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ADS schematic vs momentum

Discussion in 'CAD' started by Philip Newman, Dec 4, 2003.

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  1. Hi, my colleagues and I are trying to simulate interdigital capacitors using
    ADS. We have found our value of capacitance using the-3dB cut-off point of
    the S11 characteristics at the frequency of interest. We have also used the
    equation given in GD Alley's paper, from which we can calculate the length
    of the fingers. The width of the capacitor is limited, as is the length of
    the fingers to less that lambda/10 for operations at 10GHz.

    The problem we have found is that simulations done with the schematic view
    give very different answers to those given by momentum. The design is
    exactly the same, but the answers are different. We tested the capacitor
    using two dog bone structures coupled together with the capacitor to give a
    bandpass filter. however, the momentum sim shows that the passband is
    shifted by 2GHz to the left!

    We have updated the substrate so that everything is the same. the dog-bones
    are the same size, as is the dimensions of the capacitor.

    this problem is very annoying and is holding back our work!

    If anyone has had similar problems, or know how to fix them, then please let
    me know


  2. Mark

    Mark Guest

    I've used Ensemble instead of Momentum, but the Method of Moments
    calculations are similar.

    Have you tried increasing the number of triangles in the Momentum
    simulations? In Ensemble, I noticed frequency changes in patch antenna
    designs when the metal layers were "meshed" tighter. Ansoft's
    suggestion was to vary the mesh size until the results began to
    stabilize, suggesting that the simulation was nearing an optimum
    point. Another thing to check (especially for operation out to 10 GHz)
    is the specification of the metal layers (thickness and material).
    Conductivity will begin to add to the simulation errors.

  3. thanks, we'll try looking at the difference changing the mesh today. we
    have update the substrate from the schematic, so the same one is used, with
    the correct conductivity for copper and permittivity of substrate and that,
    so no change there, though momentum could make a difference here as it
    simulates reality more than the schematic.

    any other suggestions would be greatly appreciated


  4. Good points.
    You could also look for articles by James C. Rautio (the Sonnet guy).
    He has written about the accuracy of method of moment programs. He has
    also covered topics like what to do when metal thicknes can not be

    What else? Well, it is always a good idea to double-check the accuracy
    of the models used in the schematic-driven simulation. For what values
    of the paramters is the model valid etc.

    Remember that Momentum (and its ilk) can analyze 'any' geometry but
    to get trustworthy results you need more insight than when running an
    ordinary circuit simulator. There is quite a number of parameters
    to tweek and eventhough the default is OK in most cases it might
    not be in your case.

    Then of course you have the posibility of building a test structure and
    take it to the lab.


    Martin Schöön <>

    "Problems worthy of attack
    prove their worth by hitting back"
    Piet Hein
  5. thanks for the advice, I think we are slowing getting there!!

    I hope this works because we are getting quite annoyed with it not working!


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