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.
Smith chart: Upon which this article concentrates.In short, this is for the super-specialist. Instinct: This can be acquired only after one has devoted many years to the RF industry.Manual computations: Tedious due to the length ("kilometric") of the equations and the complex nature of the numbers to be manipulated.In addition, circuit-simulation software is not pre-installed on computers, unless they are dedicated to such an application. They also need the expertise to find the useful data among the tons of results coming out. Designers have to be familiar with the multiple data inputs that need to be entered and the correct formats.
Computer simulations: Complex but simple to use, as such simulators are dedicated to differing design functions and not to impedance matching. There are many ways to do impedance matching, including: The computational values are required to set up the type of structure and target component values. In-situ RF lab measurements, along with tuning work, have to be considered for determining the proper final values. Above a few tens of megahertz, theoretical calculations and simulations are often insufficient. The matching task is required for a proper transfer of signal and energy from a "source" to a "load."Īt high radio frequencies, the spurious elements (like wire inductances, interlayer capacitances, and conductor resistances) have a significant yet unpredictable impact on the matching network. Typically these include the antenna to the low-noise amplifier (LNA), power-amplifier output (RFOUT) to the antenna, and LNA/VCO output to mixer inputs. One is the need to match the different impedances of the interconnected blocks. When dealing with the practical implementation of RF applications, there are always some nightmarish tasks. Tried and true, the Smith chart is still the basic tool for determining transmission-line impedances. A sample matching network of the MAX2472 is designed at 900MHz using graphical methods. Examples are shown plotting reflection coefficients, impedances and admittances. Tutorial on RF impedance matching using the Smith chart.
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