On Tue, 13 Dec 2011 17:44:14 +0100, Cyril Cavel 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 ? > > Thanks for your lights, > > Cheers > Cyril The underlying motherboard samples at a fixed rate--64Msps for USRP1/B100/E1XX, and the N2XX family samples at 100Msps, so Nyquist isn't violated at all. The FPGA downconverts that sampled 6Mhz to baseband, which is then run through CIC decimators and half-band filters. In general, a Gnu Radio flow-graph deals with complex-baseband signals, which "straddle" 0Hz (that is, for a signal of B bandwidth, it stretches from -B/2 Hz to B/2 Hz).
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