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Analog Delay line in multisim

  1. Apr 13, 2017 #1
    I'm trying to construct an analog delay (series of low pass filters) and demonstrate the properties of a transmission line (reflection, attenuating etc.) using a multisim simulation (my attempt attached below). But on the oscilloscope I don't get any kind of reflection. What should I do?
    1zlzlgh.jpg
    Thank you
     
  2. jcsd
  3. Apr 13, 2017 #2

    berkeman

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    Staff: Mentor

    Can you show us the waveforms you are getting? Also, I don't think C1 will do much of anything useful if you are driving the input to the TL with an idea voltage source. What is the output resistance that you have assigned to V1? Is V1 a sine wave source or a pulse source?
     
  4. Apr 13, 2017 #3
    This is the wave form I get for sine and square pulse inputs
    347i4v9.jpg 2v0ew7p.jpg
    I get the the delay output but no reflection effects. First I tried to get the waveform for an open circuit.
     
  5. Apr 14, 2017 #4

    berkeman

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    What is the target Zo for your TL? It sure looks very lossy -- are those 1000 Ohm resistors in the TL?
     
  6. Apr 14, 2017 #5
    No such target, but I only need to demonstrate delay and reflection effects. No need to use specific values.
     
  7. Apr 14, 2017 #6

    berkeman

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    Well, you need the Zo to calculate the expected reflections of any mismatched termination, no?

    Also, why is it so lossy? The parallel resistances are meant to represent the conductance of the dielectric insulator in the TL (at the test frequency). 1000 Ohms is way too much conductance, IMO.
     
  8. Apr 18, 2017 #7

    eq1

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    I don't think the circuit as drawn will work because it's not a transmission line. The main issue is each filter bank is referenced to the same potential. To make this work you'll need inductance in the return between each too. Also, to make the lumped model approximation correct the time constant of each stage has to be >> than the edge rate of the signal (which is not the same as it's frequency). (Note: because "ground" is not the same on both sides of the filter you'll need to find the delay through the two port network. This is not the same as the equations you typically use for a garden variety LC filter.) This is definitely the hard way to do it but if you can make it work, and know why it is working, then you'll definitely have a much better understanding of transmission lines.
     
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