If you enjoy looking at signals, you will love watching digital radio signals. Many of these exist across the radio spectrum, from long waves to microwaves.
Watching digital radio signals is useful for identifying and tuning digital modes. While your digital decoding software probably contains its own spectrum displays using audio, you can also watch digital signals modulate at RF in your radio itself. For slower speed modes, it helps if you speed up the waterfall display to catch the subtle frequency changes during modulation.
The picture above demonstrates what FT8 signals look like on the 20 meter band. FT8 is a form of frequency shift keying FSK using 8 tones spaced 5.86 Hz apart. So, each signal occupies 47 Hz bandwidth. Consequently, to see these signals clearly, you must reduce the spectral display bandwidth quite a lot. Above, you can see a 3 kHz bandwidth containing more than FT8 30 signals.
Unlike the older ham digital modes (CW and RTTY) newer modes can be hard for you to identify by just listening. CW was easy with its single on-off tone. RTTY features a familiar deedle-deedle sound. I remember back in the early 1970’s, my friend Kieth VE7KW could copy most of 45 baud RTTY in his head.
If you are interested, there are a few web sites showing waterfall displays of ham digital modes. The W1HKJ site provides both waterfall and audio displays for many modes.
Watching Digital Radio Modes – Old and New
Perhaps the most famous visualization of digital modes is the “crossed bananas” display tuning indicator for radio teletype (see inset above.) This worked by passing RTTY audio through a series-resonant filter that caused the mark-space tones to undergo a shift change. Output voltages were then fed into an X-Y oscilloscope. You can watch this tuning indicator working in this video, which should bring back memories of early FSK. Later, hams started using FFT displays to identify and tune the tones on a spectrum monitor.
Today, many complex digital modulation schemes are used, especially on VHF and UHF. Then, there is also quadrature amplitude modulation QAM for digital radio broadcasting. In many cases, you will find these digital modulation schemes carried as analog or audio signals. It’s all about I/Q signals and math now. There are many tools you can use for watching digital radio modes, including constellation diagrams for quadrature and phase shift signals.