Makeing a Butterworth filter of order 2 for Lowpass to process 50 samples per second with the corner at 8 Hertz <BR><H2>Code</H2><TT>mkfilter.exe -Bu -Lp -o 2 -a 0.16 -l | gencode.exe</TT><HR><PRE>/* Digital filter designed by mkfilter/mkshape/gencode   A.J. Fisher
   Command line: mkfilter.exe -Bu -Lp -o 2 -a 0.16 -l */

#define NZEROS 2
#define NPOLES 2
#define GAIN   6.881181354e+000

static float xv[NZEROS+1], yv[NPOLES+1];

static void filterloop()
  { for (;;)
      { xv[0] = xv[1]; xv[1] = xv[2]; 
        xv[2] = `next input value' / GAIN;
        yv[0] = yv[1]; yv[1] = yv[2]; 
        yv[2] =   (xv[0] + xv[2]) + 2 * xv[1]
                     + ( -0.2523246263 * yv[0]) + (  0.6710290908 * yv[1]);
        `next output value' = yv[2];
      }
  }

</PRE><H2>Plot</H2><HR><IMG SRC="/image/temp/delme-Bu_-Lp_-o_2_-a_0.16.png" ALIGN=LEFT><TABLE><TR><TD><BR><B>Magnitude (red) and phase (blue) vs. frequency</B><UL><LI>x axis: frequency, as a fraction of the sampling rate (i.e. 0.5 represents the Nyquist frequency, which is 5000 Hz) <LI>y axis (red): magnitude (linear, normalized) <LI>y axis (blue): phase </UL></TD></TR></TABLE><P STYLE="CLEAR:left;"><H2>Parameters</H2><HR><PRE>mkfilter.exe -Bu -Lp -o 2 -a 0.16 -l
G  = 6.8811813538e+000
NZ = 2
 1.0000000000e+000
 2.0000000000e+000
 1.0000000000e+000
NP = 2
-2.5232462628e-001
 6.7102909077e-001
-1.0000000000e+000
</PRE>
</BODY></HTML>