Back in the day, it was common for some or all of a ham radio
station to be homemade. Hams would buy electronic components and
assemble them into equipment. I once built an amateur radio TV
station that could transmit still images. I mostly used parts
salvaged from old junked TV sets. I never managed to contact
anyone with it, but it was fun anyway. My real station consisted
of a commercial Hallicrafters receiver and a Heathkit transmitter built
from a kit, plus some homemade odds and ends.
With the invention of integrated circuits and robots to assemble
circuit boards, it became rare to hear homemade equipment on the
air. Today most hams use commercial equipment. The main
exceptions are QRP operators. QRP means low power. These
hams still make equipment, either their own designs or kits, and
operate CW (morse code) at a few watts output.
The development of Digital Signal Processing (DSP) combined with modern
integrated circuits has revolutionized electronics. Our analog
signals are now digitized; that is, converted to numbers. The
numbers can then be manipulated mathematically, and can be stored as
numbers or even transmitted as numbers. Audio has gone
digital. We now listen to CD's instead of LP's, and audio
production is entirely a digital process that resembles word
processing; think cut and paste. TV is about to go digital, as
analog broadcasting is being replaced with digital broadcasting.
But radio has been a holdout. Satellite radio is digital
(actually, satellite-anything is digital). In Europe there is
some
digital radio. There is a digital standard for international
broadcasters called Digital Radio Mondial (DRM), and a few broadcasters
use it. Ham radio operators also do some digital
transmitting. But today, virtually all radio, whether domestic or
international is still analog.
Radio construction is mostly analog too. Transmitters and
receivers are make mostly from analog parts. A modern ham
transceiver (combination transmitter and receiver) is mostly analog,
but
may have a digital IF; that is, some digital filters near the end of
the signal path.
But today it is possible to construct radio transmitters and
receivers using mostly digital parts, and with a minimum of analog
parts. The antenna signal is converted to numbers as soon as
practical, and all subsequent processing is done using DSP. The
result is a Software Defined Radio. It is "software defined"
because if you want to change what the radio does, you just change the
software. The small analog bit probably stays the same.
Today hams can build a software defined radio fairly easily.
The digital part consists of crunching numbers, and a ham can use a
personal computer to do that. So with a personal computer plus
some software available for free on the
Internet, plus a few analog parts, hams can again build their own
equipment. I find this to be very exciting.
Let's take an example. We need an antenna to receive radio
signals. That part of radio never changes; a good antenna makes
any radio a lot better. We connect the antenna to a black box
that spits out numbers. We send the numbers to a personal
computer that uses DSP software to tune, filter and demodulate (convert
it to digital audio). The PC then sends the digital audio to its
sound card, and then to speakers. We then have a radio that does
most of the work of receiving a signal in a PC.
So what is in the black box? A minimal cost version uses the
PC sound card recording input as part of the black box. The
signal
flow is antenna to a mixer (convert to near-audio frequency) to PC
sound card input for conversion to digital numbers. A version of
this radio for the 40 meter amateur band is the "Softrock-40" and costs
about $30. And performance is quite good. You can make this
receiver in a day. The equivalent analog version would have
dozens (maybe hundreds) of parts, and may not work as well.
Instead of a mixer and sound card, the black box could be a preamp
and a modern analog to digital converter (ADC) that
converts the antenna signals directly to numbers. It is not clear
to me which is better. I have both systems, so maybe I will find
out.
I am not claiming that software defined radios are better than analog
radios. If you look at the most expensive commercial Ham
equipment, the radios that cost about $10,000, the "front end" (the
part nearest the antenna) is all analog, and very expensive analog
parts are used. Any digital techniques are used later in the
signal chain. It is possible that as DSP parts improve, that
commercial equipment will become more digital. We will have to
wait and see.
But I am claiming that a software defined radio is a lot easier to make
at home. Besides having fewer parts, there is no need to drill
panels for buttons and switches. The control panel of the radio
is on the computer screen, and you control the radio with your mouse
and keyboard. Usually you can "see" the signals in a spectrum
window, a feature only the more expensive commercial radios have.
Is the performance as good as a $10,000 commercial radio? I doubt
it. But performance is very good indeed. It equals the
$10,000 radio except in the presence of nearby strong signals that can
overload the digital converters. If you have a location without
strong local signals, you can expect excellent performance.
Software defined radio got me interested in being a ham again. I
write computer programs for a living, so I can handle the
software. And as a teenager my hobby was electronics. Of
course electronics has changed since then, and re-learning all the
electronics was part of the fun. There are dozens of applications
for DSP in ham radio. I have touched on building homemade
stations easily and inexpensively. There are also applications in
more advanced equipment and in digital voice and data
transmission. The commercial world is exploring all this too, and
it is fun to be part of a digital radio revolution.