Matt Ettus wrote:
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
Thanks Matt.
But I can confirm that I loaded complex data into Matlab using
read_cshort_binary() provided by Gnuradio.
Then I used pwelch() function to produce the power spectrum density
estimate. According to the Matlab documentation, this function does
accept complex data. It is because the frequencies at which the PSD is
estimated is always in [0,Fsampling), the spectrum shows 0 to 2MHz, and
I think 2MHz is equivalent to -1MHz, isn't it?
I check the usrp_fft.py spectrum today again, using a sampling rate of
4M, now I can see there are adjacent channels at both sides. So I think
the undesired spectral portion is from these neighbor channels.
Regards
Kyle
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