  In spite of the complicated behavior in the time domain, long GRBs show a
simpler behavior in the Fourier domain of frequencies, represented by power
density spectra, PDS. Recently, there are some relations found between GRBs
properties and PDS parameters, modeled by power-laws. Among them, the
correlation between peak energy $E_{peak}$ and PDS slope $\alpha$ shows a clear
evidence. In this work we try to understand the origin of this correlation,
making use of synthetic pulses. We find some preliminary evidences that
$E_{peak}-\alpha$ relation can be seen as a new confirmation of the empiric
relations $E_{peak}-L$ and $t_{p}-L$ for GRBs.
