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