CHAPTER 3 - Foundations of RF & Microwave Circuit Characterisation
142
The impedance chart shown in Figure 3.6-1 (b), is called Smith chart. We will look at how it is
constructed in more detail in section 4.4 but for now let us just look at how it works!
Remember how the complex number which represents any arbitrary impedance may be expressed
in Cartesian form (1.5.3.1) as
Now if we normalise this impedance to our characteristic impedance we get
The
values of
and are the values which we can plot on (or read from) the Smith chart.
The circles which intersect the central axis of the chart (Figure 3.6-3) are used to represent the real
part of our normalised impedance,
.
Figure 3.6-3 Smith Chart Real Axis (resistance)
Figure 3.6-4
Smith Chart peripheral values represent reactance (the imaginary constant j is displayed for clarity)
0.2
0.5
1
2
5
+j0.2
-j0.2
+j0.5
-j0.5
+j1
-j1
+j2
-j2
+j5
-j5
0.0
0.2
0.5
1
2
5
+j0.2
-j0.2
+j0.5
-j0.5
+j1
-j1
+j2
-j2
+j5
-j5
0.0
Conquer Radio Frequency
142 www.cadence.com/go/awr
