AltiumLive Frankfurt 2019: Rick Hartley Keynote

Estimated reading time: 8 minutes

Tight coupling did create different line widths: the advantage with tight coupling was that it gave a narrower line for a given impedance, which made the design easier to route. The disadvantage with tight coupling was that it gave a narrower line for a given impedance, which made the design more difficult to manufacture and hence more expensive. And there were signal integrity issues associated with skin effect; sometimes, separating differential lines by greater distances and making the lines wider could be justified for reasons of signal integrity and/or cost of manufacturing.

Hartley discussed crosstalk between tightly-coupled lines sandwiched between planes, where interference from the outside world was reduced because high-frequency fields would not conduct through copper planes. But the close proximity of an aggressive signal on the same layer would result in unbalanced crosstalk, however tightly the differential pair were coupled.

Hartley also made some interesting comments about skew, which, in his opinion, was not nearly as critical as stated in the application notes. He went on to say that he never length-matched the two lines of a differential pair, even at 10-gigahertz frequencies; instead, he ran them side by side, made them approximately the same length, and they had always worked. It was much more important to reference ground on the next layer of the board.

What else could impact timing skew? Board materials. As signals travelled through the dielectric of the composite material, and the dielectric constants of epoxy and glass were different, they travelled at different speeds as they crossed the weave of the glass cloth, and the two lines of the differential pair were always effectively jockeying for position. Hartley named glass styles 1080 and 106 as the worst for this effect because of the width of spaces in the weave could result in 5 mm of skew in 75 mm of routing in a typical example, putting a different perspective on the concept of length matching and causing real signal integrity problems in high-speed designs. The message was to choose one of the newer spread-glass styles designed to minimise this effect, although there could still be some electromagnetic interference issues.

Hartley stressed that one of the biggest causes of electromagnetic interference was changing layers, and he showed an example of a signal line on layer 1 of a circuit board traversing a ground-plane on layer 2 through a via to a signal layer 3. The energy in that circuit was in the dielectric space between layer 1 and the ground-plane layer 2. If it was necessary to change layers in order to change routing direction from X to Y, then the fields would couple through the clearance hole in the plane, the fields would continue on in the dielectric space between layers 2 and 3, and everything would work perfectly with no danger of spreading fields and no electromagnetic interference problems. And for the most part, signal integrity would be maintained.

But if the fields spread out and there were other vias in that region, they would couple into those other vias, and there was a strong possibility of introducing electromagnetic interference. And if it was necessary to go from one ground plane to another, the best way to do it was to place a ground via next to it. Hartley discussed various field-spreading and coupling effects and their consequences and commented that he spent most of his consulting time solving electromagnetic interference problems, admitting that his job was so easy because the majority could be resolved simply by adding return vias or changing positions of decoupling capacitors.

People wouldn’t need to hire Hartley if they would take the trouble to gain some basic knowledge “There is no current inside of a via,” was another forthright statement. The beauty of the fields was that the return current was on the outside of the via barrel. Therefore, contrary to popular opinion, there was no justification to fill the via with conductive material; “You could use peanut butter; it doesn’t matter electrically.”

What about differential pairs? Was a return via necessary when transitioning layers? A lot of people believed not. But Hartley depicted them as two single-ended signals, referencing the plane above or below, rather than them having magical, mystical properties because they were a differential pair. Without a return via, their fields would spread in exactly the same way as a single-ended signal, and create a common-mode current in one or the other line. He illustrated the best way to change layers with a differential pair using a pair of vias, or even a single via: “But you have to take the fields through the dielectric from one dielectric layer to the next. You can’t just ignore the fields because when you do, you set yourself up for problems.”

Rick Hartley’s keynote set people thinking. He had blown away a lot of popular mythology and could support his statements and design principles with factual examples drawn from many years of practical experience. The Q&A session ran for some time.

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