At the end of the semester, it’s time to think of questions for the final exam in engineering classes. Do students really understand transmission lines? Do YOU really understand transmission lines? Here is a problem I may give my students, if I am feeling particularly cruel.

If you think you have the answer, write up a brief explanation of your answer in the comment section and in a future issue, I will post what I think is the solution.

Problem 1: In figure 1 is a simple circuit. It has an ideal voltage source that always output 2 V. There is a source resistor feeding a transmission line. The transmission line has Z0 = 50 Ohms, and TD = 1 nsec. It is connected to a switch. On the other side of the switch is another transmission line with Z0 = 50 Ohms and TD = 1 nsec. It has a resistor termination at the end.

Part 1: The switch, S1, has been in the closed position for an hour. The Rsource = Rload = 50 Ohms. What is the voltage at locations A, B, C and D in this steady state configuration?

Part 2: The switch S1 is open. After the switch has been opened for a while, what is the voltage at locations A, B, C and D in this steady state configuration?

Part 3: The switch S1 has been closed for a while. At time t = 0, the switch, S1 is opened. What is the evolution of the voltages at points A, B, C and D, over the next few nsec until steady state is reached?

Extra credit: Suppose the Rsource = Rload = 10 Ohms, repeat questions in part 1, 2 and 3.

Do you think I should give this question to my students on the final exam?