![]() ![]() The amount of reflected signal from the load is dependent on the degree of mismatch between the source impedance and the load impedance. We know the reflection coefficient is defined as the ratio between the reflected voltage wave and the incident voltage wave: The Γ L is more useful when dealing with RF frequencies. Instead of considering its impedance directly, you express its reflection coefficient Γ L, which is used to characterize a load (such as admittance, gain, and transconductance). Or, it is defined mathematically as the 1-port scattering parameter s or s 11.Ī Smith chart is developed by examining the load where the impedance must be matched. The Smith chart is a polar plot of the complex reflection coefficient (also called gamma and symbolized by Γ). The only effort required is the reading and following of values along the circles. When used correctly, matching impedances, with apparent complicated structures, can be made without any computation. What It IsĪ Smith chart is a circular plot with a lot of interlaced circles on it. This is particularly true for high-frequency environments like video lines and RF and microwave networks. In addition, for efficient power transfer, this condition is required to avoid the reflection of energy from the load back to the source. Diagram of R S + jX S = R L - jX L.įor this condition, the energy transferred from the source to the load is maximized. It is well known that, to get the maximum power transfer from a source to a load, the source impedance must equal the complex conjugate of the load impedance, or:įigure 2. This can be valid for contingencies such as RS-485 lines, between a PA and an antenna, between an LNA and a downconverter/mixer, and so forth. ![]() A Quick Primerīefore introducing the Smith chart utilities, it would be prudent to present a short refresher on wave propagation phenomenon for IC wiring under RF conditions (above 100MHz). Fundamentals of impedance and the Smith chart. They can also help the designer with such tasks as optimizing for the best noise figures, ensuring quality factor impact, and assessing stability analysis.įigure 1. Of course, matching for maximum power transfer is not the only thing we can do with Smith charts. Topics addressed include practical illustrations of parameters, such as finding matching network component values. The primary objectives of this article are to review the Smith chart's construction and background, and to summarize the practical ways it is used.
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