# Decimation filter magnitude response.

Started by June 20, 2010
I designed a CIC decimation filter, and am trying to plot its frequency response. Given below are the specifications of the filter.

Sampling Freq fclk: 1.4MHz
Decimation D: 100
Input bits: 3
Output bits: 24
Order: 3

Question:

What is the proper way to plot the magnitude the freq response of the filter?

I did the following: If I send in 102400 samples into the filter, I get 1024 samples out. In Matlab, I use the sampling frequency flck of 1.4MHz, and perform the FFT for 1024 points. For plotting, I have 512 (1024 points by 2) frequency points equally distributed between 0 and fclk/2.

The problem is that I am not getting the expected response, i.e., my plot is not showing a zero at fclk/D, and multiples of it. What am I doing wrong?
is it necessary to plot the fft result ??? ,,because if you can use simulink then,, use in SIgnal processing blockset --- in filter design --FDA tools ,,,put in values ang you will get the response magnitudes and all specs
I designed a CIC decimation filter, and am trying to plot its frequency response. Given below are the specifications of the filter.
>
>Sampling Freq fclk: 1.4MHz
>Decimation D: 100
>Input bits: 3
>Output bits: 24
>Order: 3
>
>Question:
>
> What is the proper way to plot the magnitude the freq response of the filter?
>
>I did the following: If I send in 102400 samples into the filter, I get 1024 samples out. In Matlab, I use the sampling frequency flck of 1.4MHz, and perform the FFT for 1024 points. For plotting, I have 512 (1024 points by 2) frequency points equally distributed between 0 and fclk/2.
>
>The problem is that I am not getting the expected response, i.e., my plot is not showing a zero at fclk/D, and multiples of it. What am I doing wrong?
I can't answer your question but can perhaps shed some light on why you're not seeing the results you expect.

One way of finding the freqeuncy response of a decimation filter is to use the freqz function in matlab. If vector b contains the unit impulse response of the filter, then freqz(b,1) will plot the frequency response. The catch here is that you have to take the impulse response of the filter before decimation, which isn't an option with a CIC filter since decimation occurs between the integrators and combs.

When you look at the output, all the notches get folded back to DC. You could apply an excitation containing frequencies in the band(s) you are trying to reject and then look at the FFT of the output. In this way you can infer the frequency response.

Another thing to watch is that implementations of CIC filters rely on modulo 2 arithmetic where, as long as you have long enough word lengths, overflows in the integrators get corrected in the combs. For your simulation MATLAB will probably use fixed point arthmetic unless you are specify otherwise. As the numbers grow in the integrators, the precision may deteriorate.

Steve