## Bandwidth analysis of a system with multidimensional input

HaMo

### HaMo (view profile)

さんによって質問されました 2019 年 10 月 16 日

### Star Strider (view profile)

さんによって コメントされました 2019 年 10 月 20 日
Star Strider

### Star Strider (view profile)

さんの 回答が採用されました
I am playing around with the built-in bandwidth function.
I can intuitively understand what is going on under the hood for a transfer function with scalar input and scalar output. However, how is the band width computed when the input is a vector of length > 1?
For instance, as in this example from the documentation:
A = [-2 -1; 1 0];
B = [1; 0];
C = [1 2];
D = 1;
sys = ss(A,B,C,D);
bandwidth(sys)

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## 1 件の回答 ### Star Strider (view profile)

2019 年 10 月 16 日
採用された回答

The length of the input vector is likely irrelevant, and is not even an argument to the bandwidth function. Note however that the bandwidth function only applies to SISO systems, or arrays of SISO systems (Find Bandwidth of Model Array), so it would not apply to MISO or MIMO systems.

Star Strider

### Star Strider (view profile)

2019 年 10 月 16 日
As always, my pleasure!
If you are taking a course in modern control, or linear control systems (it goes by several names), this will all be cleared up for you as the course proceeds. If you have not taken such a course, I definitely recommend it. An excellent text (in my opinion, and the one we used when I took a linear control systems course) is Chi-Tsong Chen Linear system Theory and Design (ISBN 0-19-511777-8). We used an earlier edition, and I still use it (this is the third edition) as my primary reference.
HaMo

### HaMo (view profile)

2019 年 10 月 20 日
After testing out a few different ideas in Matlab, I suddenly had an Eureka moment. This is the answer I was looking for:
A = [-2,-1;1,0];
B = [1;0];
C = [1,2];
D = 1;
% Laplace transformation:
% sX = AX + BU
% Y = CX + D
%
% (sI - A)X = BU
% X = inv(sI - A) B U
% Y = C inv(sI - A) B U + D
% H(s) = C inv(sI - A) B
sys = ss(A,B,C,D);
bandwidth(sys)
%% Built-in bode plot
figure(1)
bode(sys);
s = (i*10.^(-2:.1:2))/(2*pi); % Input frequencies
freq_response = nan(size(s)); % Frequency response vector
for i=1:length(s)
freq_response(i) = C / (s(i) * eye(length(A)) - A) * B + D;
end
figure(2)
% Magnitude plot
subplot(2,1,1)
semilogx(abs(s), 10*log(abs(freq_response)))
% Phase plot
subplot(2,1,2)
semilogx(abs(s), 180/pi*phase(freq_response));
Star Strider

### Star Strider (view profile)

2019 年 10 月 20 日
Great!
That example (from the documentation) is a SISO system.

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