# Varying transmission line parameters

• posted

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! Tobias

• posted

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...

Cheers, Tom

• posted

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

Colin =^.^=

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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, Toby

• posted

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.

Cheers, Tom

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