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+## Copyright (C) 2011   Lukas F. Reichlin
+##
+## This file is part of LTI Syncope.
+##
+## LTI Syncope is free software: you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## LTI Syncope is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with LTI Syncope.  If not, see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn{Function File} {[@var{sys}, @var{n}] =} fitfrd (@var{dat}, @var{n})
+## @deftypefnx{Function File} {[@var{sys}, @var{n}] =} fitfrd (@var{dat}, @var{n}, @var{flag})
+## Fit frequency response data with a state-space system.
+## If requested, the returned system is stable and minimum-phase.
+##
+## @strong{Inputs}
+## @table @var
+## @item dat
+## LTI model containing frequency response data of a SISO system.
+## @item n
+## The desired order of the system to be fitted.  @code{n <= length(dat.w)}.
+## @item flag
+## The flag controls whether the returned system is stable and minimum-phase.
+## @table @var
+## @item 0
+## The system zeros and poles are not constrained.  Default value.
+## @item 1
+## The system zeros and poles will have negative real parts in the
+## continuous-time case, or moduli less than 1 in the discrete-time case.
+## @end table
+## @end table
+##
+## @strong{Outputs}
+## @table @var
+## @item sys
+## State-space model of order @var{n}, fitted to frequency response data @var{dat}.
+## @item n
+## The order of the obtained system.  The value of @var{n}
+## could only be modified if inputs @code{n > 0} and @code{flag = 1}.
+## @end table
+##
+## @strong{Algorithm}@*
+## Uses SLICOT SB10YD by courtesy of
+## @uref{http://www.slicot.org, NICONET e.V.}
+## @end deftypefn
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: October 2011
+## Version: 0.1
+
+function [sys, n] = fitfrd (dat, n, flag = 0)
+
+  if (nargin == 0 || nargin > 3)
+    print_usage ();
+  endif
+  
+  if (! isa (dat, "frd"))
+    dat = frd (dat);
+  endif
+
+  if (! issiso (dat))
+    error ("fitfrd: require SISO system");
+  endif
+
+  if (! issample (n, 0) || n != round (n))
+    error ("fitfrd: second argument must be an integer >= 0");
+  endif
+
+  [H, w, tsam] = frdata (dat, "vector");
+  dt = isdt (dat);
+  
+  if (n > length (w))
+    error ("fitfrd: require n <= length (dat.w)");
+  endif
+  
+  [a, b, c, d, n] = slsb10yd (real (H), imag (H), w, n, dt, logical (flag));
+  
+  sys = ss (a, b, c, d, tsam);
+
+endfunction
+
+
+%!shared Yo, Ye
+%! SYS = ss (-1, 1, 1, 0);
+%! T = 0:0.1:50;
+%! Ye = step (SYS, T);
+%! W = logspace (-2, 2, 100);
+%! FR = frd (SYS, W);
+%! N = 1;
+%! SYSID = fitfrd (FR, N, 1);
+%! Yo = step (SYSID, T);
+%!assert (Yo, Ye, 1e-2);

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