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asymmetric stripline

Discussion in 'Electronic Design' started by John Larkin, Jan 29, 2008.

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  1. John Larkin

    John Larkin Guest

    Given a pc board like...





    ======================================= ground
    ^
    12 mils
    v
    ======= trace ----------

    ^
    24 mils
    v


    ======================================= ground




    and assuming FR-4 and 1 oz copper, does anybody know what trace width
    gives 50 ohms? My rough guess is 12 mils.

    googling online calculators yields an impressive collection of
    defunct/missing/silly web pages. I suppose I could fire up ATLC, but
    that's a heap of work.

    Help appreciated.


    John
     
  2. 12mils gives 52ohm. 13mils gives a better 50.158ohm, see here under
    simulation done with the (very good) GPL 2D impedance calculator software
    from Kirby Goulet (cf http://sourceforge.net/projects/mdtlc/) :

    Simulation pix map 400 pixels high by 1600 pixels wide.
    1921024 bytes allocated for bmp.

    INPUT PARAMETERS:

    Layer Thick Er Specifications
    Copper Plane Top 1.30 3.20 Open w=0.0 off=0.0
    Laminate Layer 1 12.00 4.20 64.3% 3.2/6.0
    Signal Layer 1 1.35 4.20 W=13.0 Etchback=0.00
    Laminate Layer 2 24.00 4.20 64.3% 3.2/6.0
    Copper Plane Bottom 1.30 3.20 Open w=0.0 off=0.0

    Rdc_trace_1= 38.67 milliohms/in 20C
    DC resistance by pixel count:
    Rdc_trace_1= 37.291 milliohm/in.
    C = 3.462 pF/in L = 8.709 nH/in.
    Er = 4.200 Delay = 173.626 ps/in.
    Zo = 50.158 Ohms

    Friendly,
    Robert Lacoste
    www.alciom.com
     
  3. john jardine

    john jardine Guest

    Good link!. Looks like fun. Thanks.
     
  4. John Larkin

    John Larkin Guest

    Thanks. I'd used Er of 4.5 for the dielectric, so my 12 mils guess is
    pretty close, maybe even a bit below 50 ohms. This is for a test trace
    that I'm piggy-backing onto a production board; I'll TDR it and post
    the results. It will include various traces and vias, and do some
    "reference plane" switching.

    That program looks like a fancier version of ATLC. It still appears to
    need an external bitmap generator. But there seems to be no HELP.

    John
     
  5. Tom Bruhns

    Tom Bruhns Guest

    My recommendation: contact your board house. They (should!!) have
    good calculation capabilities for the specific board material and
    stackup you are dealing with, and can even tell you tolerances. I've
    had very good service on this sort of thing from the couple of board
    houses we use.

    I do have a nice spreadsheet that will come close on things like this,
    and on differential traces and various other things, but when push
    comes to shove, the board house is my backup.

    Cheers,
    Tom
     
  6. me at

    me at Guest

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