How to get Log magnitude plot of psd in simulink?

Hello

I am trying to get PSD of the output in logarithm sacle. But psd block of simulink do not allow me. I am also trying to use either one of these blocks (FFT/FFT MAgnitude/periodogram) to get the power spectral density of a signal. When I export the output of any of these blocks to Matlab Workspace, I get a n*n matrix and when identical rows. Plotting each column gives a periodic thing which doesn't make sense since the signal is a periodic which three frequency components. I need to find a way to export the data from FFT/PSD block to workspace and have both frequency and corresponding magnitude.

Thanks in advance.