CIC compensation filers (FIR)

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Hi All,

Can I please have the Matlab code for the Solution 2: Multi-stage decimation in "Understanding CIC compensation filers" (Altera booklet)? I have attached this document. This matlab code produces the plot on page 15. I would like to know how to generate the impulse reponse of the compensation FIR filter that implements additional decimation by 2.

Kind Regards,

Lee

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Below is my attempt.

The compensation code is taken from above doc.

The cic formula taken from xilinx cic compiler doc as the one given in above doc does not work in my hands.

I have not added the mask

freq=linspace(0.00000001,1,1024);
H_cic = (sin(pi*freq*4)./sin(pi*freq)).^8;
H_cic = H_cic/max(H_cic);
%%%%%% CIC filter parameters %%%%%%
R = 4; %% Decimation factor
M = 1; %% Differential delay
N = 8; %% Number of stages
B = 18; %% Coeffi. Bit-width
Fs = 91.392e6; %% (High) Sampling freq in Hz before decimation
Fc = 4.85e6; %% Pass band edge in Hz
%%%%%%% fir2.m parameters %%%%%%
L = 110; %% Filter order; must be even
Fo = R*Fc/Fs; %% Normalized Cutoff freq; 0<Fo<=0.5/M;
% Fo = 0.5/M; %% use Fo=0.5 if you don't care responses are
%% outside the pass band
%%%%%%% CIC Compensator Design using fir2.m %%%%%%
p = 2e3; %% Granularity
s = 0.25/p; %% Step size
fp = ; %% Pass band frequency samples
fs = (Fo+s):s:0.5; %% Stop band frequency samples
f = *2; %% Normalized frequency samples; 0<=f<=1
Mp = ones(1,length(fp)); %% Pass band response; Mp(1)=1
Mp(2:end) = abs( M*R*sin(pi*fp(2:end)/R)./sin(pi*M*fp(2:end))).^N;
Mf = ;
f(end) = 1;
h = fir2(L,f,Mf); %% Filter length L+1
h_comp = h/sum(h); %% Floating point coefficients
plot(freq*Fs,20*log10(abs(H_cic)));
hold
 = freqz(h_comp,1,linspace(0,1,1024/4)*Fs/4,Fs/4);
H_comp = ;
plot(freq*Fs,20*log10(abs(H_comp)),'r-');
grid;
axis();
H_all = H_cic.*abs(H_comp);
plot(freq*Fs,20*log10(abs(H_all)),'g-')

Hi Kaz,

Thank you very much for the attempt and reply.

The code:

H_comp = [H_comp H_comp H_comp H_comp];

Does this line of code return the frequency response of the compensation FIR filter, performing additional decimation by 2? Do you know how to calculate the impulse response of this decimat-by-2 FIR filter?

Can you please attach the Xilinx cic compilor doc? It sounds useful!

Kind Regards,

Lee

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Hi Kaz,

The code:

H_comp = [H_comp H_comp H_comp H_comp];

Does this line of code return the frequency response of the compensation FIR filter, performing additional decimation by 2?

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Not exactly. The frequency response of compensation filter is the line of freqz.

Then I interpolated it back by 4 in the frequency domain (by repeating 4 copies) in order to compare it (and multiply) with cic filter at same Fs

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Do you know how to calculate the impulse response of this decimat-by-2 FIR filter?

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impulse response is just:

plot(h_comp);

if you mean frequency response of impulse input then it is the freqz line.

The z domain approach can also be used for both filters and overall system.

Hi Kaz,

Yes. But the fir compensation filter only compensates the cic filter which does the decimation by 4. The compensation filter did not do the additional decimation by 2. You can tell that by checking the repeating points on the plot of the fir filter, which has 22.8MHz output sampling frequency. How to do this additional decimation by 2 using the same compensation filter so the total decimation rate is 8 (4*2)? The fir ouput sampling frequency should be 11.4MHz.

Thanks a lot,

Lee

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Hi Kaz,

Yes. But the fir compensation filter only compensates the cic filter which does the decimation by 4. The compensation filter did not do the additional decimation by 2. You can tell that by checking the repeating points on the plot of the fir filter, which has 22.8MHz output sampling frequency. How to do this additional decimation by 2 using the same compensation filter so the total decimation rate is 8 (4*2)? The fir ouput sampling frequency should be 11.4MHz.

Thanks a lot,

Lee

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It is meant to compensate and decimate by 2. when implementing the compensation filter you have to discard every other sample from output. Either directly or as part of your fir implementation(to reduce resource). The response itself is referred to input Fs