Build this GNURadio Amplitude Modulation receiver as your second project. It makes use of the RTL-SDR and Ham It Up converter for complete medium and shortwave broadcast listening.
Next up, we will build a simple AM receiver for broadcast band and shortwave reception. This is another “Hello World” type of project using the RTL-SDR hardware. In addition, it shows how to set and display the receiver frequency when using an up-converter. In my case, I am using the Ham It Up converter which shifts the 0-30 MHz range up to 125 MHz.
This project is a bit different than the previous FM receiver design. It uses different decimation approaches, with AM rather than FM demodulation. Also, it demonstrates implementing a variable bandwidth filter. If you copy and expand the above graphic, you will be able to replicate the data flow program. Better yet, watch the GNURadio Amplitude Modulation Receiver video which explains everything. You can download the GNURadio Companion file below.
You don’t need much of an antenna for local medium wave reception. But for shortwave, an outdoor antenna helps a lot. Also, you will need to increase RF gain to receive distant signals. Try out the slider control to change filter bandwidth and watch the results on the spectrum scope. Lots of fun – fast, easy and cheap.
What I learned building the GNURadio Amplitude Modulation Receiver
GNURadio contains dozens of signal processing blocks, which are displayed on the right side window in Companion. They are organized into categories, which can be expanded or searched. Your main resource for documentation is the “modules” section of the GNURadio Manual. Unfortunately, this documentation is more about the underlying C++ classes. New users will get more of a start from examples and tutorials.
The good news is that a large number of the processing blocks are simple. Together with a basic knowledge of DSP you can figure out how to set up filters and do math operations on the data.
Finally, you might want to have some DSP texts handy while experimenting. A great starting point is The Scientist and Engineer’s Guide to Digital Signal Processing which is available online. This one is especially good for learning about filtering in both analog and digital domains.