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

[Yang]

I dont know if this helps. I once ran spectrum_sense.py and also got peak spur at the first several fft points. I think it might relate to the hardware communication between PC and USRP at the initial stage.

-- 
Yang
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On 2011年5月26日星期四 at 下午4:14, Lebowski80 wrote:

Thank you for answering but I cannot understand what you mean with "The USRP
is not a measurement receiver". If USRP was not a measurement receiver,
which is its utility? Moreover, if it is not a measurement receiver what is
the result memorized in m.data that is the output of usrp_spectrum_sense.py?
When I use a WiFi antenna I can receive (and measure) the energy of WiFi
channels. The problem arises without antenna becasue USRP measures a signal
that cannot exist! Furthermore, such a signal is always at the beginning of
the sensed bandwidth, so this has not any sense!

Any idea about that? Thank you!


Nick Foster-4 wrote:

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

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