Exploring GNU Radio by Eric Blossom - What Goes in the FPGA?

An FPGA is like a small, massively parallel computer that you design to do exactly what you want. Programming the FPGA takes a bit of skill, and mistakes can fry the board permanently. That said, we provide a standard configuration that is useful for a wide variety of applications.
Using a good USB host controller, the USRP can sustain 32 MB/sec across the USB. The USB is half-duplex. Based on your needs, you partition the 32 MB/sec between the transmit and the receive directions. In the receive direction, the standard configuration allows you to select the part or parts of the digitized spectrum you're interested in, translate them to baseband and decimate as required. This is exactly equivalent to what's happening in the RF front end, only now we're doing it on digitized samples. The block of code that performs this function is called a digital down converter (Figure 3, “Digital Down Converter Block Diagram”). One advantage of performing this function in the digital domain is we can change the center frequency instantaneously, which is handy for frequency hopping spread spectrum systems.

















Figure 3. Digital Down Converter Block Diagram

In the transmit direction, the exact inverse is performed. The FPGA contains multiple instances of the digital up and down converters. These instances can be connected to the same or different ADCs, depending on your needs. We don't have room here to cover all the theory behind them; see the GNU Radio Wiki for more information.