Filtering in baseband spectrum

Luca Melette luca at
Mon Aug 5 15:12:38 UTC 2013

Hi Neela,

I find your question a bit off-topic in this mail list,
but still interesting for SDR geeks.

The asymmetric spectrum is quite normal when processing
complex domain signals (I/Q).

If you are only interested in a part of the spectrum, you
would normally shift it to the origin and then apply a low-pass filter.

In your case, you can apply a positive shift of 225 KHz, and then
a filter with cutoff frequency 75 KHz.
You can easily apply this transformation with GNU Radio block.
After that, you can even downsample your signal if needed.

Other methods to obtain the same result are:
- apply an asymmetric filter (complex coefficients) and then
  shift to origin
- use an FFT filter, and possibly invert the spectrum in baseband


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