EEE 161

Applied Electromagnetics, Spring 2008.

Laboratory Exercise 11

Transmission Line Impedance Matching

Problem 1:Find the input impedance of the transmission line terminated with a load using ADS.

Method1.Using Smith Chart on paper. Print out a Black Magic Smith Chart. Find the given impedance on the Smith Chart, draw an SWR circle, follow the arrow “Wavelengths toward generator” for the length of the transmission line. Find the input impedance and the input reflection coefficient. Then use color Z-Y chart to find the input admittance.

Method2. Using ADS

  1. Select Simulations-S_Param from the pull-down menu on the top left side of the screen. Place two ports (named ‘Term’) on the schematic diagram. Change the impedance of port 2 to your given load impedance. Leave port 1 to be 50-Ohms port. Connect two ports with wire. Add an S-parameter block, select sweep type to be single point.
  2. Simulate the circuit you made in step 1, by clicking on the gear icon on the top menu.
  3. Data window should automatically open. Place S(1,1) on the Smith Chart in data window.
  4. Verify that the impedance is at the same point on the Smith Chart as you had in Method 1. Left click on the Smith Chart and select Item Options. Select the tab plot options. Select coordinate Both. Increase the line thickness of both impedance and admittance circles by selecting Grid and increasing line thickness.
  5. Select ‘TLines Ideal’ from the pull-down menu. Add a section of an ideal transmission line named ‘TLIN’.
  6. Copy and paste ports 1 and 2, ADS will automatically rename them 3 and 4. Connect the transmission line between the ports 3 and 4.
  7. Simulate the circuit you made in step 5, by clicking on the gear icon on the top menu.
  8. Data window should automatically open. Place S(3,3) on the Smith Chart in data window. Make sure you select the port where the signal generator is, not the load port.
  9. Verify that the input impedance is at the same point on the Smith Chart as you had in Method 1. Place markers on impedance points, record the impedance and the reflection coefficient and compare to the values you obtained manually.

Problem 2:Using transmission lines match a given load impedance 100+j*100 to a 50-Ohm line using following methods.

Method1.UsingSmith Chart on paper.Print out a color Z-Y Smith Chart. Find the given impedance on the Smith Chart. Plot a SWR circle (it’s a circle centered in the center of the smith chart and with the radius of the magnitude of the load reflection coefficient). Find the point where the Smith Chart crosses Z=1 or Y=1 circle. Record the length of the line. Find the impedance (or admittance) you have to add in parallel (or series) to match the circuit. Starting from the open or short circuit find the length of the stub that will produce such input impedance.

Method2. Using ADS

  1. Place two ports on the schematic diagram. Change the impedance of port 2 to your given load impedance. Leave port 1 to be 50-Ohms port. Connect two ports with wire. Add an S-parameter block, select sweep type to be single point.
  2. Simulate the circuit you made in step 1.
  3. Data window should automatically open. Place S(1,1) on the Smith Chart in data window.
  4. Verify that the impedance is at the same point on the Smith Chart as you had in Method 1. Left click on the Smith Chart and select Item Options. Select the tab plot options. Select coordinate Both. Increase the line thickness of both impedance and admittance circles by selecting Grid and increasing line thickness.
  5. Copy and paste both ports. Ports should change the numbers to 3 and 4 automatically. Connect an ideal transmission line between ports 3 and 4.
  6. Simulate the circuit. Add the reflection coefficient S(4,4) (4 stands for the port number that has 50-Ohm impedance, not the load impedance port).
  7. Select fork from the top menu. Click on the value of the line length ‘E’. The sliding ruler will appear. Once you managed to place the S(4,4) (blue dot) on the G=1 circle go to the next step.
  8. Click update schematics and close the tuning fork.
  9. Copy and paste both ports together with the circuit element you worked on in step 18. Ports should change number automatically to 5 and 6. Add a new circuit element. Place a transmission line in parallel with the existing line. This line is usually called a stub. Chenge the length of the stub by using fork. When you place the pink dot in the center of the Smith Chart stop.
  10. Click update schematics and close the tuning fork.
  11. Your circuit and the Smith Chart should look like the ones below: