How do I find the right peaks?

I need to find the two peaks that are shown below in red. The graph on the left is a periodogram.

I tried to do smoothing using the pwelch('Tukey') method (shown to the right of the periodogram), but I only get one peak in the area from 100 to 400 hz rather than the two peaks. I've tried other smoothing methods as well, with similar results. Any suggestions?

Answer 1

[Azzi Abdelmalek]

Use findpeaks

Answer 2

[Image Analyst]

Use findpeaks in the Signal Processing Toolbox. But since you did not say how general this must be and how varied the data are, you can get then for just the specific signal you gave by saying

% Find indexes of peaks around 130, 320, and 800
[~, indexesOfPeaks] = find(yourSignal > someValue); 
indexesOfPeaks(indexesOfPeaks>400) = [];  % Eliminate any peaks with x>400

I don't know what someValue is because you did not show the values along the Y axis.

[Orville]

When I use this method, I get over 1000 peaks. How do I keep it down to just the two I want? Of course I know what peaks I want, but I want the machine to be able to figure it out. I was trying to use the smoothing method to take care of this, but the smoothing is smoothing too much.

Thanks for your help!

[Wayne King]

Do you have a priori knowledge about what part of the frequency range you want to limit your search to? Can you attach your time data along with the sampling frequency

[Image Analyst]

Your peaks are extremely narrow (if you want those two precise thin lines you pointed to) so any smoothing will obliterate them. If you just want kind of the centroid of the hump, then you can smooth them, say with sgolayfilt() in the Signal Processing Toolbox, which find peaks is also in. Do you have that toolbox?