Previously, our interest was mainly listening to radio signals. Over the past fifteen years, though, we added watching radio signals in a big way. 

Above, you can see the three main visualizations of radio signals that we rely on. Most popular is the spectrum scope or panadapter at the top. These basically show the strength of all signals in a passband, usually in dBm. Next down is the waterfall display. In their simplest form, waterfall displays are just the history of the spectrum scope, with signal strength mapped to different colors. Each scan line in the waterfall is just an average of spectrum scope over a range of milliseconds. Finally, signal strength can be recorded in a time series of maybe 30-120 seconds.

Pretty much all SDR software provides both panadaptor and waterfall displays. Some also provide visualization of demodulated audio, ADC and I/Q data magnitude and phase. Once digitized, you can pretty much display anything about signals on a screen.

But let’s take a step back. Do we really understand what we are watching?

• What exactly are we seeing? What is the spectral sensitivity of your SDR?
• Are the measurement readings accurate and how are they achieved?
• Why does your software provide all sorts of adjustments, like choice of FFT window, averaging and update speed?

In this series on watching radio signals, we will answer these and other questions.

Watching Radio Signals – How I Got Started

My start in watching radio signals was with the front panel spectrum display on the ICOM 756 Pro II. Unlike most other gear until recently, ICOM was the only way to go for real time continuous spectrum display. I bought my Pro II in 2005 shortly after it was discontinued – good price!

ICOM used a separate circuit for spectrum display. The incoming passband was converted to a 13.345 MHz display IF, then mixed with a 12.79-12.99 swept oscillator and run through a Murata ceramic filter at 455 kHz. Strength was measured using an FM IF Chip with an RSSI output. Dynamic range was around 80 dB, more with AGC. All in all, ICOM engineers were able to display signals in the -20 to 60 dBuV range, with a noise floor of roughly -127 dB. Works great.

Then, in 2008, I bought my direct sampling Perseus and never looked back. Today, it’s all about doing stuff with the Fourier Transform or FFT.

So, for the past ten years, watching radio signals has been as much a part of my hobby as listening to them. More to come soon on this fascinating subject.