108K views · Oct 24, 2019 · 1:03:22 · 5 HN recommenders · 10 mentions · discussed 2021–2026
Rick Hartley's AltiumLive keynote on differential pairs in PC boards. He argues the rules from cables and twisted pairs do not carry over, and covers which pair geometry gives the best impedance control, the right spacing, routing practice and how much skew is acceptable.
Why Hacker News points people here: HN engineers cite Hartley as the intuitive, field-based alternative to cargo-cult PCB advice, especially on differential pair routing, reference planes and EMI.
“this is absolutely worth your time if you design PCBs.”HN comment, 2023
“this is absolutely worth your time if you design PCBs.”
Intuitive explanation instead of cargo cult PCB advice
Basically, go watch every Rick Hartley video on YouTube. He is awesome at explaining. It will become intuitive instead of being a bunch of cargo cult advice. (Normally I dislike long videos but this is absolutely worth your time if you design PCBs.) this video
“go watch Rick Hartley.”
Explains signals traveling on high-speed boards
The article is not that good, but whatever. I am new to electronics myself and quite fascinated how has the art progressed in the past hundred or so years. There are people remembering "we were doing it wrong". Incredible. Anyway, if you are interested in signals traveling inside those cute green multi-GHz boards you make your living off, go watch Rick Hartley. link (How to Achieve Proper Grounding) this video (What your Differential Pairs Wish You Knew)
“they are all worth watching in this order.”
Part of a recommended Hartley video sequence
I have some further video recommendations: link ("How to achieve proper grounding") link ("Keys to control noise, interference, and EMI in PCBs") this video ("What your differential pairs wish you knew") link ("Secrets of PCB optimization") These are all by Rick Hartley (from GP's comment) and they are all worth watching in this order.
“which is still one of my all time favourite lectures ever.”
A lecture the commenter counts among favourites ever
I found Phil's lab content [1] [2] indispensable for just this. Phil is a great communicator and gives in-depth explanations, so I didn't just watch most of his youtube, but also bought his mixed signals course and was very happy with it. Phil also recommends this lecture in one of his videos [3], which is still one of my all time favourite lectures ever. [1] link [2] link [3] this video
“If you want a longer talk on the topic”
Longer talk on differential pair routing myths
> First off, you want to keep both of the pair’s signals close to each other throughout their length. The closer the two signals are, the better external interference cancellation works, and the less noise they radiate (…) Wrong! This is a myth that's stubbornly difficult to weed out. The coupling between the wires of a pair is only ~12% the amount of coupling to the reference plane. On a PCB it's best to treat each signal on its own and route it as a single ended, coplanar waveguide surrounded by reference planes. This allows to use far thicker widths, which reduces inductance, which also reduces the coupling to radiating EM fields. If you want a longer talk on the topic: this video
“Some interesting material on the subject”
Material on plane splits and reference planes
> so wouldn't it be better to put power/ground on the outer layers and keep signals in the middle? Signals must never cross a break or split in the plane they're referencing (usually 0V or ""ground""). This creates huge EMI problems. Your proposal would have signals on layer 2 crossing a split in the ground plane on layer 1 (that split caused by power traces). Some interesting material on the subject: link this video link link
“I have never in my life length matched the two lines of a differential pair, including at 10 GHz. Never." - Rick Hartley”
Hartley's claim that he never length-matches differential pairs
"I have never in my life length matched the two lines of a differential pair, including at 10 GHz. Never." - Rick Hartley this video
Referenced earlier as source for fields-not-wires view of signals
> He claims that the way EE is taught is a "lie" The thing is: this is sort of true. Now, all electrical engineers are very familiar with transmission line theory, that's pretty much their bread and butter. And all EEs know that if you're not working with well-defined transmission lines (like coaxial cables), you need to use a field solver. 2D field solvers are often sufficient, but if not 3D field solvers can and will be used. And then most of those same EEs, despite having just used a field solver which clearly shows that all the power is in the fields, which are in the dielectric space between the conductors, persist in using the mental model that electrical power moves in wires. This isn't just a pedantic quibble. There are real, practical effects. If you're designing a PCB and you have two signal lines with overlapping fields, those signals are going to couple, which will create common mode current, which will cause an EMI problem. You can stop those signals coupling by making them reference different ground planes, which makes the fields no longer overlap. If you route a signal line from one side of a ground plane to the other, you have to provide a path for the fields to get to the other side of the ground plane (i.e. "route" the dielectric, generally with a ground via), because if you don't, they will find their own path anyway and you won't like the results. If you persist in thinking that electrical power flows through wires, these sorts of effects are mysterious an
“at 1:00:23 in that same video I linked before ( ), Rick Hartley says this”
Hartley says most engineers don't understand EMI fundamentals
> Is there really an epidemic of EE's who know how to use a field solver, but don't know to consider coupling between signal lines? Now, I'm not an EE myself, just someone who took undergraduate electrodynamics, decided to read up the subject, and found some truly excellent videos on youtube. But, at 1:00:23 in that same video I linked before ( this video ), Rick Hartley says this: > I spend most of my consulting time solving EMI problems because most of the engineers I meet have no clue about any of this. My job is so easy and I make such a ridiculous amount of money doing it. It's just unbelievable; I solve most EMI problems by simply adding returned vias to boards or changing the positions of decoupling caps, I mean the things are so simple and ridiculous it's amazing and if these guys would educate themselves they wouldn't need to hire me. So I gather that there is a problem. It's not that EEs don't know to consider coupling between signal lines, but because most of them persist in thinking that signals travel in wires, rather than in the fields, they don't understand when coupling will occur and when it won't. Sure, they can look at the result of the field solver and realize they have a problem, but without thinking in terms of fields they don't know how to solve it. So, they fix it either with trial-and-error until the field solver is happy, or by following design patterns that are passed down as an oral tradition, but without actually understanding why it works or what th
“I avoid application notes as if they were the plague, because frankly, more than 95% of them are incorrect.”
Hartley's opinion that most application notes are incorrect
> But focus on the two most important words in EE: REFERENCE DESIGNS. „I avoid application notes as if they were the plague, because frankly, more than 95% of them are incorrect.“ this video
