Filtering in baseband spectrum

[Neela Neela]

Hi
 
I am a beginner with RTL and observing that the baseband
spectrum is centred at 0 Hz and is asymmetric going from -1 MHz to +1 MHz. How
does one filter a part of such a spectrum? For example if signal of interest lied
in -300 kHz to -150 kHz.   The filtering techniques for typical band pass
filters work with positive frequencies and have symmetric frequency selection
response. As such a 150 to 300 KHz band pass filter will select signal in both
150 to 300 kHz band as well as from -150 to 300 kHz. Is there a special
filtering method available to work with negative frequencies and with an asymmetric
baseband spectrum.  I am sure this has been addressed as preassumably the FM and other signals of interest are filtered somehow. Anu indications will be appreciated.  


 		 	   		  
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