The ONLY thing that can cause a reflection is an impedance change. The ONLY way a signal going into port 1 can come back out port 1 is due to a reflection, somewhere in the DUT. 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. It does not store any personal data.Every RF engineer learns about the Smith Chart their first day studying S-parameters.
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These cookies ensure basic functionalities and security features of the website, anonymously. Necessary cookies are absolutely essential for the website to function properly. The Smith chart: more vital after all these years.John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE). We can extend this plot to negative values of R as well, but then the outermost circle diameter can get really big. The outermost circle shown here for R = 0 is not an absolute limit. These two annotated views of the Smith chart show the results of varying R or varying X. Varying R or varying X causes both the real and the imaginary parts of gamma to vary. If we hold a constant value of X and allow R to vary, we get a second set of curves above and below the horizontal axis which seem to emanate from that point of tangency we just spoke of. If we hold a constant value of R and allow X to vary, we get one set of curves that look like circles tangent to each other at the far right. We graphically plot the real and imaginary parts of gamma.
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