Lesson 2 : Section 7 : Smith Charts

Section 7: Smith Charts

While the Smith Chart is not the only way to look at input and output impedances of RF circuits, it has strong advantages over alternative methods. The basic Smith Chart is shown in Figure 2.7.1. All of the impedances with a positive real real value (0Ώ, 50Ώ, 1000Ώ, etc.) are mapped inside of a circle. All of the negative real values are mapped outside of the circle (-10Ώ, -50Ώ, etc).

Figure 2.7.1: Basic Smith Chart (click to enlarge)

Figure 2.7.2: Smith Chart with two impedances plotted (click to enlarge)

Figure 2.7.3 and Figure 2.7.4 show the same data presented in two different ways, a rectangular plot and a Smith Chart. Both ways of looking at the data have advantages and disadvantages. The rectangular plot has the obvious advantage that one can read off the actual impedance versus frequency directly. The Smith Chart has the obvious disadvantage that there is not frequency axis. The main advantage of the Smith Chart is that one can, with experience, quickly glean qualitative relationships. For example, at a glance one can tell from the Smith Chart that:

• 1. the circuit is matched at the center frequency

  2. the circuit has the same amount of mismatch at the edge frequencies

  3. pulling the real impedance lower (shifting the curve to the left) will improve the overall match

These are all observations that can be made from the rectangular plot, just not as quickly.

Figure 2.7.3: Rectangular plot of the real and imaginary impedance

Figure 2.7.4: Smith Chart plot of the impedance

Copyright 2010, Gregory Kiesel