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

Marcus Müller marcus.mueller at ettus.com
Thu Sep 4 15:56:51 EDT 2014


Ah! I missed the screenshots below; and thus, I agree with Ian. I
thought you were only seeing one peak ... please excuse my premature
reaction!
On 04.09.2014 21:55, Marcus Müller wrote:
> 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
>>
>>
>>
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>





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