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 routing diff pairs takes too long 

Last post Thu, Nov 20 2008 9:26 AM by redwire. 4 replies.
Started by wolf 19 Nov 2008 01:46 PM. Topic has 4 replies and 1301 views
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  • Wed, Nov 19 2008 1:46 PM

    • wolf
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    • Joined on Wed, Jul 30 2008
    • Grand Prairie, TX
    • Posts 6
    • Points 75
    routing diff pairs takes too long Reply

    When routing diff pairs manually, the field solver activates and runs calculations. the constraint manager modes are turned off and the DRC check is disabled. It still takes 1-2 miutes or more to start a route and every move of the mouse causes more calculations. Any way of turning off the field solver to speed up routing manually.

    • Post Points: 35
  • Wed, Nov 19 2008 3:20 PM

    • pcbgeorge
    • Top 500 Contributor
    • Joined on Wed, Oct 1 2008
    • Beaverton, OR
    • Posts 32
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    RE: routing diff pairs takes too long Reply
    Do you have an IMPEDANCE property on the net?
     
    If so, remove it and set the trace width using a net class.
     
    There may be an easier way that I don't know of, but this will stop the calculations and give you back your computer.  It us usually more accurate, too, since the Cadence calculated widths can be different from that specified by manufacturers.
    • Post Points: 20
  • Thu, Nov 20 2008 5:13 AM

    • wolf
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    • Grand Prairie, TX
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    Re: RE: routing diff pairs takes too long Reply

    Thank you. A fellow designer and I set this up and it works great. You are correct in that the mfg numbers for impedance are better than the Cadence numbers. I usually get close with Allegro, but not confident with it.

    • Post Points: 5
  • Thu, Nov 20 2008 9:00 AM

    • pcbgeorge
    • Top 500 Contributor
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    • Beaverton, OR
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    RE: RE: routing diff pairs takes too long Reply
    Glad I could help you.
     
    The Allegro field solver gets it a lot closer now than it used to, and it will be in the ball park if you allow it to run and have all your layer parameters set-up correctly, but it will never be exactly what your manufacturer provides (unless you are lucky) since there is still no way to allow for the tolerances and finished prepreg thicknesses and material mixes that the manufacturer will be able to plug into their calculations.
    • Post Points: 20
  • Thu, Nov 20 2008 9:26 AM

    • redwire
    • Top 10 Contributor
    • Joined on Thu, Jul 17 2008
    • Allen, TX
    • Posts 880
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    Re: RE: RE: routing diff pairs takes too long Reply

     Actually it's all about your process... we interact very tightly with our PCB vendor and as such have a limited set of suppliers.  When we take their data (we specifiy weave type on every layer for many reasons...) we can usually be within 1/2 ohm of their target impedance.

    If we weren't to interact, then your generalization is true.  I don't allow my designs to be handled in such a manner so I can in fact rely on the Cadence solver to accurately predict what the impedance is going to be. :)

     

    • Post Points: 5
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Started by wolf at 19 Nov 2008 01:46 PM. Topic has 4 replies.