Software Defined Radio

I got my first Amateur Radio license as a teenager.  Amateur radio operators are licensed (in the U.S.) by the FCC to operate radio transmitters.  We "hams" use our stations to chat with other hams around the country and around the world.

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.