Waterfall

The last project of the first year of Electrical Engineering at the University of Twente is the Transmission Project. In this project, teams have to make a radio transmitter and antenna from 'scratch' (so no IC's, unless given explicit permission). Provided was an ADALM Pluto SDR (A radio you can control from your computer), that we could use to see how well we were transmitting.

My (official) role in the team was to handle the receiving with the SDR. The course provided a couple of Matlab scripts to interface with it, but I didn't like Matlab a lot, and wanted proper control over the signal, instead on relying on a handful of abstracted away functions.

So, I learned what system libraries are used to interact with such an SDR (Libiio), and also how the Fast Fourier Transform worked, and created a basic waterfall display program, with the capability to control the 'focus' of the radio. Such a waterfall display shows the frequency spectrum along the horizontal axis, and scrolls down with time, so that you can see what frequencies happened at what point.

Here is a video of it in action.

Technical Details

I wrote a few C functions to interact with libiio to get the data from the SDR that Waterfall needs. It then compiles and statically links that C file with the main program (written in Odin). The displaying itself is done by a modified version of fenster.h, which is also compiled and statically linked. Finally, the FFT is handwritten in Odin.

At the time, I didn't know a lot about FFT's (I had only just learned how they worked), so there's no windowing function, which leads to the occasional artifact. On top of that, I didn't fully grasp Libiio either, and it would crash on buffer destruction, so there's a memory leak on resize. Fenster is quite limited; it doesn't support resizable windows, and so neither can Waterfall.

In short, Waterfall was mostly a quick and dirty tool for troubleshooting our transmitter+antenna (and those of other teams), for which it was really great to have!