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That is great!
Before getting too involved in the Quartus solution, go back to MATLAB and look at their CIC and compensating FIR designs. I think they have a downconverter example that shows how to use it, or perhaps it was the book by Losada (available on the MATLAB site). Gain an understanding of how the CIC filter works first, and then what the compensating FIR filter is being used for. Only once you understand that, should you begin the implementation in Quartus.
Cheers,
Dave
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Hi Dave,
I read through the CIC documentation and designed a CIC filter (R=4, M=1, N=9) in MATLAB as well as in QuartusII. In QuatursII, the megafunction of CIC filter generated a matlab file which output the compensating FIR filter coefficients in a .txt file and plot the response of CIC, compensating FIR and the total response. Now, I try to simulate the cascade filter (CIC followed by a FIR filter) object in matlab. The FIR filter is designed by using the output coefficients. but the frequency response of the cascaded filter block is nothing like what I expected. It's also different from the total response plotted by the matlab file. Rather, the FIR filter works nicely as a LPF filter... It really confuses me.
Allison
%%%%%MATLAB simulation%%%%%%
Fs = 50e3; %%%sampling frequency is 50khz
%%%CIC filter
r = 4; %%%decimation
m = 1;
n = 9; %%%stage
iwl=B_out; %%%input bit width
owl=B_out;
hm_cic=mfilt.cicdecim(r,m,n,iwl,owl); %%%multi-rate filter design
fvtool(hm_cic); %%% filter visual tool
%%%%FIR filter
L=64; Fc=100; %%%cutoff frequency is 100hz
myFIR_fir_comp_coeff(L, fs, Fc, true, true, 16); %%% matlab file generated by Quartus, which will output the compensating coeffcients into a .txt file and plot responses
load myFIR_fir_comp_coeff.txt
hm_fir = mfilt.firdecim(1,myFIR_fir_comp_coeff); %%%direct-form fir filter
fvtool(hm_fir);
%%%%cascading the two filters together
hm = mfilt.cascade(hm_cic,hm_fir);
fvtool(hm);
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