Integrating Software Defined Radio with USRP

Methodology

We programmed an FM receiver in GRC using the RFX2400 daughterboard

Figure 1.

Basix RX board

As shown in Figure 2, the USRP could receive clear radio signals from the targeted radio station.

Then we switched the RFX2400 daughterboard to the FLEX900 daughterboard, and we started receiving radio signals. The FLEX900 daughterboard receives frequencies around 750MHz; whereas, FM radio frequencies only range from 88MHz to 108MHz. Then we took out the FLEX900 daughterboard and put in the BasicRX daughterboard because it is supposed to read frequencies that are actually in the range. The BasicRX receives frequencies ranging from 1MHz to 250MHz. We found that the BasicRX works with our program and received radio signals and outputted them to the soundcard like we expected. The FFT plots of the radio signals without the antenna and with the antenna are shown in Figures 3 and 4. The Google Doc shown in Figure 5 explains what each block did and how it worked.

The next step was to get the I/Q data from the signals we were receiving. I/Q data show the changes in amplitude and phase of a sine wave. These changes can then be used to encode information upon a sine wave, which is a process called modulation. We saved the I/Q data as a binary file. We opened the file in Ubuntu using the terminal and programmed Octave, to plot the I/Q file and the FFT of the data (Figure 5). We then sampled the I/Q data from different radio playbacks. We plotted the I/Q data showing their slow response time and the peaks (Figures 6 and 7, 8).

Figure 2.

Layout of the GRC program

Figure 3.

FFT plots of the radio signals with the antenna

Figure 4.

FFT plots of the radio signals without the antenna