> As i am dealing with RF communication i need to know exactly what sampling > rate > the USRP2 is sampling the data and sending over the air ?
This has been detailed before on this list, so you can find a more complete explanation by searching the archives. On the transmit side, there are *two* sample rate conversions occurring in the USRP2. The DAC operates internally at 400 Msps, but is configured to interpolate samples presented by the FPGA by a factor of four, so the FPGA must present samples at 100 Msps to the DAC interface bus. This is the "DAC rate" parameter referred to above. Since it is not possible to provide the USRP2 itself with 100 Msps of data over the GbE communications interface (this would be 3.2 Gbps plus overhead), the USRP2 FPGA implements a configurable interpolation rate digital upconverter, allowing an interpolation rate between 4 and 512. In the case it is configured to interpolate by 4, then the USRP2 will consume samples from the GbE port at 25 Msps, or 800 Gbps + overhead. Since the signal sample format is complex baseband (I and Q in quadrature), the Nyquist criteria allows up to 25 MHz of signal bandwidth to be represented using 25 Msps (not 12.5 MHz, which would be the case for a "real" signal.) As the interpolation rate in the FPGA is decreased, the USRP2 consumes samples from the GbE at lower and lower rates, and the amount of RF signal bandwidth that can be represented in the sample stream goes down accordingly. In the limit, at an interpolation rate of 512, one would generate a sample stream representing ~183 KHz (100 MHz/512) for further host processing. The receive side is similar, but in the other direction, and there is only one sample rate conversion. The ADC sample rate is 100 Msps, and the configurable digitial downconverter in the USRP2 FPGA filters and decimates the digitized sample stream by a factor between 4 and 512. Thus, at the input to the FPGA, the digitized sample stream is 100 MHz wide, but the DDC reduces and resamples this to between 25 MHz (decimation 4) and ~183 KHz (decimation 512). This is a long way of saying that you should use the same decimation and interpolation rate on the transmitter and receiver to achieve the same sample rate/bandwidth to and from the host PCs. Johnathan _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
