Differential pair skew matching fails with certain dimensions

I've experience the same behaviour.

With default diff. pair dimensions, the "Tune pair skew" works perfectly. I've then changed diff. pair dimensions to 0.2mm track witdh and 0.2mm gap. Tried again "Tune pair skew" and worked fine.

But if I draw a differential pair with 0.28mm track width and 0.15mm gap then it behaves different. The "label" with the actual lenght difference and target still appears but no meander appears (And the reported lenght remains the same)

Regards

Xavi

Ups, I forgot to include the Kicad version used:

Application: kicad
Version: no-vcs-found-a7cdd94~61~ubuntu16.04.1, release build
Libraries:
    wxWidgets 3.0.2
    libcurl/7.47.0 OpenSSL/1.0.2g zlib/1.2.8 libidn/1.32 librtmp/2.3
Platform: Linux 4.4.0-112-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.2 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.58.0
    Curl: 7.47.0
    Compiler: GCC 5.4.0 with C++ ABI 1009

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=OFF
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_SPICE=ON

This seem to be true for anything less than 0.18mm gap when testing on Linux

Same behaviour on Windows 7

Application: kicad
Version: (2018-02-10 revision a04965c36)-makepkg, release build
Libraries:
    wxWidgets 3.0.3
    libcurl/7.54.1 OpenSSL/1.0.2l zlib/1.2.11 libssh2/1.8.0 nghttp2/1.23.1 librtmp/2.3
Platform: Windows 7 (build 7601, Service Pack 1), 64-bit edition, 64 bit, Little endian, wxMSW
Build Info:
    wxWidgets: 3.0.3 (wchar_t,wx containers,compatible with 2.8)
    Boost: 1.60.0
    Curl: 7.54.1
    Compiler: GCC 7.1.0 with C++ ABI 1011

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=OFF
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_SPICE=ON

From the duplicate, this also occurs with

https://github.com/cernohorsky/Modules/tree/master/CommSerial/USBHUB01A/SCH_PCB

May be related, the meander amplitude tuner geometry breaks on low values as shown on the screenshot.

I've seen that behavior, as well as other bizarre failures in which the P/N legs actually short to each other, under some circumstances with low values as well but I haven't been able to reproduce it reliably so didn't file it as a bug.

Not sure if related to this issue but certainly worth investigating.

Now that I studied it a bit more it seems to stem from forced 90° arcs. The rounded corner just does not fit there. That's why I suggest working on a 45° sharp corner implementation and a free angle arc series one. I believe the sharp corner 45° one would be the easiest for length calculation as well as DRC.

Fixed in revision 44c1f5785be0ee7a9238c9754ac4909d2748aeb6
https://git.launchpad.net/kicad/patch/?id=44c1f5785be0ee7a9238c9754ac4909d2748aeb6