The Nyquist criterion is a method for determining stability of closed loop systems using frequency response information.

• Why learn about the Nyquist Stability Criterion (NSC)?
• The Nyquist criterion lets you determine stability of a closed loop control system.  The criterion is based upon using a frequency response plot - the Nyquist plot - and does not even require you to have a transfer function for the system.  (You can use an experimentally determined frequency response plot, as long as the phase information has been measured as well as the magnitude information.)
• What should you know to learn about the NSC?
• You will definitely need to know about Nyquist plots, and the intricacies found there, including low frequency behavior for systems with poles at the origin, and high frequency behavior, in general.
• What do you need to know about the NSC?
• The Nyquist Stability Criterion can be used to determine whether or not a system is stable.
• The Nyquist Stability Criterion can be extended to provide measures of relative stability - phase margin and gain margin - that indicate how stable a system is.
• How do you use the NSC?
• The NSC is most often interpreted using a Bode' plot.
• The NSC can be applied to systems as diverse as an operational amplifier.
• Problems
• Additional Information
• Links to Frequency Response Calculators