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Halfband and FIR filter for decimation

Started by feyzakent February 11, 2011
Hi,

I want to design a decimation filter for a Sigma-Delta modulator. The modulator input is 20Khz sinus signal. And this signal is oversampled to 2.5Mhz. So decimation ratio is about 64. First I designed a CIC filter which has a decimation ratio of 16. After a halfband filter and an FIR filter. So halfband filter output is 80 Khz, FIR filter output is 40 khz. Anyway, I want to design halfband and FIR filter with Matlab firhalfband and remez functions. (6th order halfband and 11th order FIR) I'm confused to determine the passband edge frequencies. My aim is at the output 20 khz signal is passed and the noise over 50 khz is supressed. For halfband filter

b=firhalfband(6,fp);
Fs0000;
[H, f] = freqz(b, 1, 512, Fs);
mag = 20*log10(abs(H));
plot(f, mag)
For FIR filter

b=remez(11,[0,fp,fs,1],[1 1 0 0]);
Fs000;
[H, f] = freqz(b, 1, 512, Fs);
mag = 20*log10(abs(H));
plot(f, mag)
How can I determine fp and fs? Thanks