Computation of the bandwidth of a spectrum in wavelength domain

Question

Ikechi Ndamati 2021-8-3

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https://ww2.mathworks.cn/matlabcentral/answers/891542-computation-of-the-bandwidth-of-a-spectrum-in-wavelength-domain

评论： Ikechi Ndamati 2021-8-6

I have a spectrum that is plotted in the wavelength domain, and I need to find the bandwidth of the spectrum. I can't share the full code else, it will be overwhelming, so I saved the vectors. The bandwidth that the obw() function gives is innacurate. Please how can I find the correct bandwidth?

clear
filename1 =  'Ab.txt';
filename2 =  'lambda_axis.txt';
Ab = importdata(filename1);
lambda = importdata(filename2);
lambda_range = (lambda>1100e-12 & lambda<2000e-12);
plot(lambda(lambda_range),Ab(lambda_range));
xlim ([1.2e-9 1.9e-9]);
bw = obw(Ab,lambda);

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David Goodmanson 2021-8-3

编辑：David Goodmanson 2021-8-3

ok the lambda_range data has no problem but the obw result is negative, and (assuming you are staying in the wavelength domain) it appears to be on the large side by about a factor of 2. Is that the inaccuracy you have in mind?

Ikechi Ndamati 2021-8-4

@David Goodmanson I do not expect a 100% accuracy. However, the current bandwidt values are not even remotely close to a reasonable value. I need to know why my implementation gives unreasonable values so that I can try to solve the issue.

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Answer 1

David Goodmanson 2021-8-4

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https://ww2.mathworks.cn/matlabcentral/answers/891542-computation-of-the-bandwidth-of-a-spectrum-in-wavelength-domain#answer_760177

编辑：David Goodmanson 2021-8-4

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Hi Ikechi,

the problem is basically that there are not enouch points to work with.

lambda = lambdaaxis;
lambda_range = (lambda>1100e-12 & lambda<2000e-12);
lam1 = lambda(lambda_range);
A1 = Ab(lambda_range);
bw = obw(A1,lam1)
       % bw = -8.8591e-10
% add a lot of interpolated points
a = min(lam1);
b = max(lam1);
lamnew = linspace(a,b,10000);
Anew = interp1(lam1,A1,lamnew);
figure(1); grid on
plot(lamnew,Anew)
bw1 = obw(Anew,lamnew)
      % bw1 = 4.2769e-10

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David Goodmanson 2021-8-5

Hi Ikechi,

I didn't really know that it would work, I just tried it. One of the great things about Matlab is since it's an interpreted language, it's easy to try stuff. There was some reason to believe that the number of points might be involved, though. Lambda_range has 245 points. obw says it uses periodogram, and periodogram says it uses a minimum of 256 points, so increasing the number of points seemed like a worthwhile exercise.

Ikechi Ndamati 2021-8-6