitance employ closely spaced power-return plane pairs separated by a thin layer of dielectric material. In this paper, test boards with four embedded capacitance materials are evaluated. Power-bus input impedance measurements and power-bus noise measurements are presented for boards with various dimensions and layer stack ups. Unlike discrete decoupling capacitors, whose effective frequency range is generally limited to a few hundred megahertz due to interconnect inductance, embedded capacitance was found to efficiently reduce power-bus noise over the entire frequency range evaluated (up to 5 GHz).

Modeling of Shielding Composite Materials and Structures for Microwave Frequencies

Composites containing conducting inclusions are required in many engineering applications, especially, for the design of microwave shielding enclosures to ensure electromagnetic compatibility and electromagnetic immunity. Herein, multilayer shielding structures are studied, with both absorbing and re∞ecting composite layers. In this paper, flber-fllled composites are considered. For modeling absorbing composites with low concentration of conducting cylindrical inclusions (below the percolation threshold), the Maxwell Garnett theory is used. For re∞ecting layers, when concentration of inclusions is close to or above the percolation threshold, the McLachlan formulation is used. Frequency dependencies for an efiective permittivity are approximated by the Debye curves using a curve-fltting procedure, in particular, a genetic algorithm.

A Maxwell Garnett Model for Dielectric Mixtures Containing Conducting Particles at Optical Frequencies

Mathematical modeling of composites made of a dielectric base and randomly oriented metal inclusions is considered. Different sources of frequency-dependent metal conductivity at optical frequen- cies are taken into account. These include the skin-effect, dimensional (length-size) resonance of metal particles, and the Drude model. Also, the mean free path of electrons in metals can be smaller than the char- acteristic sizes of nanoparticles, and this leads to the decrease in con- ductivity of the metal inclusions. These effects are incorporated in the Maxwell Garnett mixing formulation, and give degrees of freedom for forming desirable optical frequency characteristics of composite media containing conducting particles.

Prediction of Radiated Emissions Using Near-Field Measurements

A procedure is developed to predict electromagnetic interference from electronic products using near-field scan data. Measured near-field data are used to define equivalent electric and magnetic current sources characterizing the electromagnetic emissions from an electronic circuit. Reconciliation of the equivalent sources is performed to allow the sources to be accurately applied within full-wave numerical modeling tools like finite-difference time domain (FDTD). Results show that the radiated fields must typically be represented by both electric and magnetic current sources if scattering and multiple-reflections from nearby objects are to be taken into account. The accuracy of the approach is demonstrated by predicting the fields generated by a microstrip trace within and outside of a slotted enclosure, and by predicting the fields generated by the microstrip trace close to a long wire. Values predicted from near-field scan data match those from full-wave simulations or measurements within 6 dB.

Using near-field scanning to predict radiated fields

Near-field scanning has often been used to measure and characterize magnetic fields surrounding individual integrated circuits (IC) and high speed digital electronic circuits. The paper describes the use of near-field scanning data, performed in a typical laboratory bench top environment, to predict radiated electromagnetic interference (EMI) in a typical product environment. The product environment may include enclosures and apertures. The approach begins by acquiring sufficient near-field scanning data to allow representation of an unintentional radiating source by an equivalent surface current distribution. The equivalent current distribution is used as a source in numerical full wave modeling. The agreement between direct full wave simulation results and full wave simulation results using equivalent sources works well under certain assumptions.

Power bus noise reduction using power islands in printed circuit board designs

Power islands are often used to isolate devices that put noise on a power bus from devices that may be susceptible to power bus noise. At high frequencies however, the effectiveness of these islands depends on the implementation. This paper experimentally investigates the effectiveness of different power island structures at frequencies up to 3 GHz.

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.

Reconstruction of Dispersive Dielectric Properties for PCB Substrates Using a Genetic Algorithm

An effective method for extracting parameters of a Debye or a Lorentzian dispersive medium over a wideband frequency range using a genetic algorithm (GA) and a transmission-line model is presented. Scattering parameters (S-parameters) of the transmission-line sections, including a parallel plate, microstrip, and stripline, are measured. Wave equations for TEM/quasi-TEM mode with a complex propagation constant and a frequency-dependent wave impedance are used to evaluate the corresponding S-parameters in an analytical model. The discrepancy between the modeled and measured S-parameters is defined as the objective function in the GA. The GA is used for search of the dispersive-medium parameters by means of minimizing the objective function over the entire frequency range of interest. The reconstructed Debye or Lorentzian dispersive material parameters are corroborated by comparing the original measurements with the FDTD modeling results. The self-consistency of the proposed method is demonstrated by constructing different test structures with an identical material, i.e., material parameters of a substrate extracted from different transmission-line configurations. The port effects on the material parameter extraction are examined by using through-reflection-line calibration.

Power bus isolation using power islands in printed circuit boards

Power islands are often employed in printed circuit board (PCB) designs to alleviate the problem of power bus noise coupling between circuits. Good isolation can be obtained over a wide frequency band due to the large series impedance provided by the gap between the power islands. However, power bus resonances may degrade the isolation at high frequencies. The amount of isolation also depends on the type of connection between power islands and the components on the board. This paper experimentally investigates the effectiveness of several power island structures up to 3.0 GHz.