# generating wrong Nyquist plot of tf

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keshav agrawal on 27 May 2021
Commented: Paul on 28 May 2021
i have been trying to plot nyquist plot of following transfer funtion but i am getting wrong.
sys =(s+3)/(s^2*(s-3))
when i tries to plot it using control technique i am getting different plot
Paul on 28 May 2021
What is your code and why do you think the result is wrong?
keshav agrawal on 28 May 2021
the code is
s = tf('s')
sys =(s+3)/(s^2*(s-3))
nyquist(sys)
the plot i got from this is
the nyquist plot i am expecting should be like this :-

Paul on 28 May 2021
Keep in mind that nyquist() doesn't draw the arc at infinity due to the double pole at the origin.
If you zoom in around the origin on the Nyquist plot produced by nyquist(), you'll see that the plot crosses the imaginary axis at a value of around 0.1*j and approaches the origin from the left, as in your sketch.
keshav agrawal on 28 May 2021
thank you for reply ... i am statisfied with your answer. but is there any function other than nyquist() that can show infinite semi circles.
Paul on 28 May 2021
None that I'm aware of.

Girijashankar Sahoo on 28 May 2021
Edited: Girijashankar Sahoo on 28 May 2021
sys=tf([0 0 1 3],[1 -3 0 0])
nyquist(sys)