# Nyquist diagram

Discussion in 'Homework Help' started by eclipto, Jul 2, 2016.

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1. ### ecliptoNew Member

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Hi, I have a question about the Nyquist diagrams.

Say I have a loop function L and its bode plot.
The phase of the function, say, goes from -90° to -270°, so it crosses -180°. I know the magnitude for each point (for example, I plot the bode using Matlab).
Now, I want to use the bode plot to obtain the Nyquist diagram.
I know the Nyquist diagram will cross the begative Re axis at some point.
But how do I know the exact point? In particular, how do I know if it will be more negative with respect to the (-1, 0*j) critical point or if it will be more positive?
If I look at the frequency on the bode I can read the exact magnitude in dB.
What's next?

2. ### MrAlWell-Known MemberMost Helpful Member

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Hi,

Are you talking about the Nyquist criterion?
In that case you would be concerned with the mapping of F(s)=1+P(s)
which changed around a little is P(s)=F(s)-1
so you are looking for the number of encirclements of the (-1,0) point
and how they relate to the number of poles.

Have you ever tried Root Locus, or do you have to use Nyquist?