Thread: Power spectra

Hi,
I am using MacSac 10.6f-grh104 and trying to compute a power spectra
from 10 seconds synthetic seismograms.
Is there a simple way of doing that in sac or do I have to take the
fft and multiply it by it's conjugate?
I can't get the PDS (in the SPE sub-process) command to work properly.
I might be using bad widow length. Is there a rule of thumb as to what
the length should be?
Thanks