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As the title already states, is there a way to change the width and espacially the gap of a differential pair after routing?

In Altium Designer you can easily change the width of both of the differential pair tracks, but if you do so the gap of the diff pair changes depending on the change of the width. I'm searching for a way to adjust the width of both tracks and maintain the gap between them.

I searched the Altium documentation and googled a bit but couldn't find any solutions. I'm afraid there is no easy way to do this, but first let's hear your suggested solutions.

Thanks in advance.

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  • \$\begingroup\$ You might want to try to find a geometry that meets your needs, but instead of adjusting the width and keeping the gap the same, adjust both of them in a way that keeps the center-to-center distance the same. \$\endgroup\$
    – The Photon
    Jun 23, 2021 at 6:12
  • \$\begingroup\$ @ThePhoton By changing only the width of both tracks the center-to-center distance keeps the same, but this is not what I want to achive. \$\endgroup\$
    – Euler
    Jun 23, 2021 at 6:22
  • \$\begingroup\$ What exactly are you trying to achieve? If the goal is to adjust the characteristic impedance, then you can likely do it by changing both width and spacing (keeping center-to-center the same) just as well as by changing the width without changing the spacing. And doing so would avoid having to re-route the tracks, which is what you asked for. \$\endgroup\$
    – The Photon
    Jun 23, 2021 at 6:23
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    \$\begingroup\$ There should be a way to not adjust the width by as much, but keep the center-to-center distance the same, and meet your impedance goal. The only problem comes if this reduces the gap to something not manufacturable. If you share the actual geometry, I could demonstrate with real numbers. \$\endgroup\$
    – The Photon
    Jun 23, 2021 at 14:51
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    \$\begingroup\$ If you're using a full-service fab shop, you could even ask them to calculate the new geometry for you. Adjusting the characteristic impedance without changing center-to-center distance is something they do a lot. \$\endgroup\$
    – The Photon
    Jun 23, 2021 at 14:58

1 Answer 1

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If you update the design rules, the Retrace feature can somewhat intelligently update the traces. See:

https://www.altium.com/documentation/altium-designer/post-route-glossing-and-retracing-ad#!retrace-your-routes

It tends to work better in recent versions of Altium (20, 21), and requires good design-rule discipline. It can also do some stupid things at times, especially when entering pads or with fly-by connections, so it requires close supervision.

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  • \$\begingroup\$ This is pretty much exactly what I have searched for. Thank you! Unfortunatly I don't get it to work. As I have read in the documentation the retrace feature will retrace the track with the preferred track witdh/gap specified by the design rules. In my design rules I have specified a width of 0.25 mm for a 50-Ohm track. I select a track on my design (which is affected by that rule) with a width of 0.255 mm and select Route -> Retrace Selected. But it doesn't change anything. \$\endgroup\$
    – Euler
    Jun 25, 2021 at 10:39
  • \$\begingroup\$ I have tested some other tracks with other rules and specified widths/gaps, but it doesn't work for me. Any ideas why it might fail? \$\endgroup\$
    – Euler
    Jun 25, 2021 at 10:39

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