Please consider to set the spectrum type correctly:
[___] = plomb(_,<https://de.mathworks.com/help/signal/ref/plomb.html#buio4ln-1-spectrumtype spectrumtype>) specifies the normalization of the periodogram.
Set spectrumtype to 'psd', or leave it unspecified, to obtainpxx as a power spectral density.
Set spectrumtype to 'power' to get the power spectrum of the input signal.
Set spectrumtype to 'normalized' to get the standard Lomb-Scargle periodogram, which is scaled by two times the variance ofx.
The documentation for theplomb function allows you to specify the output as'psd','power' or'normalized' (see the section on thespectrumtype (link) argument). Since the'psd' option is the default, that will give you the result in units ofdb/Hz. If your original data are in Volts (amplitude) the output will be in terms of Watts (power). Choose the output option you want.
At least that is how I always understood power spectral density calculations.
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