Since the telephone only allows 4 kHz through the phone line, sampling for voice is done 8000 times per second. Nyquist Theorem Harry Nyquist. Mathematician who studied the theory of sampling. Notice that in this plot the Y-axis is negative and that each point on the Nyquist plot is the impedance at one frequency. Simple Equivalent Circuit with One Time Constant Figure 2 has been annotated to show that low frequency data are on the right side of the plot and higher frequencies are on the left.
Nyquist plots have one major shortcoming.
When you look at any data point on the plot, you cannot tell what frequency was used to record that point. The Nyquist plot in Figure 2 results from the electrical circuit of Figure 3. Electrochemical Impedance plots often contain several time constants. Often only a portion of one or more of their semicircles is seen. A metal covered with an undamaged coating generally has a very high impedance.