Checking High speed signals

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1

Hello all,

I have a board that I am working on which contains high speed HDMI, Ethernet, and USB. I have the signals routed and I used the calculator to ensure that my parameters meet the impedance spec (track width and separation) for the specific standard.

Next, I want to check if I need to implement length tuning for my diff pairs. I was doing some searching and found the following spec for length tuning on the signals:

HDMI: ± 118 mil
USB ± 20 mil
Ethernet ± 20 mil

I am not sure how to use KiCAD to ensure that the tracks are within spec. I should also note that I am new to high speed routing. So I am learning as I am going about it. I do have some basic understanding on transmission line stuffs. But there is certainly more for myself to learn.

2

Hi,

I think the length of a selected track is shown in the status bar, also when inserting a tuning element (Route/tune length of a … track) or modifying it.
I recall a related discussion on DDR interfacing that used a spreadsheet to manage the different constraints.

Note return current vias https://www.ti.com/lit/an/snla387/snla387.pdf?ts=1727198587832 Fig. 2-3 for high-frequency signals, especially for single-ended ones.

3

Hello,

Thank you, that is a good resource. so when it says that the trace should not be more then 2000 mils long, is that the entire net or from one “component” to another? (ex. trace from via to another via)

4

Older versions of KiCad had bugs in measuring differential track lengths. It was difficult to measure skew (track length differences in a differential pair)

Toradex has some good examples about designing high speed interfaces. For instance

This gives requirements of various interfaces.

Raspberry Pi Forum might also have some info. Their Compute module IO boards have high speed interfaces, and it is supposed to be easy to modify. Ha. (It is, after some study.)

2 Likes
5

Very helpful, indeed.
Thanks a lot!

6

Ok so it sounds like that the biggest thing here is that the lengths for the +/- tracks have to be the same length within tolerance

7

Hello everyone,

Quick question about checking the lengths. I feel I ran across this somewhere in the forums. But in checking the lengths for the nets, I am getting 3 different measurements:

  1. In the DRC with my custom rules setup
  2. The highlight net shortcut to get the length
  3. The “inspector” tool thing before modifying the differential track.

So like, which one should I be referencing off of?