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

[Marcus D. Leech]

>
> 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!
>
>   
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