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Superimpose Nichols chart on Nichols plot




ngrid superimposes Nichols chart grid lines over the Nichols frequency response of a SISO LTI system. The range of the Nichols grid lines is set to encompass the entire Nichols frequency response.

The chart relates the complex number H/(1 + H) to H, where H is any complex number. For SISO systems, when H is a point on the open-loop frequency response, then

is the corresponding value of the closed-loop frequency response assuming unit negative feedback.

If the current axis is empty, ngrid generates a new Nichols chart grid in the region –40 dB to 40 dB in magnitude and –360 degrees to 0 degrees in phase. If the current axis does not contain a SISO Nichols frequency response, ngrid returns a warning.


Plot the Nichols response with Nichols grid lines for the system.


H = tf([-4 48 -18 250 600],[1 30 282 525 60])

These commands produce the following result.

Transfer function:
- 4 s^4 + 48 s^3 - 18 s^2 + 250 s + 600
  s^4 + 30 s^3 + 282 s^2 + 525 s + 60



See Also

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