topic Matching/calculating propagation delay of internal traces and external traces in EAGLE Forum

Matching/calculating propagation delay of internal traces and external traces

grahamUTKMN — Mon, 27 Jun 2022 08:54:46 GMT

Hi,

I'm routing a board where I need to match some internal traces and external traces. I know ideally they should be on the same layer, they I could just match the lengths, no. of vias etc, but the routing just isn't viable.

I'm trying to match 'time' here, or I guess propagation delay is a better term. As signals travel faster on external layers, is there a calculator somewhere that I can use to work out what length the external traces need to be to match the internal traces? What other variables do I need from my PCB supplier? I don't see anything built into Eagle to support this? How about Fusion 360?

Many thanks,

G

Re: Matching/calculating propagation delay of internal traces and external traces

jorge_garcia — Mon, 27 Jun 2022 19:38:06 GMT

Hi @grahamUTKMN,

I always like to use the Saturn PCB Toolkit. Google it.

Now symmetry is your friend here, if you can make the various traces involved as symmetrical as possible you may be able to get away with the length match meander.

Let me know if there's anything else I can do for you.

Best Regards,

Re: Matching/calculating propagation delay of internal traces and external traces

grahamUTKMN — Tue, 28 Jun 2022 08:34:40 GMT

Hi @jorge_garcia ,

Thanks for the reply.

What do you mean by symmetry is my friend?

Unfortunately I'm on a Mac and that seems to be windows only.

Does the Saturn Toolkit actually allow me to work out how much longer my stripline traces need to be to match my microstrip traces?

Do you know of any other resources?

Is this feature in Fusion 360 at all?

Thanks,

Re: Matching/calculating propagation delay of internal traces and external traces

jorge_garcia — Wed, 29 Jun 2022 18:35:57 GMT

Hi @grahamUTKMN,

I hope you're doing well. I believe you can run Windows software under Mac using Parellels. That calculator isn't at all heavy on system resources so it should be performant. You could also try checking the calculators here https://www.microwaves101.com/calculators . This site is a treasure trove of information.

My symmetry comment is a general one. If you can share a visual description of what you are trying to do that can give me something more solid to talk about. Basically, is the absolute delay important or is it just that all the signals should have the same delay? If it's the second one then if you can make all of your traces of interest similar then you only have to worry about keeping any deviations between the traces as small as possible.

Fusion 360 doesn't have any calculators built into it.

Best Regards,

Re: Matching/calculating propagation delay of internal traces and external traces

grahamUTKMN — Fri, 01 Jul 2022 16:21:31 GMT

Hi @jorge_garcia

I'm laying out DDR3 and these signals *should* be on the same layer ideally and matched length (but matched length only to match the timing), but I can't route them.

So what I'm trying to do is work out how much longer the outer layer tracks need to be to match (time wise) the inner layer tracks.

Thanks,

Re: Matching/calculating propagation delay of internal traces and external traces

jorge_garcia — Fri, 01 Jul 2022 18:42:35 GMT

Hi @grahamUTKMN,

I hope you're doing well. OK, so like we've mentioned there are no tools built-into Fusion or EAGLE to handle this. If by any chance you have access to a tool like Ansys, then you could export the design to Ansys to do the calculations. This export is currently only available through Fusion 360.

If Ansys is not an option, then what I would try to do is make sure all the traces have some portion on a surface layer and some part on an internal layer and make sure they all have the same number of layer transitions(vias). Then length match the traces. That's as close as you'll can get without more sophisticated sim tools.

Best Regards,

Re: Matching/calculating propagation delay of internal traces and external traces

grahamUTKMN — Thu, 07 Jul 2022 10:51:03 GMT

Hi Jorge,

I don't have access to a tool like Ansys.

I'm now not sure what I can do. It's not possible to route all traces across both layer.

Thanks,

Re: Matching/calculating propagation delay of internal traces and external traces

jorge_garcia — Thu, 07 Jul 2022 18:16:12 GMT

Hi @grahamUTKMN,

Here's what I'm going to recommend. When we route we should prioritize critical signals first. The best thing to do now may be to rip up some of your traces get these routed and then continue from there. It may be the case that the traces that are in the way are critical too. Without looking at the design it's hard to be specific about what you could do.

I know this advice would require you to redo some of your work, but with the information I have there isn't much more I can recommend.

Best Regards,