Re: [Discuss-gnuradio] Unexpected energy peak sensing thermal noise and using usrp_spectrum_sense.py

[Nick Foster]

On Wed, 2011-05-25 at 08:51 -0700, Lebowski80 wrote:
> Hello everyone
> 
> I’m using an USRP with a XCVR2450 daughter board working at 2.4 - 2.5 GHz. I
> want to find a threshold between thermal noise and signals energy using the
> script usrp_spectrum_sense.py and sensing from 2.400 GHz to 2.500 GHz. To do
> that, I replaced the WiFi antenna with a resistor of 50 Ω to measure the
> noise. I obtained a very strange peak at the beginning of the bandwidth
> (around 2.405 GHz) and this result was totally unexpected because I supposed
> to sense only thermal noise without any particular peak. So, I repeated the
> experiment from 2.420 GHz to 2.500 GHz and I achieved the same peak always
> at the beginning of the bandwidth (in this case around 2.425 GHz).
> Therefore, I'm quite sure this is a problem linked to USRP or to the script
> usrp_spectrum_sense.py, isn’t it? Can someone give me a hint about that
> please?

The USRP is not a measurement receiver, and some small spurs are to be
expected. You can plan for this, or work around it by changing the LO
offset at a given center frequency using the advanced tuning parameters:
http://www.ettus.com/uhd_docs/doxygen/html/structuhd_1_1tune__request__t.html

--n

> 
> Regards 
>