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
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