[Discuss-gnuradio] wbx usrp1 sampling spectrum distortion

[Kyle Zhou]

Hi
I use WBX + USRP1 to capture a DAB+ digital radio broadcast signal at 
frequency 204.64MHz and bandwidth 1.4MHz.
Sampling rate is 2Msps (decimation=32)
The spectrum shown by usrp_fft.py is normal and clean. However, after 
capturing the data using usrp_rx_cfile.py and loading the data to Matlab 
and show the spectrum, I found an unexpected portion of spectrum around 
the Nyquist frequency (1MHz in this case).
The parameters I use for usrp_rx_cfile and usrp_fft are the same: -f 
204.64M -d 32 -g 6.
I uploaded a snapshot of the spectrum:
http://lh4.ggpht.com/_ozkfEhDY6Mg/S81p3QihHkI/AAAAAAAAAsE/KJ8vtRo4EI4/spec_samp.jpg
I can filter out the undesired part easily but that cost computational 
resource and I do want to find out the cause.
Regards
Kyle