PSD plot help with basics

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rg21

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I am involved in a project dealing with in Transit Vibrations. From what i understand as of now, There are 3 methods of getting the final result i am looking for which is a PSD plot.
1. Taking fft of the signal and squaring the magn. divided by frequency.
2. fft of autocorrelation method.
3. pwelch function. using different windows.

I tried all the three and am getting different results for all of them.
The difference is of the magnitude of ten times(the power on y axis)
The signal i took was a random one and shuffled by using matlab itself.

Can anyone please enlighten me about the reasons about these differences and how i can go forward and reduce them for better results.

Does doing these processes for random signal reduce their efficiency ?
 

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