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Symbolic (Analytical) Transfer function from Cadence (XF/PZ) for Simulink modeling

willTspriggs

I am trying to extract a symbolic transfer function from a circuit I have so that I can use that transfer function in Simulink. I am designing a multi-stage op amp and wanted to be able to use the Simulink control system toolbox to optimize my op-amp and take some of the guess work out. I know how to run the XF and PZ analyses, but I'm not sure how to use them to accomplish my goals (or if I can accomplish my goals using PZ and XF). So here are my goals

Produce a symbolic expression or numerical representation of a subcircuit such that: 

  1. The representation can be used to build (with the models of other subcircuits) and optimize the full circuit in Simulink
  2. In addition to meeting (1), the representation convers all possible DC operating point
  3. In addition to meeting (1) and (2), the representation covers the expected frequency range

I have had trouble meeting all these goals so far. XF only covers small signal analysis and I'm not sure if it actually does produce a symbolic expression. Also, with the PZ analysis, I guess I could conceivably do a frequency and voltage sweep, but I am not sure if this is any better. The PZ analysis doesn't seem to cover every frequency. So here are my questions

  1. Does XF analysis produce a symbolic transfer function?
  2. If XF produces such a function, how can I export it?
  3. Does PZ analysis cover all input/ouput frequencies? (The name seems to imply it would)
  4. If PZ does not cover all frequencies, what would be the best way to use it to generate a transfer function in Simulink (using pole-zero_block)?
  5. Is cosimulation a better method? I believe I have the tools to run cosimulation.
  6. If cosimulation is the best method, can someone give me directions or point me to tutorials that can be describe my specific simulation needs?

  • Answering your questions:

    1. No - it gives you the transfer function from any input source to a single specified output, as a voltage gain, current gain, transimpedance or transconductance versus frequency plot. So that's not really a "symbolic transfer function"
    2. Since it doesn't produce a symbolic transfer function, you can't export it
    3. Yes, it does. There is a "frequency" you specify in the analysis, but this is the frequency to use for any frequency-dependent components (note, a capacitor or inductor is not normally a frequency dependent component). From the spectre help it says:
      Note: A frequency-dependent component means that the capacitance or
      conductance-equivalent representation of the component varies with the
      frequency. Examples are transmission lines or bjts with excess phases. A linear
      capacitor is not a frequency-dependent component.
    4. It does, so this is a moot point
    5. Co-simulation will be a lot slower, and potentially more complex to set up than characterising the block and reusing in the external tool. 

    You could export the data from pz by copying and pasting from the log file, or using OCEAN functions - some examples in this article of mine (plus look at the related article link at the bottom of this article): Simple calculator functions for accessing pz (pole zero) results

    Andrew.

  • in reply to Andrew Beckett

    Thank you very much for the information. Are transistors considered frequency-dependent components?

  • in reply to willTspriggs

    Generally not, because the capacitances are not frequency dependent (as it says in the note, transmission lines and bjts with excess phase are some of the few).

    Andrew.

  • in reply to Andrew Beckett

    ok, though I would confirm just because the gave me a warning about having MOS transistors in schematic being frequency dependent.

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