USRP2 noise floor generation

We can "usrp2_siggen_gui.py" located at gnuradio/gr-util/src/python
where we can set the parameters like central frequency, interpolation rate, amplitude of noise level.

In order to avoid some potential problem in USRP2, it is highly recommend to modify the amplitude level by changing the default at the function of "usrp2_siggen.py".

Generating the noise signal at one USRP2,
sudo python usrp2_siggen_gui.py -m 00:50:c2:85:32:97

Verifying the PSD by doing the FFT
sudo python usrp2_fft.py -m 00:50:c2:85:32:96

Some example data are collected:
in WiSeR lab environment, the background noise level is around -55dB(+30 = -22dBm). Same results are observed for antenna and SMA connector.
It seems like USRP board generating a large noise itself.