how to get a certain frequency spectrum using rtl_power

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