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Varying transmission line parameters

Discussion in 'Electronic Design' started by bazhob, May 25, 2006.

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  1. bazhob

    bazhob Guest

    I read that the electrical properties of transmission lines can be
    described using their RLCG parameters (line resistance, inductance,
    capacitance and dielectric conductance per unit length).

    How can one elegantly introduce "noise" to transmission lines, so that
    their properties vary within a given interval, ie +-100 Ohms, +-100uH,
    +-100pF ... ? Would one use a combination of controlled resistors (ie
    JFETs), inductors and varactors or is there a simpler solution?

    It would also be sufficient if one could adjust those parameters
    specifically (ie. add 10 Ohms, subtract 10pF)

    Thank you!
  2. Tom Bruhns

    Tom Bruhns Guest

    Did you want to build a lumped model of a line, with parameters you can
    adjust at will, or a simulation model, or a real physical line? How
    rapidly did you want to adjust the parameters, and with what accuracy?
    Are you looking to do the same thing all along the line so it remains
    uniform, or did you want to simulate what happens to a line if, for
    example, the center conductor becomes un-centered periodically along
    the length of the line?

    In short, exactly what are you trying to accomplish?

    I'd note that 10 ohms is a huge variation for most practical lines...

  3. colin

    colin Guest

    With a balanced line you could arrange it so you could vary the distance
    between the 2 wires, that should vary the parameters.

    Colin =^.^=
  4. bazhob

    bazhob Guest

    I want to build a real physical (PCB) layout. The adjustment would vary
    only every 5 seconds or so (this is why I would prefer actually setting
    parameters instead of using a noise source).

    I am experimenting with the use of "evolutionary algorithms" in circuit
    design. If there is no variation, the circuit will over-adapt to one
    specific board. The tests are done on real hardware, simulation is not
    an option.

    I sort of want to simulate *in real* that the length of the
    transmission line between two components has been extended, ie. by
    physical wires, or that a 60-Ohm closed switch in the line has been
    removed. I am aware that this can only be solved by approximation.

    Thank you,
  5. Tom Bruhns

    Tom Bruhns Guest

    Hi Toby,

    Well, in theory you can cascade a bunch of sections of series L and R
    and shunt C and G, and with enough sections it will be a decent
    approximation of a TEM transmission line. The problems I see are that
    it takes a lot of sections, and in a real line R and G are typically
    pretty small, so it may not be easy to find parts that will be
    controllable and cover the right range of values. In addition, a real
    line has a frequency dependence that will be very difficult to emulate
    with lumped components. You can use varactors for variable
    capacitance, as long as the signal levels you are transmitting aren't
    very large, and you can similarly use something like saturable reactors
    for the series inductance. All in all it doesn't sound like a very
    practical thing to try to do. If the bandwidth of the signal you're
    transmitting isn't too broad and the line is "long" enough, I'd say it
    might well be easier to digitize the input signal, apply signal
    processing to emulate the desired line, and convert back to analog at
    the other end. That takes care of the transmission characteristics in
    one direction at least, but not the load and source impedances without
    lots of additional work.

    Good luck.

  6. Marc Popek

    Marc Popek Guest

    you are describing more of a monte carlo analysis then adding noise. try
    seeking the monte carlo analysis.
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