On Tue, Dec 13, 2011 at 8:44 AM, Cyril Cavel <cyril.ca...@gmail.com> wrote:
> Hi all, > > I use a TVRX daughterboard to acquire VHF signals. I believe (but I may be > mistaken) that the TVRX daughterboard downconverts RF signals to a baseband > of 6MHz bandwidth before going to the ADC. The function usrp_rx_cfile > (which I use to acquire signals) has a minimum decimation factor of 8 for a > sampling rate of 64Ms/s. So it gives a maximum sampling rate of 8MHz. > > Nyquist theorem tells us that to acquire properly a signal occupying a > band between 0 and 6MHz, the sampling rate shall be at least 12MHz, which > is then unreachable by the TVRX daughterboard. > > So, are my figures wrong, or am I missing something ? > Nyquist says that you need 2 samples per second for every 1 Hz of bandwidth you want to sample. However, we use complex (I and Q) samples, which count double. So if you want to sample 6 MHz of BW, you would need 6 MS/s complex or 12 MS/s real. You are best off oversampling by about 25%, so the 8 MS/s complex of a USRP1 or B100 device can give you the full bandwidth. Matt
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