ARLA/CLUSTER: Episódio 204 da série "Foundations of Amateur Radio": Rádio Definido por Software é fundamentalmente uma maneira diferente de olhar para o espectro de rádio

[João Costa > CT1FBF]

Foundations of Amateur Radio #204

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

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