The Nyquist plot of a transfer function 
is the image curve 
in the complex plane
as 
traverses a Nyquist
contour 
.
The part obtained from 
records the frequency response as the real
frequency 
varies. The full plot also includes the images of the closing
arc and of any small indentations around poles
on the imaginary axis. The Nyquist
criterion reads the winding number of
this plot about the critical point 
. Equivalently, the image curve is the oriented set of complex
numbers traced by 
as 
moves around 
. The point 
is critical because 
makes the closed-loop characteristic function 
vanish.
Like a Smith chart, a Nyquist plot traces a complex-valued frequency response. A Nyquist plot is used
to infer feedback stability from winding around 
, whereas a Smith chart
maps normalized impedance to a reflection coefficient
and is used primarily for impedance matching.
