[Solved] Differential pair: what is the difference between the "Tune Skew" measurement and the "Routed Length" mesurement?

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1

Hi,

I have a differential pair I0_P and I0_N which I want to adjust the skew. On the status bar, I can read that the routed length are IO_P = 30.0625 mm and IO_N = 29.9039 mm which give a skew of 0.1586 mm:

kicad-lengths
kicad-lengths1155×817 212 KB

But the “Tune Skew of a Differential Pair” tool gives a skew of 3.1493 mm:

kicad_skew
kicad_skew770×496 80.3 KB

I know that the “Routed Length” takes in account my board stack up which is 0.305 mm, but I don’t understand the skew given by the “Tune Skew of a Differential Pair” tool.
Anyone knows how this tool measure the skew?

KiCad version 8.04 ; Windows 10

Thank you

2

The problem was that one of my segment was not correctly connected to the center of a via.
Both measurements are equal now…

3

There is more to it. A single via would not have resulted in an over 3mm difference.
There is another current issue for this length tuning:

4

Yes there are a number of issues with the algorithms.

5

This afternoon I’ve been reading https://docs.kicad.org/8.0/en/getting_started_in_kicad to see if there are chapters I can add something useful to, and it also states a similar discrepancy.

https://docs.kicad.org/8.0/en/pcbnew/pcbnew.html#differences_between_net_inspector_and_length_tuner