Mini Lab for EE117L—Propagation of Pulses in Transmission Line

Professor M.A. Lieberman
Professor S.E. Schwarz
Developed by: Tim Chou, Wilson Lee

Overview:

Pulse propagation and impedance matching are important concepts in the transmission of signals and information in communication electronics. Reflections and power loss can distort and destroy signals. In this lab, you will learn to predict the reflection characteristic at the termination of a transmission line in different configurations based on the following equation for the reflection coefficient,

ro = (ZL - Zo)/(ZL + Zo)

while the transmission coefficient,

t o=2ZL/(ZL + Zo )

Various configurations including single and dual transmission lines and single and multiple resistors will be explored.

In this part of the EE117 laboratory, propagation of electrical pulses and waves will be analyzed in detail using computer software. The computer program will help you visualize waves traveling in transmission lines. For this experiment, we will be using Travis transmission line software. Travis is a high-level computer program that allows the user to customize the characteristics of the transmission lines.

This lab is composed of three parts. In the first part, you will construct a transmission line circuit. And in the second and third parts, you will be studying two pre-made transmission circuits.

    1. Simple single transmission line circuit with a resistive load
    2. Single transmission line with multiple resistive loads
    3. Double transmission line with multiple resistive loads
The files required for this lab are: single.pro and multiple.pro

 Pre-lab:

Please read the attached pages of the manual before the coming to the laboratory and complete the following questions.

Figure 1: Dual Transmission Line Circuit

A double transmission line circuit is different from previous cases because at anytime, the voltage and current amplitudes are a superposition of transmission and reflections from both lines. In this system, pulses will superimpose constructively or destructively. To illustrate this, analyze the previous 50W characteristic impedance double transmission line in figure 1.

To further understand this, we must realize the details of the experiment:

Determine the length of each transmission line assuming the signal velocity is the speed of light:

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What is the reflection coefficients at R2 and R3?

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What is the reflection coefficients at Vout looking from the left transmission line? How about from the right    transmission line? (Hint: the left and right transmission lines have the same characteristic impedances so the coefficients should be identical)?

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What is the transmission coefficients at Vout looking from the left transmissions line? How about from the right transmission line? (Hint: the left and right transmission lines have the same characteristic impedances so the coefficients should be identical)?

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What is the equivalent impedance Req at Vout, i.e. looking into the transmission lines from the source?

( Hint: the two transmission line are in parallel ) _____________________________________________

What is the voltage at Vout with respect to ground, what is the drop across R1?

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What is the voltage at V2 and V3 when the pulse emitted from the source first reaches it? (Hint: the front part of the pulse that is reflected overlaps the back part of the pulse that is traveling toward the resistor)

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 Now that you have adequate knowledge of this system, draw the signal that is detected at Vout for time t=0us to 100us. Include a voltage and time scale.
 
 

Part I.

Procedure:

    1. Please make sure that the computer is turned on. See your T.A. if there is any problem

    2. Start the Travis Program that is located in the STARTà PROGRAMS.

    3. You should now have the Travis main circuit window opened. Please make sure the words Edit circuit appears on the left corner of the window. This means that the program is ready for the user to start constructing or editing a circuit. **Please make sure you have done your pre-lab readings before going on.

      4. Your are now ready to construct the transmission line circuit.

    4. Move your cursor to the top menu, select PLACE à VOLTAGE SOURCE. The words Place Component should appear on the right corner of the main circuit window. A voltage source icon will appear in the window. Drag the voltage source to the appropriate position and right click to place the voltage source

    5. Move your cursor to the top menu, select PLACE à DRAW WIRE. The words Click to start wire should appear on the right corner of the main circuit window. Right click at the appropriate position to start drawing wire, left click to end.

    6. Move your cursor to the top menu, select PLACE à TRANSMISSION LINE. The words Place Component should appear on the right corner of the main circuit window. You can now see the transmission line icon. Right click and select PLACE to place the transmission line. You can also select ROTATE or MIRROR to correctly position the selected transmission line.

    7. Move your cursor to the top menu, select PLACE à RESISTOR. The words Place Component should appear on the right corner of the main circuit window. Right click and select PLACE to place the resistor. You can also select ROTATE or MIRROR to correctly position the selected resistor.

    8. Now, you can complete the circuit by connecting all components by wires.

    9. Move your cursor to the top menu, select PLACE à GND. The words Place Component should appear on the right corner of the main circuit window. This is the ground node. Right click and select PLACE to place the ground node. You can also select ROTATE or MIRROR to correctly position the selected ground node.

    10. Your circuit should appear as Figure 2.

      11. Figure 2: Single Transmission Line Setup

    11. Move your cursor to the top menu, select SEE RESULTà PLACE ANIMATION PLOTS ONE AT A TIME. The words CLICK t-LINES TO TURN ON ANIMATION PLOTS should appear on the right corner of the main circuit window. Right click the transmission line. Now, the transmission line is darkened. This means that the transmission line is ready for animation.

      12. You have completed the construction of the transmission line circuit. We now need to change some of the parameters of the transmission line circuit.
    12. Left click the voltage source. You should see the parameters of voltage source listed. For our experiment, make the following changes to these variables in the "tran" line.

      13. tr = 0.1us
      tf = 0.1us
      pd = 2.0 us
      T= 30.0 us

      such that it reads tran=pulse(0,1,0,0.1u,0.1u,2u,30u)

      Click Okay to save changes

    13. Left click the transimission line and make the following change.

      14.              td= 5.0 us

      Click Okay to save changes.

    14. Move your cursor to the top menu, select ANALYSIS à SETUP TRANSIENT ANALYSIS… A window should appear. The START TIME should be set to 0.00 and the STOP TIME should be set to 200us and the NUMBER OF POINTS set to 300.

      15.  

      Click Okay to save changes.

    15. Now, you will run the circuit. Move your cursor to the top menu, select ANALYSIS à RUN TRANSIENT ANALYSIS. Wait for the computer to complete the analysis.

    16. To view the animation, select SEE RESULTSà PUT PLOTS ON ALL TRANSMISSION LINES. To scale the animation, select SEE RESULTSà SETUP TRANSIENT ANIMATION PLOTS. Hit the button AUTOSCALE ANIMATION PLOT LIMITS in the lower right hand corner to autoscale the plot and then hit OK. This allows you to use the full height of the animation box for graphing.

    17. Move the cursor to the top menu, select SEE RESULTSà ANIMATE TRANSIENT ANIMATION PLOTS. A window should appear, click PLAY to play the waveforms. To see this waveform on the transmission line circuit, double click the transmission line. You should have another window open, with an X moving in the window. The movement of the X symbolizes the propagation of the wave in the transmission line.

      18.  

      Now, you have finished the first part of the laboratory. Below are some questions designed to help you better understand the laboratory.

Whether the terminating impedance is large, small, or identical to the characteristic impedance of the transmission line, it will determine what will happen to the signal when it reaches a termination. The reflection coefficient ro = (ZL-Zo )/(ZL+Zo) determines the ratio between the incident wave function and the reflected wave function.

For 50W characteristic impedance cable, what is the reflection coefficient for each of the following cases?

shorted termination, ro=____________

open termination, ro =____________

matched termination (50W ), ro =____________

How does the voltage amplitude of the reflected signal change with the change in impedance of the termination?

Now that you have run the simulation once, rerun the simulation using different values of resistance for the terminating resistor. What do you expect to happen for large and small impedance loads?

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What voltage do you expect to see at the terminating resistor when the pulse reaches it? Double click the wire right above the terminating resistor to see the voltage at that point. Is this what you expect to see in comparison to the source voltage?

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Part II.

Figure 3: Single Transmission Line Setup with source resistance

Procedure:

  1. Move your cursor to the top menu, select FILE ® OPEN PROJECT. Select the file Single.Pro and press ok.

  2. If the transmission line is not highlighted, repeat step #12 of part I.

  3. Before running the program, you will have to make some changes.

  4. Left click the voltage source. You should see the parameters of voltage source listed. For our experiment, make the following changes to these variables in the "tran" line.

    5. tr = 0.1us
    tf = 0.1us
    pd = 2.0 us
    T= 30.0 us

    such that it reads tran=pulse(0,1,1u,.1u,.1u,2u,30u)

    Click Okay to save changes

  5. Left click the transimission line and make the following change.

    6. td= 5.0 us

    Click Okay to save changes.

  6. Move your cursor to the top menu, select ANALYSIS à SETUP TRANSIENT ANALYSIS… A window should appear. The START TIME should be set to 0.00 and the STOP TIME should be set to 200us and the NUMBER OF POINTS set to 300.

    7. Click Okay to save changes.

  7. Now, you will run the circuit. Move your cursor to the top menu, select ANALYSIS à RUN TRANSIENT ANALYSIS. Wait for the computer to complete the analysis.
  8. To view the animation, select SEE RESULTSà PUT PLOTS ON ALL TRANSMISSION LINES. To scale the animation, select SEE RESULTSà SETUP TRANSIENT ANIMATION PLOTS. Hit the button AUTOSCALE ANIMATION PLOT LIMITS in the lower right hand corner to autoscale the plot and then hit OK. This allows you to use the full height of the animation box for graphing.
  9. Move the cursor to the top menu, select SEE RESULTS à ANIMATE TRANSIENT ANIMATION PLOTS. A window should appear, click PLAY to play the waveforms. To see this waveform on the transmission line circuit, double click the wire near the transmission line. You should have another window open, with an X moving in the window. The movement of the X symbolizes the propagation of the wave in the transmission line.

    10. Now, you have finished the second part of the laboratory. Below are some questions designed to help you better understand the laboratory.

For 50W characteristic impedance cable with both a source and load resistance, what would happen to the pulse after it has reflected off the load? Does the source resistance affect the initial pulse amplitude?

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What would happen when the source and load resistances are matched to the transmission line’s impedance?

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 Now rerun the simulation with various source and load resistances. Do the source and load resistances both cause reflections?

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Part III.

Figure 4. Dual Transmission Line Setup

Procedure:

    1. Move your cursor to the top menu, select FILE ® OPEN PROJECT.   Select the file Multiple.Pro and press ok.

    2. If the transmission line is not high lighted, repeat step #12 of part I.

    3. Before running the program, you will have to make some changes.

    4. Left click the voltage source. You should see the parameters of voltage source listed. For our experiment, make the following changes to these variables in the "tran" line.

      tr = 0.1us
      tf = 0.1us
      pd = 3.0 us
      T= 200.0 us

      such that it reads tran=pulse(0,1,0,0.1u,0.1u,5u,200u)

      Click Okay to save changes

    5. Left click the transimission lines and make the following change.  For the left and right lines, make td=30u and td=20u respectively.

      Click Okay to save changes.

    6. Move your cursor to the top menu, select ANALYSIS à SETUP TRANSIENT ANALYSIS… A window should appear. The START TIME should be set to 0.00 and the STOP TIME should be set to 200us and the NUMBER OF POINTS set to 300.

      7. Click Okay to save changes.

    7. Now, you will run the circuit. Move your cursor to the top menu, select ANALYSIS à RUN TRANSIENT ANALYSIS. Wait for the computer to complete the analysis.

    8. To view the animation, select SEE RESULTSà PUT PLOTS ON ALL TRANSMISSION LINES. To scale the animation, select SEE RESULTSà SETUP TRANSIENT ANIMATION PLOTS. Hit the button AUTOSCALE ANIMATION PLOT LIMITS in the lower right hand corner to autoscale the plot and then hit OK. This allows you to use the full height of the animation box for graphing.

    9.   Move the cursor to the top menu, select SEE RESULTS à ANIMATE TRANSIENT ANIMATION PLOTS. A window should appear, click PLAY to play the waveforms. To see this waveform on the transmission line circuit, double click the wire next to the transmission line. You should have another window open, with an X moving in the window. The movement of the X symbolizes the propagation of the wave in the transmission line.

Compare your hand drawn graph with that of the simulation.