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.
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
Cheers,
LM
Hi Luca
Thanks for your reply.
Perhaps I am too old school. But how would one make a frequency shift of 225 KHz without running into issues with the folded image spectrum causing a havoc?
Any more indications on asymmetric filter (complex coefficients) and then shift to origin. is there a link to any page that addresses this in detail.
While I am being skpetic about these, I am sure everyone is doing it. How thowugh?
Regards
Date: Mon, 5 Aug 2013 17:12:38 +0200 From: luca@srlabs.de To: baseband-devel@lists.osmocom.org Subject: Re: Filtering in baseband spectrum
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
Cheers,
LM
baseband-devel@lists.osmocom.org
-
Luca Melette -
Neela Neela