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Frequency response functions (in the xf format) for linear systems.


xf = qbode(a,b,c,d,w)
xf = qbode(ss,w)
xf = qbode(num,den,w)
XF = qbode( ... ,'struct')
     qbode( ... ,'iiplot ...')


For state-space models described by matrices a, b, c, d, or the LTI state-space object sys (see Control System Toolbox), qbode uses an eigenvalue decomposition of a to compute, in a minimum amount of time, all the FRF xf at the frequency points w

  xf = [C] (s [\ I \ ] − [A])−1 [B] + [D]

The result is stored in the xf format . 'iiplot "Test" -po -reset' can be used to display results in iiplot(see section 2.1.2). The option -po is used to save poles in ci.Stack{'IdMain'} so that they can be displayed. -reset reinitializes the curve stack.

qbode will not work if your model is not diagonalizable. A specific algorithm was developed to deal with systems with rigid-body modes (double pole at zero associated to non-diagonalizable matrices). This algorithm will not, however, indicate the presence of incoherent b and c matrices. In other cases, you should then use the direct routines res2xf, nor2xf, etc. or the bode function of the Control System Toolbox.

For the polynomial models num, den , qbode computes the FRF at the frequency points w

num (jω)
den (jω)


See also

demo_fe, res2xf, nor2xf, and bode of the Control System Toolbox

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