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Measuring VSWR in a Simulation?

Discussion in 'General Electronics' started by Stutzer, May 18, 2004.

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  1. Stutzer

    Stutzer Guest

    I would like to measure the VSWR (or the small signal output impedance)
    of a circuit I simulate (using PSpice A/D by Cadence).
    What would a circuit look like, that indicates the VSWR of the circuit
    loaded with a given impedance, or how would I have to calculate it using
    measurable values?

    I don't konw if the concept of standing waves applies to infinitely
    small lenghts of the "transmission-lines" resp. wires (what is probabely
    assumed for simulation).
    My actual goal is to get a good matching of the circuits output and the

    So far thanks for reading, I hope you can help me.
    D. Stutzer
  2. Mantra

    Mantra Guest

    SPICE's generally can't do it explicitly because SPICE is a
    lumped-equivalent-based simulator. Some harmonic balance simulators
    can because they are distributed-model-based. It's probably easiest
    to just using the formulae and/or a Smith chart.

    rho = (Zload - Zo) / (Zload + Zo)

    VSWR = (1 + |rho|) / (1 - |rho|)

    or the load is purely real (no reactance):

    VSWR = Rload < Zo ? Rload/Zo : Zo/Rload (in C)

    and if you have a slotted line to measure:

    VSWR = Vmax/Vmin anywhere along the transmission line

    Standing waves are relevant any place 1) you can *not* used the lumped
    equivalence model, that is, you must use the distributed impedance
    model, 2) where you have any significant impedance discontinuity, and
    3) where the reflected power is big enough to "matter". Pretty much
    all transmission lines and antennae qualify #1. #2 depends on your
    equipment and cables. #3 depends on equipment specs and your choice
    to worry about it.

  3. Stutzer

    Stutzer Guest

    Thanks for the information.
    Actually, the goal is to match an antenna to the output of a (audio-)
    FM-Oscillator. So what I need is the output impedance of the circuit (at
    its resonance freq.).

    Thanks again and best regards,
  4. Stutzer

    Stutzer Guest

    I'm sorry but I did not post this message really correctly. Therefore
    there is the same discussion going on at sci.electronics.cad.
    Please take a look at it as well if you read my article.
    Thanks and please pardon my mistake.
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