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Measuring S - Parameters in Spice

Discussion in 'CAD' started by Paul Burridge, Aug 6, 2004.

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  1. Hi all,

    I flicked through Google last night and saw some quite simple set-up
    whereby, given an accurate enough transistor model, one might measure
    S - parmaters for any device under consideration purely via
    simulation. Is this feasible? Or is it really only possible to
    reliably do this empirically with real components?


  2. I dont follow what you are asking for. A bit of confusion between
    "measurement" and "simulation" here.

    *All* the parameter sets (h, s, abcd etc) are mathematicly *identically*
    equivalent. A far as simulation goes, it don't care a toss what set you

    S - parameters came into use for R.F, because it is easier to *measure*
    S parameters in the real world at high frequencies. By and large, S
    parameters themselves, in my opinion, are a pain in the arse. If you can
    get a good spice model at h.f, you are better using it, especially,
    because this will allow one to do transient simulation as well.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  3. I think you know. I've since Googled a bit more and found that
    "gwhite" (can't recall his first name off hand, but he's the author of
    the VHF/UHF DX Handbook) posted to one of the radio amateur groups
    that he'd had more accurate results 'measuring' S-parameters from
    Spice simulations than the manufacturers provided in their datasheets.
    He was able to 'adjudicate' between the sheets and Spice by virtue of
    having access to a decent VNA.
    True enough.
    Indeed? I'm surprised. Measuring them might be a pain in the arse, but
    once you have them, they do make it much easier to visualise any
    mis-match when subsequently plotted on Mr. Smith's chart.
    Whatever, but what I'm trying to arrive at is a circuit simulation
    which will enable the modulus and phase-angle components of reflected
    waves to be displayed via Spice. If you or anyone else can come up
    with an accurate way of doing so, then I'd be interested to hear about

  4. Robert

    Robert Guest

    Easily done.

    The biggest problem is the S Parameters vary with bias on the Transistor.
    But you should get reasonable results at different bias points off a good
    Spice Model.

    I have an old 2 page App Note from Wes Hayard (W7ZOI) in DOC format that
    describes a setup that will work in all the various Spices. If you provide
    an address (suitably camouflaged) I'll send it.

    But essentially you drive the device you want to measure the S Parameters
    with a 2 volt AC source through a 50 ohm series resistor. The other end of
    the source is tied to ground. That is, if the impedance of your system is 50

    In shunt with this connection to the device is one side of another 1V AC
    source, set so it subtracts 1V of the applied 2V signal on the side not
    connected to the device input.

    That side of the 1V AC source is tied to ground through a high value (1Meg)

    Then the voltage at the top of that high value resistor will be S11.

    If you tie the output connection of the device you're trying to measure the
    S Parameters to 50 ohms to ground (use a coupling cap if necessary) then the
    voltage at the top of that 50 ohm load to ground will be S21.

    Turn this around to measure S22 and S12.

  5. Chaos Master

    Chaos Master Guest

    Robert, Robert, Robert.... did you really write this article
    (<aScRc.12272$>) in newsgroup
    (sci.electronics.cad) at the date of (Sat, 07 Aug 2004 22:39:02 GMT)?

    [S Parameters in SPICE]
    I am interested in this app note, can you send it to me?

    renan_tdb AT yahoo DOT com DOT br

  6. Sounds just what I'm looking for. Thanks, Robert. Address is
    with no mods needed to reach me. I just don't care!

    Best regards,

  7. I don't use Mr. Smith, that's an even bigger pain in the arse.
    Why? These "waves" do not "exist", i.e they are imaginary, in a lumped
    component model such as a spice one.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  8. I know what you mean, Kev, but this is the very reason I'm posting the
    question. It *is* possible to contrive a simulation to establish a
    device's S -parameters in Spice, notwithstanding real-world
    distributed reactances. I simply would like to know *how* it's done!
  9. Jim Thompson

    Jim Thompson Guest

    It's in this list of app notes:

    ...Jim Thompson
  10. mehran gupta

    mehran gupta Guest

  11. Many thanks, Mehran. That's just the kind of info I'd be trawling the
    'net for and been unable to find.

    Regards, Paul
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