I am working with power spectral density function (PWELCH) in matlab. I converted time series of data to frequency domain by using PWELCH. However, the frequency obtaining from this function is incorrect sometimes. I will attach an example which I have 4 set of time series data. All of them contain the same time series data (you can refer to the .txt file attached). However, after using PWELCH and plot PSD vs frequency. You can find that some plot has the shift in frequency. Can anyone please help me?