Re: [Discuss-gnuradio] undersampling with USRP

[Martin Dvh]

Jonathan Gill wrote:
> Hi all,
> 
>  
> 
> I'm having some problems with sub-sampling signals using with the USRP.  I
> currently have the board connected to a signal generator with a sinusoidal
> input at 0dBm.  I'm using the usrp_rx_cfile.py script from USRP examples
> with the -8 -s and --no-hb flags and a decimation of 4 and -f 0 (no down
> conversion).
> 
>  
> 
> This gives me an output file with interleaved I and Q samples (with one set
> being empty due to -f 0), this works fine up to Nyquist frequency (8MHz, as
> sampling frequency is 64/4 = 16MHz) but beyond this the operation is not as
> expected.  At 10MHz I expect to see a single sinusoidal component at 6MHz,
> however this is corrupted by a strong component at 2MHz, and at any
> frequency above 10MHz no signal is recorded.  Any suggestions?  (I have
> checked the signal generator, it is working fine)
The sampling freq is not 16 Mhz but 64 Mhz
So niquist is not 16/2 but 64/2=32 MHz
The cic-decimator resamples at 16 Mhz
But it also filters away anything outside of the new bandwidth.
Ideally you should not see anything beyond 8 Mhz.
Since the cic-decimator is not ideal, it doesn't have perfect steep slopes, you 
still see some niquist mirroring signals up to 10 Mhz.
This is why the halfband filter was introduced, to filter out the garbage at 
the bandwith edges (which you disabled to get 16 Mhz samplerate)

You do still get niquist mirrors around the actual niquist frequence of the  AD 
converter
example:
If you input a signal at 56 Mhz you will see it back as a single 6 Mhz 
component if using -f 0.

If you input at 36 Mhz you will get a signal at 28 Mhz.
To see this signal you would have to use a -f 26.0e6 or something like that.
Then you would see the signal at 2 Mhz.
(if you would use -f 28.0e6 you would see the signal at 0.)
If you would use -f 0, you would see nothing because 28 Mhz is outside the 
bandwith ofthe cic-decimator.

I hope this helps,
Martin



> 
>  
> 
> Thanks for your help
> 
>  
> 
> Jon
> 
>  
> 
>  
> 
> --========================================
> 
> Jon Gill
> 
> address@hidden
> 
> CAA Institute of Satellite Navigation
> 
> University of Leeds
> 
> Tel: +44 (0) 113 343 2025
> 
> ==========================================
> 
>  
> 
> 
> 
> 
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