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Impedance setting for ports in Spectre

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Be aware, when using ports in Spectre simulations, a real and imaginary impedance can be defined in the properties menu for the device. When a reactance is specified, this imaginary part is valid only when running small signal analyses like sp, ac, and noise. The reactance will give erroneous results if pss, psp, and pnoise analyses are run. We are reporting this to Cadence as an action item.


Originally posted in cdnusers.org by jmedina

  • Actually the imaginary impedance does work if you're using Flexible Balance.

    A complex impedance can be simulated in the frequency domain, but in a time domain based analysis such as PSS in shooting mode, a non-varying complex impedance is non-causal. If you think about it, it is completely non-physical, and representing this in the time domain is impossible - you simply do not get constant complex impedance over all frequencies, which is what the time domain representation would have to do.

    In a frequency domain simulation, the fact that it is non-physical does not present problems (of course, the results may not make any sense outside a relatively narrow band, depending on the application).

    So it can't really be fixed for PSS in shooting mode, for the reasons I've stated above.

    Regards,

    Andrew.


    Originally posted in cdnusers.org by adbeckett

  • I believe this to be an erroneous conclusion. A simple experiment can verify this. Use two ports, one port for a source (port1) and a second source for a load (port2). Give port1 and port2 a real impedance, say 50 ohms. Give port1 an imaginary impedance and give port2 the conjugate match of that impedance. If port1 has a source power of "Psource", maximum power should be transferred to port2 since they have a conjugate match. I have tried this using flexible balance and shooting in PSS and neither yields the correct answer. Cadence identifies only ac, sp, and noise as being supported analyses with port reactance since the release of MMSIM6.0_USR2.
     
    If you perform the same experiment using real port impedance and external L and C to create imaginary parts for the impedance, you do get expected results when the ports are congugately matched.


    Originally posted in cdnusers.org by jmedina

  • Andrew, In theory it seems there is agreement. The reactance should be ingnored when running a time-based analysis. The documentation even states this. But, a plot of S11 after running a PSP analysis (with the PSS using shooting method) results in an incorrect result when a reactance is defined in the port element. Trying to run a PSP analysis (with PSS using flexible balance)fails because the PSP never runs stating the flexible balance is not supported. Error found by spectre during PSP analysis `psp'. FB small signal currently only handles ac, xf, noise and stb So, going back to the shooting method and removing the reactance from the port element, PSS give the proper and expected result. All we are saying is the reactance is not truly ignored as it should be. Cheers, Kris


    Originally posted in cdnusers.org by kdonate

  • I think it _ought_ to work with flexible balance (you might need to set tstab to 0?), but if it isn't working, please report it to Cadence so that this can be investigated.

    Unfortunately I am travelling at the moment so don't have the chance to test this.

    Regards,

    Andrew.


    Originally posted in cdnusers.org by adbeckett

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