We call this the reflection coefficient and also, for historical reasons, the return loss. It is the ratio of the reflected wave to the incident wave. When there is only one port, the only S-parameter term is S11. We label each specific S-parameter term based on the port number of the going in port and the coming out port. Regardless, their properties and the information we extract from them, are the same. S-parameters can come from measurements or simulations. Establishing the incident signal, which is the same as Vref, and the reflected signal from a DUT. Figure 1 illustrates this idea of an incident and reflected sine wave signal from a DUT.įigure 1. The DUT can be anything, even a discrete component like a resistor or capacitor, or an extended structure like a transmission line, traces on a board or an entire channel. For now, we will consider only one port connected to the interconnect, or device under test (DUT).

Let’s Start with ReflectionsĮach S-parameter is really the ratio of the sine wave voltage signal coming out of the end of an interconnect, relative to the sine wave voltage signal going in. But there are some valuable insights a Smith Chart can illuminate. It’s an important tool for RF applications. Every RF engineer learns about the Smith Chart their first day studying S-parameters.