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- 20th September 2005, 00:58 #1

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## freqz matlab

I am trying to use the freqz function in matlab to plot an FIR filter (coefficient b=[.5 .5 .5 .5])magnitude and phase response. I tried two different method to plot the magnitude, but I got different results. anything wrong?

1) directly use freqz(b,1) , it automatically plot both of magitude in db and phase.

2)I use [h,w]=freqz(b,1)

plot(w, 20*log(abs(h)))

For these two methods, I got different magnitude plot. What's wrong with my code?

I suppose they will get same plot.

- 20th September 2005, 00:58

- 20th September 2005, 03:03 #2

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## matlab freqz

The plot is the same. The normalized frequency is what is different in the plots.

In one it is normalized from -pi*pi or 0 to pi as it appears.

In the other it is normalized from -0.5 to 0.5 or 0 to 1 as it appears

Either way is correct but take care , once using one approach, keep on this.

- 20th September 2005, 03:03

- 21st September 2005, 07:24 #3

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## freqz function in matlab

I guess if u add the following line before the plot line they will be the same:

Code:w = w / pi;

amraldo.

- 21st September 2005, 07:24

- 21st September 2005, 11:54 #4

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## freqz plot

There is another thing that I didn't quote in the first answer. Typing the commands exactly how you presented, there is also a difference in magnitude in the plots. Actually that was your question.

This is because you did type >> plot(w, 20*log(abs(h)))

you should type >> plot(w, 20*log10(abs(h))) otherwise you are gonna have the Neperian Logarithm and not the base 10 log.

Well, now I think the answer is completed.

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