Question

In Nyquist criterion roots of the characteristic equation are given by _____.
a.
Zeros of closed loop transfer function
b.
Poles of open loop transfer function
c.
Poles of closed loop transfer function
d.
Zeros of open loop transfer function

Answer 1

The correct answer is option (d) Zeros of the open-loop transfer function.

What is the formula of the Nyquist criterion?

• The Nyquist equation is Z=N+P. (for any system, whether it is stable or unstable). For a stable system, Z=0, meaning that the RHS of the characteristics equation shouldn't include any roots. N thus equals -P for the stable system.

• According to the Nyquist Theorem, C = N + O, and C = 0 denotes that the closed-loop system is stable. According to this, "the number of encirclements of (1 + j0) point by the locus of G(j), + in the counterclockwise direction is equal to the number of unstable open-loop poles for a system to be closed-loop stable."

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