Francesco Chirico

IIR Filter with SDK doesn't work properly

Discussion created by Francesco Chirico on Feb 4, 2008
Latest reply on Feb 8, 2008 by Pascal Irrle

Dear to all forum,
I have a some problems with implentation of an IIR filter via SDK DSP library. 
I started a new project with these :

1) Motorola  MC56F8357EVM board
2) CodeWarrior 5.5.1v
3) Processor expert v 2.93 for Motorola 56800 family.

So I generated a simple sinusoid at 50Hz with 8000 Hz of sample frequency and then I filtered it but sometime the filter doesn't not work well.

I extract the following IIR filter from help docs of processor expert.

/* The order of coefficients : a2,a1/2,b0,b1,b2 for each biquad

   Note that |b0|,|b1| and |b2| ahould all be lesser than 1.

   Test Case Filter Specifications:
   Type: Chebychev2
   Order: 4
   Passband edge: 1000Hz
   StopBand Edge: 2000Hz
   Maximum Ripple in PassBand: 1db
   Attenuation in Stopband : 30db
   Sampling Frequnecy: 8000Hz

const Frac16 IIR_Coef_F1[] =
FRAC16(-0.1310),     /* a2*/
FRAC16(0.27805),     /* a1/2*/
FRAC16(0.1808),      /* b0*/
FRAC16(0.2133),      /* b1*/
FRAC16(0.1808) ,     /* b2*/

FRAC16(-0.6107),     /* a2*/
FRAC16(0.4944),      /* a1/2*/
FRAC16(0.3892),      /* b0*/
FRAC16(-0.1566),     /* b1*/
FRAC16(0.3892)       /* b2*/

The first problem is: How I can generated these coefficient? I tried with QEDESign Lite sw but the results  doesn't macth? Why?

The second problem is when I set the sinusoid amplitude to 1 value. Infact the filter output is something strange not so close to be considered a sinuosid.

 Are there some problem with use of Frac16?  Some saturation problem are possible?
This problem disappear if I set the amplitude sin to 0.5 or less.

The third problem is the following. When I generated a sinusoid outide the passband filter (for example 1500Hz) the output isn't close to zero value. So the filter doesn' t kill the sin.

I attached the entire project  generated vith processor expert. It is in zip  format

Thank in advance for any help you will give to me!
Please It's very urgent.

Sw Eng
Francesco Chirico
Mer Mec Spa

Message Edited by t.dowe on 2009-10-15 05:41 PM