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Topic: 8th order Butterworth not working? :-( :-(
Replies: 1   Last Post: Jun 23, 2011 1:25 PM

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Peter Mairhofer

Posts: 38
Registered: 8/18/10
8th order Butterworth not working? :-( :-(
Posted: Jun 23, 2011 12:10 PM
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I am trying to simulate a system which uses an 8th order low pass. But
instead of a flat band, I only get noise. This is the simple code:

fnyq = 10e9;
L = 195;
N = 100000;
[b5,a5] = butter(5, 1/L);
[b7,a7] = butter(7, 1/L);
[b8,a8] = butter(8, 1/L);
[h5,f] = freqz(b5, a5, N);
[h7,f] = freqz(b7, a7, N);
[h8,f] = freqz(b8, a8, N);
plot(linspace(0, fnyq, N), 20*log10(abs([h5 , h7 , h8])));
xlim([0, fnyq/L * 3]);
legend('5 order', '7 order', '8 order');

And this is the result:


5th order works fine, 7th order has some noise and 8th order (which I
would need) is completely useless!

Unfortunately I need to simulate an 8th order Butterworth (and not Cauer
etc.) so I have no chance :(

Any hints?


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