Re: [Discuss-gnuradio] Distorted Spectrum after applying gr.freq_xlating_fir_filter_ccc

[Michael Dickens]

On Jan 19, 2009, at 5:46 AM, Bruhtesfa Ebrahim wrote:
> Details:
> - The sampling rate of the original recieved signal from USRP was  
> 64MHz/128=500KHz, so I used decimation factor of 500 in the xlating  
> filter such that the output baseband signal will have a sampling  
> rate of 500KHz/500=1KHz.
> -  since the frequency offset of the signal in 1st figure is  
> -7.68KHz, i used the opposite freq_offset=7680 in the xlating filter.
> - i used cut off frequency =500Hz and width of transition band=200Hz.

Everything looks OK "on paper", but I think your filtering will be  
tight given the sample rate of the output of the xlating filter.  It  
very well might be that the offset needs to be the actual value (not  
the negative of it) ... I really don't remember any longer.

My advice is to play with the xlating filter using more bandwidth:  
reduce the decimation to, say, 50 (BW = 100k), and change the filter  
to be wider and more relaxed (say, BW=25k, transition=15k, in order to  
encompass both the original signal as well as the shifted copy of it  
if the offset is not exactly correct).  I think if you play with the  
variables a bit using more bandwidth, you'll figure out what's going on.

Good luck! - MLD