Re: [Discuss-gnuradio] undersampling with USRP

[Matt Ettus]

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)

In complex sampling, 16 MS/s gives you a frequency range of +/- 8 MHz. A 
signal at +10 MHz would alias to -6 MHz, not plus 6.

In any case, the [digital] antialiasing filters are removing the signal 
once it goes beyond 8 MHz. You can sub-sample signals at the ADC since 
there is no anti-aliasing there (assuming you use a BasicRX), but 
digitally we have antialiasing filters in the decimation.

Matt