[SAC-HELP] Power spectra

[Shahar Kadmiel שחר קדמיאל]

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