The Nyquist criterion is a method for determining stability of closed loop systems using frequency response information.  Nyquist stability analysis (i.e. applying the Nyquist Stability Criterion) is most often done using Bode' plots rather than a Nyquist plot even though the criterion is developed using reasoning about the Nyquist plot.

• Why learn to use Bode' plots for Nyquist analysis?
• Bode plots have a number of advantages over Nyquist plots that make Bode' plots the vehicle of choice for Nyquist analysis.
• In Nyquist plots frequency is a parameter, and it is difficult to determine the frequency of a point on a Nyquist plot, where in Bode' plots, magnitude (in db) and phase are plotted against frequency and that problem does not arise.
• In Nyquist plots you can only see a limited range of gain values on a typical plot.  On a Bode' plot you can see large ranges of gain because the db scale presents information differently.  For example, the open loop frequency response plot for an operational amplifier has a DC gain of over 100,000 (i.e. 100 db), but you can still clearly see the point where the gain drops to 1 (i.e., the zero-db crossing) on that plot.
• What should you know to learn about using Bode' plots with the NSC?
• You first need to know about the Nyquist Stability Criterion.
• You also need to be knowledgable about Bode' plots.
• What do you need to know about using Bode' plots with the NSC?
• You need to be able to interpret what the Bode' plot is telling you about the Nyquist plot for the system.
• You will need to learn some "rules of thumb" about the NSC, particularly this one.
• A system is usually stable if the zero-db crossing occurs at a lower frequency than the -180^o crossing.
• You will need to be able to determine phase margin and gain margin on a Bode' plot.
• How do you use the information you get from applying NSC analysis on Bode' plots?
• When you are setting phase and gain margins, you are doing part of the design for a closed loop system.  The whole concept of using Bode' plots for NSC analysis is really a valuable design tool for closed loop systems.
• Problems
• Additional Information
• Links to Frequency Response Calculators