Software Defined Radio is fundamentally a different way of looking at radio spectrum

Foundations of Amateur Radio

We think of radio as operating on a specific frequency. We select an antenna resonant on a single band. We configure the radio for that same band and then turn the dial or the VFO, or Variable Frequency Oscillator to a particular frequency within that band.

All of our language is geared towards this concept of tuning, of picking out, selecting one special tuned, resonant frequency and listening to it.

I've said this before, but that's not actually what's happening.

Your radio is receiving all RF frequencies, all of them, all at the same time, all the time. Your antenna is better at hearing some frequencies than others, but that doesn't stop it from hearing everything at once. Your radio is getting all that RF information at the antenna connector. After that, every step along the way is removing unwanted information, first it removes all the bands you're not listening to, then the VFO selects which part of what remains to let through to the decoder and the result finally arrives at the loudspeaker.

Ultimately, all your radio lets you play with is what's left over. Say about 3 kHz bandwidth. Using traditional radio, if you want to listen to two repeaters, you either need to switch back and forth quickly, or you need two receivers.

Now without going into how precisely, imagine an SDR with a bandwidth of 3 MHz, one thousand times larger than your traditional radio. Before you think I'm being fanciful, a $25 gadget can do this. This means that you could process most if not all of the 2m amateur band and then pick out which bits you'd like to decode. You could decode all the local FM repeaters, an overflying satellite, the International Space Station SSTV, a beacon, Morse, Packet, RTTY and simplex contacts, WSPR, APRS, EME, whatever is happening on 2m, all at the same time.

Let me say that again. All of the 2m band, all at the same time.

The point is that all this information is there, all the time. We can opt to decode or ignore the information. In a traditional radio, you can only decode one signal at a time, but on an SDR, you can extract as much or as little as your computer can handle. Some SDR language talks about using multiple receivers, but a better description is multiple decoders.

This means that software defined radio is fundamentally a different way of looking at radio spectrum. Instead of filtering out everything we don't want to decode, we select which decoder to apply to which part of the spectrum.

With an SDR you could represent the 2m band as a 3 MHz slice of spectrum as a series of measurements. There is no loss if you reuse the numbers, so if you process the same data multiple times, you have no loss of signal, no deterioration, no extra noise.

All we do is feed the same data into each decoder, pick out the bit we want to decode and have at it.

There is a misconception that you need serious computing power to do this. That's not strictly accurate. A $5 Raspberry Pi single board computer is more than powerful enough to do this. You can argue that this is serious computing power, compared to what we used to land on the moon it is, compared to your mobile phone, it isn't.

I fully intend to go into the maths behind this, but it's not scary, despite what you might think or have been taught. My week has been about the maths and it's become clear to me that there are lots of explanations around, each trying harder than the next to scare you away.

If you feel the need to run screaming for the hills when you hear the words Nyquist, Shannon and Fourier, then get it out of your system and come back when you're ready.

I'd like to mention that I've been working on how to explain this over much of the week, I've lost count of the number of drafts I've written, but it keeps coming back to the words that are almost as old as I am: My god, it's full of stars.

No doubt you might be convinced that I've lost my marbles and that I'm going well outside the Foundations of Amateur Radio, but I have to confess, this is what radio is today, and I'm thrilled to be here learning more about how this all works. Hopefully you are just as thrilled.

I'm Onno VK6FLAB