1 files changed, 47 insertions, 1 deletions
diff --git a/help/en_US/invfreq.xml b/help/en_US/invfreq.xml
index 2f10f8e..38f2da3 100644
--- a/help/en_US/invfreq.xml
+++ b/help/en_US/invfreq.xml
@@ -17,7 +17,53 @@
 
   <refnamediv>
     <refname>invfreq</refname>
-    <refpurpose></refpurpose>
+    <refpurpose>Calculates inverse frequency vectors</refpurpose>
   </refnamediv>
 
+
+<refsynopsisdiv>
+   <title>Calling Sequence</title>
+   <synopsis>
+   [B,A] = invfreq(H,F,nB,nA)
+   [B,A] = invfreq(H,F,nB,nA,W)
+   [B,A] = invfreq(H,F,nB,nA,W,[],[],plane)
+   [B,A] = invfreq(H,F,nB,nA,W,iter,tol,plane)
+   
+   </synopsis>
+</refsynopsisdiv>
+
+<refsection>
+   <title>Parameters</title>
+   <variablelist>
+   <varlistentry><term>H:</term>
+      <listitem><para> desired complex frequency response,It is assumed that A and B are real polynomials, hence H is one-sided.</para></listitem></varlistentry>
+   <varlistentry><term>F:</term>
+      <listitem><para> vector of frequency samples in radians</para></listitem></varlistentry>
+   <varlistentry><term>nA:</term>
+      <listitem><para> order of denominator polynomial A</para></listitem></varlistentry>
+   <varlistentry><term>nB:</term>
+      <listitem><para> order of numerator polynomial B</para></listitem></varlistentry>
+   </variablelist>
+</refsection>
+
+<refsection>
+   <title>Description</title>
+   <para>
+Fit filter B(z)/A(z) or B(s)/A(s) to complex frequency response at frequency points F. A and B are real polynomial coefficients of order nA and nB respectively.  Optionally, the fit-errors can be weighted vs frequency according to the weights W. Also, the transform plane can be specified as either 's' for continuous time or 'z' for discrete time. 'z' is chosen by default.  Eventually, Steiglitz-McBride iterations will be specified by iter and tol.
+   </para>
+   <para>
+</para>
+</refsection>
+
+<refsection>
+   <title>Examples</title>
+   <programlisting role="example"><![CDATA[
+[B,A] = butter(12,1/4);
+[H,w] = freqz(B,A,128);
+[Bh,Ah] = invfreq(H,F,4,4);
+Hh = freqz(Bh,Ah);
+disp(sprintf('||frequency response error|| = %f',norm(H-Hh)));
+
+   ]]></programlisting>
+</refsection>
 </refentry>