# Filtering in baseband spectrum

Leif Asbrink leif at sm5bsz.com
Thu Aug 8 22:16:57 UTC 2013

```He Neela,

I think the most efficienmt solution is to first shift the frequency.
The average of -300 and -150 kHz is -225 kHz so you need to multiply
the samples by the appropriater sine and cosine functions.

After that your signal will be centered at 0Hz and it will extend
to +/- 75 kHz. Now you can apply low pass filters with a bandwidth of
75 kHz. Probably it would be clever to resample in the same procedure.

For 150 kHz bandwidth you would not need much above 150 kHz sampling
rate. The rtl should run at the highest possible rate because the
dynamic range improves when sampling is faster.

Regards

Leif

>  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

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