No obvious maximum in power spectral density graph

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sarah Abdellahi 2018-9-19

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https://ww2.mathworks.cn/matlabcentral/answers/419877-no-obvious-maximum-in-power-spectral-density-graph

评论： Star Strider 2018-9-19

采纳的回答： Star Strider

I am extremely new to signal processing.

I have sampled the wind velocity downstream of a fan at 100 HZ and here is the time history:

I would like to plot the power density. Using pwelch I get the following plot which doesn't seem meaningful to me. I simply use pwelch(V,[],[],[],100). Based on Matlab documentations, I should get a curve with a peak! Can someone help me figuring out what the problem is? Or if this odd-shaped pwelch plot is actually meaningful and I can get it?

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Answer 1

Star Strider 2018-9-19

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在 MATLAB Online 中打开

You are seeing a peak, just not the one you’re anticipating. The reason is that you have a significant d-c (constant) offset in your signal.

To Illustrate —

t = linspace(0, 1);
V1 = sin(2*pi*t) + randn(size(t));
V2 = sin(2*pi*t) + randn(size(t)) + 10;
figure
pwelch(V1,[],[],[],100)
figure
pwelch(V2,[],[],[],100)

See if subtracting the mean of your signal produces the result you want:

pwelch(V-mean(V),[],[],[],100)

I¹m thinking it will.

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sarah Abdellahi 2018-9-19

编辑：sarah Abdellahi 2018-9-19

Thank you for your response. I see your point but When I subtract the mean, I still see the same graph. Here is the graph: I mean I dont see an obvious peak at the middle of the spectrum!

I can also attach the vector if that would help to solve the problem.