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Brooks' Bits
Column from: Douglas Brooks, PhD.
Douglas Brooks, PhD, is the founder of UltraCAD Design, Inc. He has written numerous articles in several disciplines and has held signal integrity seminars around the world. He has spent most of his career in the electronics industry in positions of engineering, marketing, management, and as CEO of several companies. Brooks finally retired from daily work in 2013. Prentice Hall has recently published his latest book, PCB Currents: How They Flow, How They React, and a 7-hour educational video series on signal integrity.
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December 06, 2018
Brooks' Bits: Internal Trace Temperatures—More Complicated Than You Think
One of the most interesting findings in IPC-2152 was that internal traces are cooler than external ones for the same size and current. Independent experimentation was not done on internal traces in earlier charts. Internal traces were merely assumed to be hotter than external traces, and the external trace data was derated by 50% by that assumption. Traces are heated by Joule, or I2R, heating. They are cooled by a combination of conduction through the dielectric, convection through the air, and radiation. It had previously been assumed that convection conducted heat away from the traces and cooled the traces more efficiently than conduction through the dielectric. Hence, the assumption that internal traces were hotter than their external trace counterparts.April 30, 2014
Brooks' Bits: Electromagnetic Fields, Part 3 - How They Impact Coupling
In Part 1 and Part 2 of this series, Doug Brooks talked about how helpful it can be to recognize what the electromagnetic field looks like around a conductor or trace and how that field may change as the stackup or trace parameters are changed. In Part 3, he looks at how changes in the electromagnetic field relate to changes in coupling between traces or between a trace and the outside world.November 13, 2013
Brooks' Bits: Electromagnetic Fields, Part 2: How They Impact Propagation Speed
In Part 1, Doug Brooks suggested that thinking in terms of what the electromagnetic field looks like around our traces might offer significant insight into how circuits might be performing. In this column, he makes similar observations about signal propagation speed.October 23, 2013
Brooks' Bits: How Electromagnetic Fields Determine Impedance, Part 1
When a current flows down a conductor an electric field and a magnetic field radiates away from that conductor. Collectively, this is called the electromagnetic field. What is important to note is that this field always exists. Furthermore, the electromagnetic field and the current are inseparable.May 08, 2013
Trace Currents and Temperature, Part 4: Via Heat
In this final installment of his four-part series, Doug Brooks suggests a new method for dealing with vias. Part 1 hypothesized that trace heating was a function of the i2R power dissipated in the trace, and trace cooling was a function of surface area. Can these same fundamental principles be applied to vias when looking at current-carrying capacities?May 01, 2013
Trace Currents and Temperature, Part 3: Fusing Currents
This is the third of a four-part series on trace currents and temperature from Douglas Brooks. Part 1 discussed the role of resistance and formulated a basic model for analysis. Part 2 explored various results empirically obtained. Part 3 explores using the melting temperature of a trace to your advantage, and Part 4 will suggest a way to deal with vias.February 13, 2013
Trace Currents and Temperature, Part 1: The Basic Model
This first of a four-part series on trace currents and temperature covers the role of resistance and then formulates a basic model for analysis. Subsequent parts will explore various results that have been empirically obtained, how we can use the melting temperature of a trace to our advantage, and how to deal with vias.July 25, 2012
The Skinny on Skin Effect, Part 3: Crossover Frequency
It is instructive to look more closely at the formula for skin depth that we discussed in Part 2 of this series. First, note that it does not depend on the dimensions of the conductor, or even the conductor's shape! Skin depth is purely a function of frequency. That leads to some conclusions that are not particularly intuitive.July 11, 2012
Brooks' Bits: The Skinny on Skin Effect, Part 2
In Part 1 of this series, I described how skin effect is all about current density. If we multiply current density by cross-sectional area, we get current. In this column we'll take a look at two propositions that are well accepted in electronics, and they are useful models, even if they are not exactly correct, as we shall see!June 20, 2012
Brooks' Bits: The Skinny on Skin Effect, Part 1
Why do we care about skin effect? First, it impacts any calculation that involves Ohm's Law. Thus, the voltage drop across a conductor and the power dissipated within the conductor (and therefore loss to the circuit) will increase with frequency. Second, skin effect can impact trace current/temperature effects. Doug Brooks has the first column in a series.March 07, 2012
Crosstalk, Part 3: More On Shapes and Amplitudes
In Part 2 of the series, I showed that a step-function change in the aggressor signal results in two crosstalk components, forward and backward crosstalk, neither one of which resembles the aggressor signal. In this column, Part 3 of the series, we will explore the shapes of these components and their amplitudes a little more fully.February 01, 2012
Crosstalk, Part 2: What It Looks Like
In Part 1 of this series, I pointed out that many people seemed to find the concept of crosstalk difficult to wrap their arms around. This column, Part 2, of the series, we will look more closely at the shapes of the two crosstalk components. In Part 3 of the series, we will look even more closely at the shape and the magnitude of the backward crosstalk component.January 04, 2012
Crosstalk: Why It's Difficult to Understand, Part 1
Why is crosstalk difficult to understand? Let's see: Crosstalk has two different fundamental causes, which generate two different signals. These two signals flow in opposite directions, but the signals can interact with each other. These two signals have significantly different shapes, which behave differently as a function of coupled length. And neither shape resembles the "aggressor" signal that caused the crosstalk in the first place!December 07, 2011
Current Flow on Traces, Part 2: Common Mode and Mode Shift
If we have a differential trace pair, and the signal on the return trace is exactly equal and opposite to the signal on the forward trace, there is no common-mode component. If they are not exactly equal and opposite, there will be a common-mode component to the currents on the trace. Let's carry that idea further.November 09, 2011
Coupling's Effect on Impedance: Why is Zdiff Less Than 2*Zo?
Assume we have a trace (T1) with a controlled impedance equal to Zo. Now bring a second trace (T2) near it (and parallel to it) so that the signal on T2 couples into T1. Consider this statement: The impedance of T1 (Zo) changes as a result of that coupling. Why is this true, and why do we only worry about it in the case of differential-mode or common mode-signals?October 12, 2011
Current Flow on Traces, Part 1: Transmission Lines
Current propagates down transmission lines by utilizing the distributed capacitance between the lines and the return path. There will always be reflections from impedance discontinuities along, and at the end of, the line. Do we care about such reflections? We may care, depending on the relationship between the propagation time down the trace and the rise time of the signal flowing on the trace. By Doug Brooks.August 31, 2011
Rise Times and Harmonics: Introducing Mr. Fourier
In very general terms, frequency relates to information and rise time relates to how quickly we can process that information. A circuit only needs to have a rise time fast enough (and not faster) to process the information flow. Bandwidth refers to how wide a frequency range a circuit (or PCB) needs to handle without distortion. So, how wide a bandwidth do I need to pass my signals?June 07, 2011
Rise Time vs. Frequency: What's the Relationship?
A circuit must have a fast enough rise time to accommodate the signal being processed. If it does not, information in the waveform or circuit timing may be lost or distorted. But here is the clincher: a circuit does NOT have to have a faster rise time than is required by the waveform. Faster is not necessarily better!May 11, 2011
Propagation Speed in Microstrip: Slower Than We Think
How fast is propagation speed in microstrip? First of all, we must remember, the propagation speed is not determined by how fast the current can travel down a wire; it is determined by how fast the electromagnetic field can propagate in the medium it is in. And propagation times are much more controllable in a stripline environment.April 20, 2011
What Does Voltage Refer To, and Why Do We Care?
If you do a search for voltage on the Web, you will find lots of definitions. For our purposes here, we can consider it to be the force that causes current to flow. Since current is the flow of electrons, and since electrons are negatively charged, we can consider voltage to be the difference in charge between two points. Remember that for the quiz!December 15, 2010
A History of Signal Integrity: One Man's Perspective
The stages of signal integrity can be broken down into stages, not unlike the stages of grief. But there's no denial or bargaining here. The only way to handle signal integrity issues is to be prepared. Remember, the more information you have, the better off you'll be if you find yourself at Stage 4!November 17, 2010
Resistance, Reactance and Impedance, Part 3
Congratulations! You've made it to Part 3. Part 1 focused on resistance. That was pretty simple; there is a single component value, it does not depend on frequency, and there is no phase shift. Part 2 covered reactance, which is much more complicated. This installment on impedance ties it all together.October 20, 2010
Resistance, Reactance And Impedance, Part 2
Capacitors and inductors are almost exactly opposite in their effects. Both "impede" current, but capacitance impedes current at low frequencies, inductance at high frequencies. Capacitance causes voltage to lag current by 90 degrees, while inductance causes voltage to lead current by 90 degrees.September 30, 2010
Resistance, Reactance and Impedance, Part 1
New columnist Doug Brooks brings us Part 1 of a three-part primer on resistance, reactance and impedance. Most of us are familiar with resistance, but few really understand reactance and its relationship to resistance. And few really understand the relationship between impedance and the other two properties. This series will tie them all together.October 23, 2013
How Electromagnetic Fields Determine Impedance, Part 1
When a current flows down a conductor an electric field and a magnetic field radiates away from that conductor. Collectively, this is called the electromagnetic field. What is important to note is that this field always exists. Furthermore, the electromagnetic field and the current are inseparable.November 13, 2014
Electromagnetic Fields, Part 2: How They Impact Propagation Speed
In Part 1, Doug Brooks suggested that thinking in terms of what the electromagnetic field looks like around our traces might offer significant insight into how circuits might be performing. In this column, he makes similar observations about signal propagation speed.April 30, 2014
Electromagnetic Fields, Part 3 - How They Impact Coupling
In Part 1 and Part 2 of this series, Doug Brooks talked about how helpful it can be to recognize what the electromagnetic field looks like around a conductor or trace and how that field may change as the stackup or trace parameters are changed. In Part 3, he looks at how changes in the electromagnetic field relate to changes in coupling between traces or between a trace and the outside world.
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