MATLAB: Filters: Differences between fvtool and bode

fvtool bode freqz filter response butterworth transfer function

Hi,

I am willing to make a complex filter by multiplying the transfer function of several elementary filters (Butterworth for example).

While doing this, I found several questions about what is the differences in the plot of the functions bode and the one performed in fvtool .

Maybe, I'm not understanding the way fvtool express the filter. For example, the transfer function of a Butterworth 2nd order is 1/(1 + 1.41s + s^2), and bode plots its response correctly.

In the other hand, fvtool calculates a different expression for a Butterworth 2nd order, and the figures drawn by it has nothing to do from those done by bode – Even when I intgroduce the same transfer funcion!

Can anyone tell me what am I missing?

Thank you in advance

Best Answer

I'm not sure how you are using bode, but from your transfer function, you are working with analog filter while fvtool is for digital filters, so you may not be comparing apple to apple.

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MATLAB: Daubechies filters: Frequency response

Hi Lorena, you have to determine the equivalent filter at each level of the wavelet tree. As a simple example, I'll show you how to compare the db4 scaling filter at level 1 and level 2.

Note to get the equivalent scaling filter at level 2 you have to lowpass filter at level 1 followed by downsampling by two and then lowpass filter for level 2.

[LoD,HiD] = wfilters('db4');
[H1,W1] = freqz(LoD);
% getting the equivalent impulse response for level 2

scal2 = conv(upsample(LoD,2),LoD)./sqrt(2);
[H2,W2] = freqz(scal2);
plot(W1,abs(H1),'k'); hold on;
plot(W2,abs(H2),'r'); 
legend('Level 1 Scaling Filter','Level 2 Scaling Filter');
xlabel('Radians/sample'); ylabel('Magnitude');

This is for 1st and 2nd level db4 wavelet (detail) filters.

[LoD,HiD] = wfilters('db4');
[H1,W1] = freqz(HiD);
% getting the equivalent impulse response for level 2
wav2 = conv(upsample(HiD,2),LoD)./sqrt(2);
[H2,W2] = freqz(wav2);
plot(W1,abs(H1),'k'); hold on;
plot(W2,abs(H2),'r'); 
axis tight;
legend('Level 1 Wavelet Filter','Level 2 Wavelet Filter', ...
    'Location','SouthEast');
xlabel('Radians/sample'); ylabel('Magnitude');

MATLAB: Butterworth passband filter transfer function in matlab

The butter function returns the numerator polynomial ‘b’ and denominator polynomial ‘a’ of the transfer function as vectors if you request it:

[b,a] = butter(n,Wn,ftype)

See the documentation for details.

Best Answer

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MATLAB: Daubechies filters: Frequency response

MATLAB: Butterworth passband filter transfer function in matlab

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