[lmilcin]

Fun to introduce general population to some interesting physics but please, don't call it "settling physics debate".

There is NO debate. Not physics debate, not even practical debate because that knowledge is in practical everyday use.

We know signals in wires take time to propagate and I have to take this into account when I design my circuits. Things like matching lengths of traces (especially differential pairs) so that critical signals take same time to propagate or matching length of trace and return to improve signal integrity are staple for anybody that is working on anything over couple hundred megahertz.

All of this only because it takes time for signal to actually travel through the length of the wire.

[AlabasterAxe]

It's less "settling a debate" and more "clearing up major misconceptions than most people will come away with from watching Veritasiums video". I've taken several physics and electronics courses in my life and after watching Veritasiums video, I was under the impression that the majority of the current would be flowing through the lightbulb immediately which is not the case. So this video is just demonstrating that there's an initial "residual" current induced in the lightbulb from the electric field but the majority of the current is propagating at the speed of light through the wire.

[foxfluff]

"I have never in my life length matched the two lines of a differential pair, including at 10 GHz. Never." - Rick Hartley

https://youtu.be/QG0Apol-oj0?t=2765

[lmilcin]

It is right if you are only interested in making the damn thing work.

But if you don't want it to broadcast noise like crazy you want it tight and matched as much as possible.

The value of differential pair comes from the fact that they have constant common component. That only works if both components aren't shifted in time. If that happens, you will get strong common signal and say bye to nice quiet differential pair.

[foxfluff]

Are you saying your differential line is causing EMI if it's not length matched?

Or are you saying that the receiver won't do a good enough job cancelling common mode noise if you're not length matched? For that, I'd like to have a reference. Otherwise I'm not convinced this is more critical than the max skew to keep your crossings in the right region.

[lmilcin]

Differential pair minimises EMI by having constant common mode potential over the length of the line (and wires close to each other). Meaning if you look at the pair from afar the average of their voltage at any point in the line is constant. Or you might say that the noise from both lines cancels each other. From afar, it looks like there is no signal traveling the length of the line (to an approximation, because there is necessarily some distance between the wires meaning the cancelling is not perfect).

If the signals are shifted in time, though, that is no longer true. From afar you will see places in the line that have different common mode potential. Where one signal edge lags behind the other the average will no longer be the same as for the rest of the line. From afar this looks like high bandwidth signal travelling down the line.

[jeffbee]

I mean ... he's saying right there on the slide there's less than 1mm of length error budget at 10GHz.

[foxfluff]

And that's gobs. What's your typical space between the lines in a pair? Five mils? You run them side by side and you'd be hard pressed to make a length error significant enough to matter. Yeah you do need to be aware of it enough to ask whether you're good when you're routing a really weird loop.. but for the vast majority of things and sane routing, you can just eyeball it or don't need to care at all.

Point is, a lot of people seem to think length matching is critical ("for anybody that is working on anything over couple hundred megahertz") and application notes would have you believe as much but the industry experts and theory I've found suggest it's way overstated.

[jeffbee]

I've always followed Howard Johnson's rule of thumb to match them within 1/10 of the rise time, but it's not like it's a lot of work: the software does it automatically.

[soneil]

That's fairly intuitive. The lower current being present before the signal arrives is not intuitive.

[sega_sai]

I just arrives through the air. The electromagnetic wave will propagate with similar speeds through the air and wire.

[soneil]

I didn't finish the video (yet, no fault of the video, just bad timing), but I don't think it's EMI - the "pre-current" looked fairly flat, and EM doesn't like flat. I'd expect EMI to show as a spike timed to match the leading edge of the signal.

[foxfluff]

I think the takeaway is that EMI looks flat for a while if you run half a kilometer of wire next to a half-a-kilometer long source of EMI.

The leading edge isn't there for one nanosecond and then gone, it travels down and continues to induce a current to the adjacent conductor, a current which must continues to flow.

[charcircuit]

>There is NO debate

I'm guessing you never checked the comment section.

[lmilcin]

Bring any topic however widely known and thoroughly proven to a large enough general audience and you will always find people with no actual knowledge that will question it.

Try to find a post on HN with a lot of upvotes and comments where everybody would agree on something.

If you want to know if there is "physics debate" ask people who know physics, not commenters on Youtube or HN.

[charcircuit]

Why are random commenters not allowed to have a debate regarding physics?

[BenjiWiebe]

They are allowed to.

However, saying the phrase "a <n> debate" -usually- leads people to believe there exists a debate about <n> in the field of <n> between experts who study <n>.