Filtering in baseband spectrum

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. Any indications will be appreciated. Regards