how to get a certain frequency spectrum using rtl_power

[393775602]

Hi,
   i am doing a project to get a certain fequencyspectrum  . And thanks to the 
wonderful tool rtl_power, i can get some dbm data over a very wide area of the 
frequency specturm.
 and i read the source code in order to change it to get a certain frequency 
specturm.Sadly, i know little about digital signal processiong and getting a 
hard time solving it.
 does anyone know how to get a certain fequencyspectrum using rtl_power or some 
other tool.
 
   sincerely
   Chen Zujie