On the right side of the window is a list of the blocks that are available. By expanding any of the categories (click on triangle to the left) you can see the blocks available. Explore each of the categories so that you have an idea of what blocks are available.

You can also click on the magnifying glass in the upper right side of the window and simply type a search term (e.g. receiver) to search all categories. A small text window will appear above the list of blocks in which your search term will be entered. This will filter the list leaving only blocks with receiver in their name. Try a few searches such as filter and source to see what comes up.

  • The FFT Sink acts as a Spectrum Analyzer by doing a short time discrete Fourier transform (STFT). Review the theory of the Spectrum Analyzer

  • Close the scope and change the sample rate back to 32000. Add a WX GUI FFT Sink (under Instrumentation->WX) to your window. Change the Type to Float and leave the remaining parameters at their default values.

  • Connect this to the output of the Signal Source by clicking on the out port of the Throttle and then the in port of the WX GUI FFT Sink. Generate and execute the flow graph. You should observe the scope as before along with an FFT plot correctly showing the frequency of the input at 1KHz. Close the output windows.

  • Explore other graphical sinks (WX GUI Number Sink, WX GUI Waterfall Sink, and WX GUI Histo Sink) to see how they display the Signal Source

    • The number sink is typically used to monitor slowly-changing signals such as the RMS input level. In this example, the sine wave changes too fast for the numbers to keep up.

    • The waterfall sink is used to display amplitude vs. frequency vs. time with amplitude represented as a variation in color. The waterfall is a time frequency diagram with time on the vertical axis. Note that for a single 1 Khz sine wave input, the frequency does not change with time, thus a vertical line is displayed at 1 Khz.

    • The histo sink displays a histogram of the input values, which can be used to monitor the symbol distribution in a digital signal or the distribution of a noise source.

  • Create the flow graph shown in Figure 9.


  • The Audio Sink is found in the Audio category. The Audio Sink block directs the signal to the audio card of your computer. Note that the sample rate is set to 48000, a sample rate that is usually, but not always supported by computer audio hardware. 44100 is supported by every sound card. Other commonly-supported rates are 8000, 11025 and 22050. Some audio hardware may support higher rates such as 88200 and 96000. Also note that there is no Throttle block. This is because the audio hardware enforces the desired sample rate by only accepting samples at this rate.

    • It is worth noting that sample rates, especially related to audio output are a common source of frustration in these labs so it is worth spending some time to ensure the concepts are understood here. More information about how GNU Radio communicates with the computer's audio hardware can be found here

  • Generate and execute this flow graph. The graphical display of the scope and FFT should open as before. However, now you should also hear the 1 kHz tone. If you do not hear the tone, ensure that the output from the computer is connected to the speakers and that the volume is turned up. Experiment with changing both the overall sample rate in the flow graph as well as the sample rate in the audio sink to see how the tone is affected.