new autoregressive signal model for dft resonance estimation

matlab/AR_estimate.m

@@ -0,0 +1,73 @@
+function [val_ar t_ar f_val_ar] = AR_estimate( t, val, freq, nu, mu, expand_factor)
+% [val_ar t_ar f_val_ar] = AR_estimate( t, val, freq, < nu, mu, expand_factor >)
+%
+% apply autoregressive signal model to improve dft results
+%  
+% t     : time vector
+% val   : time domain values
+% freq  : frequency vector for dft
+%
+% optional
+% nu    : AR order (default 40)
+% mu    : number of timesteps to train the model (default 3*nu)
+% expand_factor : increase signal length by this factor (default 5)
+%
+% 
+% openEMS matlab interface
+% -----------------------
+% Author: Thorsten Liebig, 2011
+%
+% See also ReadUI, DFT_time2freq
+
+if numel(t) ~= numel(val)
+    error 'numel(t) ~= numel(val)'
+end
+
+if (nargin<4)
+    nu = 40;
+end
+if (nargin<5)
+    mu = 3*nu;
+end
+if (nargin<6)
+    expand_factor=5;
+end
+
+if (mu<=2*nu)
+    error 'mu should be larger than 2*nu'
+end
+
+if (expand_factor<=1)
+    error 'expand_factor must be larger than 1'
+end
+
+dt = t(2)-t(1);
+
+M = numel(t);
+
+if (M<0.6*mu)
+    error 'signal is to short for AR estimate --> decrease AR order'
+end
+
+for n=1:mu-nu
+    b(n) = val(end-n+1);
+    for m=1:nu
+        A(n,m)=val(end-n+1-m);
+    end
+end
+
+a = ((A'*A)\A')*b';
+
+val_ar = val;
+t_ar = t;
+for k=M:expand_factor*M
+    val_ar(k) = 0;
+    t_ar(k) = t_ar(k-1)+dt;
+    val_ar(k) = sum(a.*val_ar(k-(1:nu))');
+end
+
+if (max(val_ar(M:end)) > max(val))
+    error 'estimated signal appears to be unstable --> use a larger mu'
+end
+
+f_val_ar = DFT_time2freq(t_ar, val_ar, freq);

matlab/ReadUI.m

@@ -1,5 +1,5 @@
-function UI = ReadUI(files, path, freq)
+function UI = ReadUI(files, path, freq, AR_order)
-% function UI = ReadUI(files, path, freq)
+% function UI = ReadUI(files, path, freq, AR_order)
 %
 % read current and voltages from multiple files found in path
 %
@@ -14,6 +14,8 @@ function UI = ReadUI(files, path, freq)
 % freq (optional):
 %     frequency-domain values will be calculated according to 'freq'
 %     if 'freq' is not given, a FFT will be used
+% AR_order (optional):
+%     order to use within an auto-regressive model to improve FD accuracy
 %
 % e.g.
 % U = ReadUI({'ut1_1','ut1_2'},'tmp' );
@@ -22,6 +24,8 @@ function UI = ReadUI(files, path, freq)
 % openEMS matlab interface
 % -----------------------
 % author: Thorsten Liebig
+%
+% See also DFT_time2freq, AR_estimate
 
 if (nargin<2)
     path ='';
@@ -51,6 +55,10 @@ for n=1:numel(filenames)
         [UI.FD{n}.f,UI.FD{n}.val] = FFT_time2freq( t,val );
     else
         UI.FD{n}.f = freq;
-        UI.FD{n}.val = DFT_time2freq( t, val, freq );
+        if (nargin<4)
+            UI.FD{n}.val = DFT_time2freq( t, val, freq );
+        else
+            [val_ar t_ar UI.FD{n}.val] = AR_estimate( t, val, freq, AR_order);
+        end
     end   
 end