wxBounce can be downloaded at the bottom of the page.
This is a 1D FDTD code which shows voltage wave bounces and calculates the return. This code simulates three transmission lines, plus an adjustable termination. The source impedance is matched to the first transmission line.
In Figure 1 below, the dashed green line is the boundary between the first and second transmission lines. The red dashed line is the boundary between the second and third transmission line.
The transmission line impedances should be kept between 25 ohms and 400 ohms; also, the impedance ratios from highest to lowest should be kept under four to keep run times reasonable.
The transmission line lengths are set to be the electrical wavelengths set for 1 GHz, however the display is set for physical lengths. The speed of propagation is allowed to vary with the impedance (the capacitance is considered fixed), and therefore the electrical length and the physical length are not directly coupled. The middle length is set such that the electrical length and the physical length are define to be equal: the electrical length is four times longer for impedances that are four times smaller.
The loss is the shunt resistance along the line.
Figure 1: Initial start of wxBounce
The Run and Pause button controls will launch a voltage wave, and the waves can be seen reflecting and resonating along the transmission lines. When you pause the run, the Copy button will copy the display to the clipboard.
Figure 2: Voltage waves bouncing along the transmission line
When the voltages along the lines drop below a sufficient level, or when the Stop button is pressed, the return loss is displayed: the frequency dependent S-Parameter is calculated based on the voltage waves that have reflected back to the source.
Figure 3: Return loss for the quarter wave matching network defined above
The load impedance can be defined to be either "0" for a short, or "inf" for an open.
Figure 4: Return with resistive loss
Copyright 2012, Gregory Kiesel