Q: What tools can be used facilitate determining the matching component options?Ī: A graphical tool called the Smith chart, named after its creator Phillip Smith, is most commonly used. In other words, there is no single “optimum” choice instead, it depends on many conflicting factors. You can focus on the fewest components, smallest overall footprint, the lowest cost, parts ones which are already on the BOM (bill of materials), component trimmability, or the path which is most tolerant of component variations and temperature coefficient, among many design considerations. It’s the same with determining the path from A to B for the matching network it it’s not just a simple matter of observing the closest-fit curved path between the two points. Do you take the fastest route, the shortest route, the one that is most scenic, the cheapest route, the one you have traveled before and with which you are comfortable, or one where you can stop along the way and see some sites? Consider an analogy: you need to travel from point A (the source impedance) to point B (the load impedance). But the choice of “best” topology among the possible ones is a function of design priorities. Certainly, the component values can be worked out for a given matching-circuit topology. Q: Why is this difficult? Isn’t this just a matter of detailed calculations which can be automated?Ī: Yes and no. There are many topologies for implementing networks that can do this, but determining the specific values of the components within these networks is very difficult. Q: Once you know the source and load impedance, what’s the next step?Ī: The next step is to devise a matching network so that the source sees an impedance equal to its complex conjugate, and the load sees a source with impedance equal to its own complex conjugate. The VNA sweeps across the frequency range of interest, measures the impedance, and then provides a table or graphical representation of results. The difficulty of assessing these impedances increases with frequency.Ī: A sophisticated test instrument called a vector network analyzer (VNA), as well a test suite are used, Figure 1.
In these cases, impedances must be measured. In many cases, impedances must be determined by a detailed model of the circuit and its many elements, which is a difficult task for high-frequency RF. In some cases, such as individual components, the vendor provides this data however, parasitics and other in-circuit factors will change these values. Q: What are the challenges in impedance matching?Ī: The first challenge is figuring out what the source output impedance and load impedance actually are. The Smith chart is a very helpful graphical tool for developing the matching network.
THE SMITH CHART SOFTWARE
It may seem that doing a few calculations using a software package is all that is needed, but the reality is far more complicated.
Once the need for an impedance-matching network is determined – and it is very likely needed – the next challenge is defining and creating this network. Smith (1905–1987) and independently by Mizuhashi Tosaku, is a graphical calculator or nomogram designed for electrical and electronics engineers specializing in radio frequency (RF) engineering to assist in solving problems with transmission lines and matching circuits.Part 1 looked at impedance matching and the need for a complex conjugate impedance at the load, compared to the source impedance. Graphical calculator or nomogram designed for electrical and electronics engineers specializing in radio frequency (RF) engineering to assist in solving problems with transmission lines and matching circuits