[USRP-users] Not getting white noise in a USRP N210

Marcus Müller marcus.mueller at ettus.com
Thu Sep 4 15:55:07 EDT 2014


Hi Harold,

>When using USRP N210 without an antenna I would expect to receive white
noise, right?. 

Not really; I'd expect you to see whatever that open port picks up --
which might actually be received by traces on the PCB, connectors...

> But consistently I am getting signal at center frequency
well over 20 dB compared to other frequencies and 30 or dB over the signals
near the edge.
> Anyone knows what is happening here? Is something wrong in the USRP or in
GNU Radio? or me? Thanks.

That would be the local oscillator. Every direct downconversion
architecture suffers from LO leakage; so there's nothing wrong with you,
the USRP, or GNU Radio.

Luckily, the USRPs give you the possibility to construct a
tune_request_t that specifies an RF frequency, where the LO will be, and
an offset, which it uses to digitally shift the signal; see the UHD
doxygen for tune_request_t for further details. An example:

uhd.tune_request(center_freq, rf_freq=(center_freq +
     lo_offset),rf_freq_policy=uhd.tune_request.POLICY_MANUAL)

To elaborate on this: the N210 always samples a 100MHz bandwidth; from
this 100MS/s it only extracts the sampling rate you request (up to
25MS/s, more doesn't fit through Gigabit). Simultaneously, it's able to
digitally shift the baseband signal (multiplication with complex sine)
in frequency to get you the sub-band you actually desire.

With best regards,
Marcus Müller

On 04.09.2014 21:24, Harold Daniel Moreno Urbina via USRP-users wrote:
> Hello,
>
> When using USRP N210 without an antenna I would expect to receive white
> noise, right?. But consistently I am getting signal at center frequency
> well over 20 dB compared to other frequencies and 30 or dB over the signals
> near the edge.
>
> Anyone knows what is happening here? Is something wrong in the USRP or in
> GNU Radio? or me? Thanks.
>
>
> I am using USRP N210, GNU Radio 3.7.2.1 (version of last friday) over
> Ubuntu 14.04.
>
> I uploaded some screenshots of the FFT of the received signal, always
> 10Msps and getting about 5 minutes of signals. I used the example included
> in GNU Radio uhd_fft.grc and a custom flowgraph icluding only a USRP source
> and a FFT sink.
>
> ----------------------------------------------------
> uhd_fft.grc
> ----------------------------------------------------
>
> center frequency 2.45 GHz:
> https://mega.co.nz/#!WFE3UTRZ!AO04usGbl9TBa5-mCUPz7FYW2IabY5nbo5zZqSWwaM4
>
> center frequency 2.42 GHz:
> https://mega.co.nz/#!Hc0UzBJA!fSfgScdXPJ_phKdIWUKcNseonlKQt3m64yvYMR0bDhs
>
> ----------------------------------------------------
> Custom USPR2FFT
> ----------------------------------------------------
>
> center frequency 2.45 GHz:
> https://mega.co.nz/#!Hd9RyApS!FzsY5yxr5e6dJaAAO7Ww2O4BFefyCk6MDPN6ffcOs_g
>
> center frequency 1.80 GHz:
> https://mega.co.nz/#!LQcTkDjb!NQ7rIe1PDgRFCp547_U3mhxxxzsvQilPV-esTo7nHa4
>
> center frequency 900 MHz:
> https://mega.co.nz/#!eZ1TFbSY!nkt-n1mlC54aFfDwvqoJIJd7-a7gMB2GBLLhyclxxcs
>
> center frequency 300 MHz (out of SBX used operational band):
> https://mega.co.nz/#!mJ8EESwR!lI_0r4RvvTncxwwMnpEVAvrwsVhcHzLX7H6C7TA4Cj0
>
>
>
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

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