Re: [Discuss-gnuradio] wbx usrp1 sampling spectrum distortion

[Matt Ettus]

On 04/20/2010 02:51 AM, Kyle Zhou wrote:
> 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


2 MS/s complex should give a spectrum of -1 to +1 MHz.  Your Matlab plot 
show 0 to 2 MHz, so I think you are loading the samples as reals instead 
of floats.

Matt