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Software Defined Radio (SDR) for Hackers, Part 2: Building Our First SDR Radio (FM)

Welcome back, my aspiring RF hackers!


In part 1 of this series, we set up the HDSDR software and the RTL-SDR hardware to work together to create our software-defined radio. Now that we have those elements functioning, let's use our radio initially for some simple, basic radio signal capture such as your local FM radio station.


Step #1: Sampling


The first step is to setup our sampling rate. Radio signals are continuous and analog. To use them, we need to take discrete samples of this continuous process. In order words, we need to capture pieces of the analog signal at a fixed time interval and feed that to our system.


As you can see in the diagram below, the continuous wave of audio is broken into a sample at a fixed time interval.







These samples can then be used to retrieve the original signal by sending them through a reconstruction filter.


Let's click on the bandwidth button in HDSDR as seen below.


This opens a window to set the sampling rate. We can set both the input sampling rate and the output sampling rate. You can set the sampling rate at the level of your choice but most audio engineers believe that the human ear can not distinguish differences in sampling rates above 48khz (48000). Since we will be sampling FM radio, a sampling rate above 48khz will not make a distinguishable difference to the quality of the signal.


Step #2: Set the Tuner


To listen to your local FM radio, click on the FM mode icon near the top of the panel.

Now, go down to the Tune section (see above) and set the tuner to the frequency of your favorite local radio station. You can also use the slider to adjust the frequency of your captured signal. For the best reception, place the frequency slider in line with the peak here.




Once you have done so, you should now be able to hear you radio through your speakers. To adjust the volume, you can use the volume slider as seen below.


Congratulations! You have just built your first software defined radio! Enjoy your local FM radio station and experiment with the various buttons and sliders in HDSDR and watch what happens.


Summary


Software Defined Radio is the leading edge of cybersecurity research. Now that we have completed our first software defined radio, look for future tutorials as we look to capture satellite signals, aircraft signals and so many more! As we develop our skills, we will advance to transmitting, replaying, and decoding signals from a multitude of sources.






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