Reflections in Transmission Lines

It is a real head ache for the one who designs transmission lines that carry high speed signals. The mismatch or change in the impedance of the transmission lines will cause reflections that the signal at the receiver gets corrupted. In this post you will find a simple SPICE simulation to understand the reflections.

Creating SPICE netlist
You must have been familiar with program called SPICE simulate the electronic circuits. SPICE is a powerful program for circuit simulation. You can find many flavors of SPICE free to download from the Internet. The one used here is LTSPICE the one available from Linear Technology. To run the spice netlist give in this post get one and install it in your computer.


Open notepad and paste the code given below:

T1 N001 0 N002 0 Td=1n Z0=50
V1 N001 0 PULSE(0 5 10ns 1ns 1ns 10ns 20ns 5) Rser=0
R1 N002 0 100
.tran 100ns
.end

Save it as filename.cir
The schematic of the netlist is given below

In the simulation we are going to simulate three cases
(1) R1 = 50 ohms
(2) R1 = 25 ohms
(3) R1 = 75 ohms

You will find that only in case (1) signal is reproduced at the receiver as launched into the transmission line.

To run the simulation, open the file.cir in the LTSPICE SwitcherCAD window and click run. After the simulation is over, waveform window will open. Now right click on it and select add traces. Now you will be able to select the signal at the input and the output of the transmission line. V(n001) and V(n002) are the voltages at the input and output of the transmission line respectively.

Case (1)
Now the impedance at the load is matched with the impedance of the transmission line and thus there is no change in the signal received at the receiver. This is the ideal condition for the signal to be transmitted over a transmission line.


Case (2)
This is the case where the receiver impedance is not matched to the transmission line impedance. The impedance at the receiver is lesser than the transmission line impedance. This is one case in which reflection occurs. As the signal propagates the signal sees the change in the impedance. Whenever it sees a change in the impedance it gets reflected. The reflected and the launched signals are shown in the following figure.


Case (3)
This is the case where the receiver impedance is not matched to the transmission line impedance. The impedance at the receiver is more than the transmission line impedance. This is one case in which reflection occurs. As the signal propagates the signal sees the change in the impedance. Whenever it sees a change in the impedance it gets reflected. The reflections add to the original signal that the overshoots appear. The reflected and the launched signals are shown in the following figure.

To avoid reflections ideally you have to avoid impedance variations in the transmission line that carries your signal.

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