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Allegro Impedance Calculator problem

Hossein1357
Hi,

I am working on my board Stack-up to tune Single/Differential impedances of my board in Allegro PCB 16.6. I see that Dielectric Constant is 4.5 (FR-4) ! even for Conductor/Plane layers. I don't see if I have to change it to 1 or not? I googled and found some online calculators. Unfortunately, each one has its own approximations and consequently they're giving different results for a single set of configurations!. I am confused which one is correct - Allegro Impedance Calculator or online ones? I asked our engineers in RF department and they used AWR - Microwave Office- software which gave us different results from Allegro or online calculators. According to new calculations it seems that I have to set Dielectric Constant to 1 for Conductor/Plane layers!. If I change it to 1 I'll see that Allegro & AWR results are almost the same. However, it seems that new results are strange and different from what I see in boards of famous manufacturers? For example, Xilinx has used 9 Mil spacingas well as 3 Mil width - Mid layers - in SP605 boards for DDR-3 differential signals. These specifications result to reasonable impedances with 4.5 Dielectric Constant of Conductor/Plane layers, while differe completely from AWR results?! :-(

I am looking for comprehensive information about Allegro PCB Impedance Calculator. What shaould I do? Using Dielectric Constant of 1 for Conductor/Plne layers or continue with that default value - 4.5 - and neglect differences with other claculators.

Any comments are welcome.

Hossein
  • Hossein1357
    Urgent help is needed. I wish anyone could help.
  • kabalee

    HI,

    don't change the any dielectrical contant.you need to get all the details from Manufacturer.you need to get trace widht and trace thickness,impedance value for your trace (single and Differential).

  • Hossein1357
    I am curious to know the differences. I know what manufacturers want to control impedance. I see serious differences between what Allegro calculates and other reliable famous software like AWR. I hope if someone can tell me the differences.
  • Dennis Nagle

    The impedance calculator in the Allegro stack-up editor uses a static 2D field solver by default (bem2d). It has been correlated many times. You can see for example, Matthew K. Herndon's (Apple) paper from DesignCon 2003 "Noise Allocation for Near-end Crosstalk and for Uncertainty in Characteristic Impedance".

    For more details on using bem2d and the full wave optional solver ems2d, see the doc: <install_dir>\doc\BP_si_solver\BP_si_solver.pdf

    Regards,

    -Dennis 

  • Hossein1357

     Hi Dennis,

    Thnak you for your information. However, this is not what I am looking for. As I further mentioned I see differences between AWR calculations and Impedance calculator of Allegro PCB!. Another strange thing I see is that we need to give Dielectric Constant for conductor layers! while we know it should be 1 for conductors!. I hope this will make sense. Any information regarding this topic is highly appreciated.

     Thanks,

    Hossein

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