Differential pair routing length/skew adjustment question

Hello,

I’m a beginner in KiCad and I’m trying to route 3 differential pairs from a connector on the bottom of the PCB to a connector on top of the PCB. For me it seems like, the Length/Skew function is somewhat broken or i have a understanding problem. I’m using version 5.99 currently.

  1. If i want to adjust the skew of a differential pair and match the two traces I get the following result. Trace D2+ is shown with a length of 36,3019mm. Trace D2- with 34,9782mm. I’d think that the length difference is about 1,3mm. But the skew tool tells me, the shorter Track is about 5,5mm too short. If i add the meanders as kicad recommends, the resulting D2- track is then shown with a length of 40,5288mm. I’m not sure where these 5,5mm come from. I created a screenshot to visualize this:
  2. I have the same effect for length tuning. If I want to adjust the length of another differential pair to match the 36,3mm of my longest track, I set the target length to 36,6mm. Then i add the meanders and the resulting track length is then 46,7mm. I added another screenshot to show my result: Screenshot2 (Sorry, for imgur, I’m only allowed to add one picture as new user)

Am I missing something or using this tool wrong? Or is it maybe a bug in the nightly build of kicad I am using?

Thank you

Christian

You can try to find out if there are any relevant open bugs: https://gitlab.com/kicad/code/kicad/-/issues?scope=all&utf8=✓&state=opened&label_name[]=router (differential pairs and length tuning are part of the router).

Ok, thank you. I’ll have a look! This might be my issue, will check. https://gitlab.com/kicad/code/kicad/-/issues/7809

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Yes, I think there is a bug. If you have overlapping tracks, the skew adjustment is calculating the wrong length.

The second error occurs, because KiCad is only measuring to from Pad to the Via. Not the distance from Pad to Pad. So with some math I should be able to get to the correct length.

Lenght stats action plugin migh be of some help

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