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
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