Unexpected energy peak sensing thermal noise and using usrp_spectrum_sense.py

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?

Regards


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On Wed, 2011-05-25 at 08:51 -0700, Lebowski80 wrote:

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

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 F.-4 wrote:

that, I replaced the WiFi antenna with a resistor of 50 Ω to measure the
usrp_spectrum_sense.py, isn’t it? Can someone give me a hint about that
Regards


Discuss-gnuradio mailing list
[email protected]
https://lists.gnu.org/mailman/listinfo/discuss-gnuradio


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Any idea about that? Thank you!

You should perhaps do some diagnosis using something other than
spectrum_sense.py.

Try a simple FFT display with your tuner tuned to different frequencies.
Does an apparent
spur appear in those cases as well?

A general observation is that essentially-all receivers will experience
spurs in the RF, IF and
occasionally baseband stages. These are internally-generated, small,
signals narrowband in
nature that come from the internal operations of the receiver itself.

A “measurement receiver” is one that has been engineered specifically to
drastically reduce spurs, and
also to have a calibrated (as in laboratory-quality calibrated)
response that is predictable,
specified, and flat across the entire tunable bandwidth of the
receiver. Providing that kind of
functionality requires design and construction techniques that are
very, very, expensive.

However, that doesn’t mean that the hardware can’t be used for
measurement, you just have to
be aware of things like changes in response at different frequencies,
and calibrate them yourself.

I would suggest that you use a simple FFT display at various frequencies
to help with diagnosis.


Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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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