UltraCAD Design, Inc

Mentor Graphics Article Series

Doug Brooks, president of UltraCAD, has agreed to write a series of articles for Mentor Graphics. The articles are posted on the Mentor web site and are available for download from there. The following is a list of articles written to date, along with their abstracts. More articles will be added to this list as they are posted on the Mentor site. You can now also download them from this by clicking the article title. Note: There is a bug in some versions of Adobe Acrobat that sometimes prevents a pdf file from opening. If you have this problem, right-click the link and select "Save Target As...". Then save the file on you local hard drive and open it there.

bulletESR and Bypass Capacitor Self Resonant Behavior: How to Select Bypass Caps. Improvements in bypass capacitor fabrication and assembly techniques have resulted in higher self-resonant frequencies. The results have sparked many debates on how to best take advantage of these higher frequencies and lower ESRs, while other issues (such as anti-resonant peaks) have been largely ignored. This paper explores the effects of lower ESRs, the true value of impedance minimums, and how to obtain better results using more capacitors. Several examples based on various cases are given, each supported by calculations, results, and illustrations. Information on achieving flat frequency responses is provided in detail, with a link where you can download UltraCAD's Bypass Cap Impedance Calculator that was used in the analysis. Detailed analysis, graphs, and results are provided in the appendices.  Note: This article is a copy of the one already available from the Article page on UltraCAD's web site. 
bulletBasic Transmission Lines; Why Use 'Em At All?  Signals traveling down a trace are compared to all other forms of general communication. Reflections from unterminated lines are analogous to echoes in poorly designed rooms. But just as we can acoustically engineer a room, we can electrically engineer a trace to absorb, and not reflect, echoes. The special nature of transmission lines is discussed, as are the special characteristics that are obtained when we design traces to look like transmission lines and then terminate them in their characteristic impedance.
bullet Propagation Times and Critical Length: How They Interrelate.  Signal propagation speed is directly related to the relative dielectric coefficient of the material(s) surrounding the signal trace. Traces are considered short, from a reflection standpoint, if the signal can travel to the end of the trace and return to the driver in a time period shorter than the rise time of the signal. Long traces are those where the round trip propagation time is longer than the rise time. In our industry, the critical length – the length where we need to consider using transmission line design and termination techniques – is generally considered to be the length where the round trip propagation time of the trace equals the rise time of the signal. 
bullet Differential Design Rules: Truth vs Fiction. There is no shortage of design rules available when people talk about differential traces on circuit boards. At various times you can hear people argue that there is a need for, or there is no need for, a variety of special rules regarding continuity of ground planes underneath the traces, equal length traces, equal separation between traces, differential impedance control, etc. So let’s set the record straight. NONE of these rules are inherently required by the fact that we are using differential signals! But some of them might be required if we are worried about signal integrity issues in our designs. This article looks at these individual types of rules from the standpoint of various signal integrity issues to see when, if ever, they need be applied.
bullet Transmission Line Terminations: It's the End That Counts.  Termination strategies are effective in eliminating, or at least controlling, transmission line reflections. There are five types of termination strategies commonly used with transmission lines (parallel, AC, Thevenin, Series, and Diode.) This paper looks at each strategy and summarizes its strengths and weaknesses. In addition, simulations are illustrated for two of tem (AC and Series), helping to illustrate their unique characteristics.
bullet Microstrip Propagation Times; Slower Than We Think. Most of us have been using incorrect values for the propagation speed of our microstrip traces! The correction factor for Er we have been using all this time is based on an incorrect premise. In particular, it results in a value for propagation speed that is independent of variations in trace width and height above the reference plane. But the propagation speed for microstrip traces depends significantly on such variations. This article explains why and develops a superior model for estimating propagation speeds and propagation delays for microstrip configurations.
bullet Termination Placement: How Much Does It Matter?  When we use transmission line techniques to control reflections on circuit board traces, we must terminate the lines. Typically we do so with resistors placed at the beginning (series termination) or at the end (parallel or Thevenin termination) of the trace. An interesting question is, “Where do we place these terminating resistors?” The more obvious assumption that we place them “as close as possible” to the end of the trace may not be the best answer. This article looks closely, with the aid of some simulations, at precisely where terminating resistors should be placed, and why.
bulletElectromagnetic Fields; the Good, the Bad, and the Ugly.  PCB designers, and others who don’t have a lot of background in EMC issues, usually don’t have a good understanding of electromagnetic fields. So electromagnetic fields often get a bad rap. But in fact, they are neither good nor bad, per se. ANY (AC) signal traveling along a trace generates an electromagnetic field. It is the effect of this field that can be good or bad. This article looks at the basic concepts of EMI, EMC, crosstalk, inductance, ground bounce, and RF communications and shows how they are all interrelated. The article also shows why there only a relatively few PCB design rules we have available to us for controlling the signal integrity issues related to electromagnetic field radiation, but why those rules can be effective.
bulletCrosstalk Part 1; Understanding Forward vs Backward.  Crosstalk can be a difficult phenomenon for PCB designers to grasp, particularly since there are two types of crosstalk, forward and backward, which behave quite differently. Although the magnitude of forward crosstalk increases as the length of the coupled region increases, its pulse width remains nearly constant and independent of the length of the coupled region. Backward crosstalk, on the other hand, has a nearly constant magnitude that is independent of the length of the coupled region (as long as the coupled region is “long enough”). But its pulse width is twice as long as the coupled region. This article is Part 1 of a two-part series on crosstalk.
bulletCrosstalk Part 2; Simulating Crosstalk Effects. It is known that forward crosstalk increases (for all practical purposes) with increasing coupled length, but has a pulse width that is constant. Backwards crosstalk, on the other hand, rises quickly (within the critical region) to a constant maximum, but has a pulse width that increases with increasing coupled length. Simulations using Mentor's HyperLynx LineSim tool show this very effectively and clearly. The tool can illustrate how impedance loading of the victim trace can impact the magnitude, and even the polarity, of the backward crosstalk pulse. The HyperLynx tool also can be used to clearly illustrate how the backward crosstalk pulse is 2X the propagation time through the coupled region plus one rise time, how the crosstalk signal is impacted by the relationship between the traces and their reference planes and also with each other, and how an aggressor AC signal’s period can interact with the length of the coupled region to create some surprising crosstalk effects.
bullet Crosstalk Coupling: Single-ended vs. Differential.  The paper begins with four propositions:  First, the effects of crosstalk coupling decrease with trace separation.  Second, crosstalk coupled to a differential pair has meaning only for the differential component of the crosstalk on the differential pair, not the common mode component of the crosstalk. Third, crosstalk caused by a differential pair would be equal and opposite, and therefore cancel, on a victim trace were it not for the (perhaps only slight) separation of the differential traces themselves. Finally, differential pair coupling to another differential pair would combine these last two effects and should be quite small. The relationships between these four propositions are quantified and then tested against four PCB structures using the Hyperlynx simulation tool. The results of the simulations are as predicted for microstrip configurations and for stripline configurations when the traces are close together relative to the second reference plane. But single-ended coupling drops off more quickly than predicted with increased spacing for stripline environments. Differential coupling, however, does not drop off in the same manner for stripline configurations.
bullet Adjusting Signal Timing (Part 1).  It is becoming a routine requirement for PCB designers to tune traces on boards. Such tuning can be relative (i.e. traces are equal length) or absolute (i.e. traces must be a proscribed length.) This article, Part 1 of a two-part series, addresses why traces need to be tuned at all, how to determine signal propagation speeds and times so that proper trace length can be determined, and how sensitive propagation time (and therefore tuning) is to factors such as er, trace length, trace pattern, etc. The special case of differential traces is mentioned, and some examples of tuning on a high-speed computer motherboard are illustrated.
bulletAdjusting Signal Timing (Part 2). When a signal passes through a serpentine trace with coupling between the legs, there is an apparent speed-up of the signal. That is, the signal appears to pass through the serpentine section faster than the trace length would otherwise indicate. This apparent speed-up is caused by crosstalk coupling between the legs of the serpentine traces. The amount of apparent speed-up is directly related to the coupling strength between the legs and inversely related to the rise time of the signal passing through the section. The apparent speed-up of the signal is not directly related to the coupled length. For long coupled lengths (those longer than the critical length) signals may become distorted as they pass through the serpentine section, but the degree of distortion is a complex function of the frequency of the signal. Signals pass relatively undistorted through short coupled serpentine sections.
bulletControlling Impedances When Nets Branch Out:  It is not uncommon for a driver to drive numerous receivers. In some designs it is impractical, or undesirable, to drive every receiver from a single trace segment. In such cases it is common to design a trace that branches out into two or more branches, each serving a select number of receivers. The question becomes, then, where to place the branch point. Improper placement of the branch point can have serious implications from an impedance discontinuity standpoint, resulting in reflections that can have signal integrity consequences. This paper describes several ways to deal with the branching problem in designs if they come up.
bulletWhat is This Thing Called "Current:" Electrons, Displacement, Light, or What?  The generally accepted definition of electrical current is “the flow of electrons.” But some people criticize this definition on the basis that: (1) it cannot explain how current (a signal) flows at the speed of light, (2) it cannot explain how current “flows” across the plates of a capacitor, and (3) it cannot explain how current can be induced in a conductor some distance away. This article shows that these criticisms are not valid. Electron flow, properly understood, can occur at the speed of light, and some of the component laws on which Maxwell’s Equations are based are fully capable of explaining the other phenomena


As additional articles are published, they will be made available from here. Keep watching for updates.

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