Forums

comb filter and distortion

Started by egan_nc July 3, 2003
Hi,
I am designing a 3 stages decimation filter for a second order 1 bit
delta sigma modulator.
The sampling rate of DSM is 1.536Mhz. The bandwidth for the input
signal is 4Khz.
For the first stage I used matlab's function cicdecimate with the
following parameters:
m = 1;
n = 3 ;
r = 32;
q = quantizer([8 0],'fixed');
out1a = cicdecimate(m,n,r,in1a,q); %in1a is the output of DSM
For the second and third stage I used matlab's function remezord
with
the following parameters:
Second stage:
rp = 0.1; % Passband ripple
rs = 85; % Stopband ripple
fs3 = 96e3; % input frequency from comb filter
f = [4e3 20e3]; %cutoff freq is 24-4
a = [1 0];
[n1,f1,a1,w1] = remezord(f,a,dev,fs3);
h1=remez(n1,f1,a1,w1)
out2a=filter(h1,1,out1a);
out2b=downsample(out2a,4);
Third stage:
rp = 0.1; % Passband ripple
rs = 85; % Stopband ripple
fs3 = 24e3; % input frequency from comb filter
f = [4e3 8e3]; % Cutoff frequencies 12-4=8
a = [1 0];
[n1,f1,a1,w1] = remezord(f,a,dev,fs3);
h2=remez(n1,f1,a1,w1);
out3a=filter(h2,1,out2b);
out3b=downsample(out3a,2);
My simulation result shows this filter only works for input up to
3Khz, for 3.3Khz or 4Khz input, the simulation result shows a low
freq noise. For the 3.3khz how do I determine if the output is
acceptable?
Can somebody please explain to me what did I do wrong?
According to some articles sinc filter will introduce distortion in
the low-freq part of signal band. Can somebody please elaborate on
this issue? Where is the distortion come in for my sinc filter? How
do I equalize it on the next stage filter?
Any help is greatly appreciated.
Thank you,
EngGan