Measure noise level

Hi,

I am trying to measure noise. I am connecting ‘usrp.source_c’ to ’
gr.fft_filter_ccc’ to ‘probe’ where ‘probe’ is
‘gr.probe_avg_mag_sqrd_c’. I
am trying to print out the value returned by ‘probe.d_level()’ and it
always prints out zero. Is there anything I am missing?

Thanks for the help in advance
M