Re : [Discuss-gnuradio] spurious sidelobes with WBX

[Mathias Coinchon]

Hi,

Thank you for your answer. However I am not sure to understand how you changed your ant-alias filter because this is something that is implemented in the USRP FPGA isn't it ?

As I said when monitoring the baseband IQ signal produced by the modulator beofre going to USRP, I don't see these sidelobes, they only appear after USRP.

Is it some rounding problem ? and where does this aliasing come from as the signal is centered on 0Hz with no further energy after +/- 750Hz. A sampling frequency of 3.2MHz (complex) should be enough. I can try use a lower interpolation and so use 4MHz instead of 3.2MHz.


Best regards

Mathias

---

De : Charles Brain <address@hidden>
À : Mathias Coinchon <address@hidden>
Cc : gnuradio <address@hidden>
Envoyé le : Mer 4 août 2010, 8h 32min 40s
Objet : Re: [Discuss-gnuradio] spurious sidelobes with WBX

On 03/08/2010 21:50, Mathias Coinchon wrote:

Hello,

I am using the USRP1 + WBX to create a DAB OFDM signal (1.5MHz bandwidth) in VHF 
band.
The sampling rate used is 3.2MHz (interpolation 40)

However, there's something I don't understand. On the picture attached, you can 
see 2 spurious sidelobs that appear at about 2.5MHz from the center (red arrow).

They are not present when monitoring the spectrum of the baseband IQ signal 
before going to USRP. 

They also don't appear when sending a tone signal.

Is there any explanation for this ? and any hint to avoid or reduce it ?

Thank you for your help.

Mathias Coinchon
opendigitalradio.org

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It looks a bit like this doesn't it.
http://tinyurl.com/2vhge7j
These ears on my DVB-T signal were caused by aliasing.
when I changed my anti-alias filter they went away

- Charles