Voices May 14, 2019
When is a radio not a radio? When it’s ‘software defined’
About a year ago, a friend, Kent Johansen, who happens to be the designer of the sensors that power the earthquake early warning system in our schools, gave me a device he called a software defined radio – SDR for short.
Although I have had a lifelong fascination with all aspects of radio, I had ignored developments in the field in recent decades as I focused on my work in computer technology and physics.
When he handed the device to me, it was the first time I had heard mention of the term “software defined radio,” let alone seen one.
I was completely befuddled. A radio, to me at least, had dials, lights, perhaps even meters or a digital display of some sort. This device had none of that. In fact it was just a little bigger than one of my thumbs. It had a USB plug at one end, and a very small brass terminal near the other.
Accompanying it was a small antenna that attached to the brass terminal. “Here you go”, Kent said to me, “a complete radio that can tune from 25 MHz all the way up to nearly 2 Ghz.”
Well, I remember thinking, what exactly was I going to do with this little device? Most of my experience with radio in my teenager years was with radio signals below 25 MHz, namely AM radio, which centres around 1 MHz (for example, Vancouver sports station TEAM 1040 operates at 1.040 MHz, and shortwave radio, which typically runs from around 5 MHz to 25 MHz (part of what is called the HF, high frequency, band.
Frequencies above 25 MHz were mostly unfamiliar to me, with the exception of the aircraft band (108 - 137 MHz), and the FM band (88 - 108 MHz). My friend made vague allusions to using the SDR for listening to the International Space Station, or to signals bouncing off meteor trails, and he referred to signals from weather satellites.
Having obtained an amateur (ham) radio licence several decades ago, I was also vaguely familiar with something called the 2-metre band (144 - 148 MHz), part of what is called the VHF (very high frequency) spectrum that runs from 30 to 300 MHz.
It took me a while to actually begin experimenting with the SDR. I did a lot of background reading, learning that SDR really had military origins as it made it possible to listen to wide swaths of frequencies very easily. The basic concepts for SDR evolved in the 1970s, but did not become mainstream until relatively recently when the concept was adapted to TVs.
My SDR dongle, made by NooElec, is a particularly good unit, costing under $30. I have found it to be a solid performer that can be left running for days, if not weeks on end, gathering for instance data from satellites (more on this aspect in a future column).
Unfortunately, as with many electronic devices, knock-off models with dubious performance are to be found of many brands of SDRs. I now also have a second SDR, made by SDRplay, a model called the RSP2. It is considerably more expensive than the NooElec model, but it covers the entire spectrum from 0 to 2 GHz.
When I first plugged in the NooElec SDR it did nothing. I didn’t
realize I actually needed some software to unleash its potential – specifically
I needed SDR software, basically a utility that could use the circuitry in the
SDR itself to tune and demodulate (make audible) radio signals.
There are numerous versions of SDR software. Three that I have found particularly good are SDRuno, SDR Console V3, and SDR# (pronounced SDR Sharp).
Once I had one of these free packages running and connected to the NooElec the magic of software defined radio became apparent. Something called the waterfall allows users to see entire sections of spectrum spread out over a time interval. For instance, if watching the aircraft band, where communications come in brief bursts, a user can spot frequencies where there has been activity and quickly tune there.
When looking at FM stations I could see from the waveforms and waterfall those that have HD add-on signals (you may for instance hear AM station News1130 promoting its HD presence on 96.9 FM HD2).
Where an SDR stands out from physical radios is the ability to tune a broad range of frequencies and modes. Example include marine and aircraft communications, data signals such as ACARS and ADS-B (a form of home aircraft radar system) from aircraft, television broadcasts, and amateur radio broadcasts.
Through this experimental phase I also learned more about antennas, and how different bands required different types of antennas, a subject for another column.
In the meantime, for more on SDR, check out the wealth of material at RTL-SDR.com.
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