This video explains how to manage noise, interference, and EMI in PC boards by understanding the underlying principles of electromagnetic fields rather than just following rules. The key takeaways include:
Importance of Understanding Why: Focus on understanding why EMI problems occur to effectively prevent them, rather than just applying rules.
Rise Time vs. Clock Frequency: Rise and fall times of signals are the critical factors causing EMI and signal integrity issues, not clock frequency.
Lumped vs. Distributed Elements: High-speed circuits behave as distributed elements, leading to reflections, crosstalk, and EMI if not properly designed.
Energy in Fields: Electrical energy travels in electromagnetic fields within the dielectric material of the PCB, not directly in the copper traces.
Return Path Design: A low-impedance, well-defined return path, ideally a solid ground plane, is crucial for controlling EMI and ensuring signal integrity.
Proximity for Inductance and Capacitance: Keeping signal traces and their return paths in close proximity significantly reduces inductance and increases capacitance, leading to better impedance control.
PCB design showing component placement and routing
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Recommended Reading and Core Philosophy [00:00:00]
The speaker, Rick Hartley, emphasizes the importance of understanding the "why" behind EMI and signal integrity issues, rather than just following rules. He recommends several books:
"Signal Integrity Rules Have Outlived Their Usefulness" by Ralph Morrison [00:01:00]
This book is considered the most important publication in the last 30 years for understanding how energy moves in circuits.
Morrison's work helped Hartley shift from teaching design rules to explaining the underlying principles of electromagnetic phenomena.
Example: A metal building with lightning arrestors still experienced machinery resets during lightning strikes because the metal walls provided a lower impedance path to ground than the arrestors, leading to energy spreading across the floor.
"PCB Design for Real-World EMI Control" by Bruce Archambeault, James Drewniak, David Hockanson, Roger Hill, etc. [00:03:57]
Bruce Archambeault worked for IBM and is a senior fellow with IEEE.
This book is lauded for its practical solutions to real-world EMI problems.
"Principles of Power Integrity for PDN Design" by Larry Smith and Eric Bogatin [00:04:28]
This book, about a year and a half to two years old, is considered "fabulous" and crucial for the future of EMI and signal integrity control, especially for gigabit designs.
It focuses on proper power bus design.
"Digital Designs for Interference Specification" by David Terrell and Kenneth Keenan [00:05:40]
This book is a compilation of 30 years of experience from an EMI test facility, explaining why units fail EMI testing and how to fix them.
"Electromagnetic Compatibility" by Henry Ott [00:06:35]
Henry Ott wrote the first EMI control book in 1976. This book is a rewritten and updated version of his original work.
A 1989 IEEE survey revealed that only 5 out of thousands of North American engineering schools taught EMI control, a situation that hasn't significantly improved by 2014.
Circuit application notes from IC manufacturers should be assumed wrong until proven right.
This quote by Lee Ritchey highlights that many engineers who write these notes understand circuit theory but lack understanding of proper circuit board layout.
Examples of bad advice include "no 90-degree corners" (which are not the main problem) and "split the ground."
The Impact of Circuit Frequency on Design Methods [00:12:32]
Historically (1950s-mid-1980s), PC boards could be laid out in almost any manner due to low frequencies.
All components behaved as lumped elements.
A 14x16 inch VMEbus board with 200 ICs designed in 1984, with traces running all over, worked without issues because all elements were lumped.
Lumped Elements: Characterized by long rise times compared to propagation time; energy travels the entire line before rising significantly, reflections are absorbed, and crosstalk is a non-issue.
Distributed Elements: Characterized by short rise times compared to propagation time; energy moves as a wave, causing reflections, ringing, crosstalk, and EMI at impedance discontinuities or line ends.
Rise Time is Critical, Not Clock Frequency [00:17:37]
EMI failures and harmonics are observed at clock frequencies because those are where the peak energy levels are in a digital square wave, but the fundamental cause is the rate of change of energy (di/dt and dv/dt), which is related to rise/fall time.
Case Study: Elevator Controller Company [00:18:42]
A company designing elevator controllers experienced EMI and operational problems when their ICs underwent a die shrink, resulting in faster rise times, even though their clock frequency remained low (500 kHz). The board lines transitioned from lumped to distributed, causing ringing and crosstalk.
It is crucial to match propagation times, not physical lengths, because propagation velocity varies between outer and inner layers, and with load capacitance.
Calculating Distributed Length Elements [00:29:19]
Die shrinks (reducing IC size) increase speed and reduce cost, leading to faster rise times.
This can lead to significant problems if board layout is not updated for the new rise times.
Historically, purchasing departments prioritized lower cost, often resulting in lower quality boards that could not handle faster signals.
Circuit Transmission Lines and Electromagnetic Fields [00:48:30]
A transmission line is any pair of conductors moving energy from point A to B, often of controlled impedance.
Impedance (Z0): An inherent property of every trace, determined by inductance (L), capacitance (C), resistance (R), and conductance (G).
$Z_0 = \sqrt{(R + j\omega L) / (G + j\omega C)}$
Below 1 GHz, R and G (losses in copper and dielectric) are often negligible in digital circuits, simplifying the impedance calculation to $\sqrt{L/C}$.
A waveguide is a metal tube used to transmit electromagnetic waves through air. Similarly, a microstrip trace above a plane acts as a waveguide, with energy traveling in the dielectric.
Substrate Integrated Waveguides (SIW) are created on PCBs using via strings to form a "wall of copper" that contains fields.
The distance between vias in a string needs to be small enough (frequency-dependent) to create a continuous copper wall.
This understanding enabled better EMI control in avionics.
When energy is applied to a transmission line, e-fields couple to the planes, causing current flow in the copper.
At high frequencies, due to skin effect, current flows only on the surface of the copper, leading to increased resistive losses because the fields have to work harder to induce current in a thinner layer.
Loss tangent is the energy loss in the dielectric material. FR4 is a "lossy" material, which can be advantageous for power delivery but detrimental for high-frequency signal integrity.
For high frequencies, critical signals should be routed on inner layers to better contain the fields and minimize coupling.
For short traces that are "lumped elements," routing on outer layers is acceptable, avoiding unnecessary vias.
Return Current Path at Different Frequencies [01:46:16]
At DC, return current takes the path of least resistance (straight line).
As frequency increases, the path of lowest impedance shifts to directly under the trace due to increasing inductance and decreasing capacitance.
For circuits above audio frequencies (kHz to MHz), the return current will predominantly flow directly under the signal trace.
In low-frequency analog circuits, where currents can spread out, wider return traces directly under the signal trace are used to channel the fields and prevent coupling between circuits.
This ensures that, even at low frequencies, sensitive circuits do not interfere with each other.
