Software Defined Radio

The Next Great Thing Or A Passing Fad?


Software Designed Radio (SDR) did not pop into existence out of nowhere a few years ago. It started as a concept in the 1980’s and got taken up by the military and their deep pockets to help develop it for practical purposes. Moore’s law has made this technology possible for us common folk without the Pentagon’s budget.

SDR is a communication system whose hardware components have been replaced by software and high performance solid state devices. Modulation, demodulation, equalization compression, decompression, companding, passband filtration, noise reduction are all done in the digital domain.  Very much like the pluses and negatives of your personal computer – with the right software, it can do magic, with no software it can be a very expensive door stop!

Most of the magic under the hood is located in an SDR’s receiver section, and many purely SDR Receivers are available and more are being announced regularly.  The underlying radio receiver technology used in most receiver sections has not changed since the 1930’s! Look up Super-Regenerative and Super-Heterodyne. They are used in most radios to this day!  Where SDR Receivers innovate, is by taking what used to be a completely analog signal and converting it into a digital one – where the latest and greatest silicon IC’s can rapidly manipulate it in any way the human mind can imagine.

SDR - 1

SDR - 2

SDR - 3

SDR - 4

One of the first SDR’s built for the Amateur Radio Operater in mind was the SDR-1000, by Gerald Youngblood, now K5SDR as documented in the pages of QEX in the July/August thru March/April issues of 2002-3. Gerald also cobbled together software to do most of the magic, an early open source version of PowerSDR.  There was so much demand for this technology that Gerald decided to make a business of it and Flex Radio was born.

SDR - 5

The state of the art presently can’t do away with all analog circuits.  The antenna feedline supplies the receiver’s front end with RF that needs to be put through analog RF amplifiers or attenuators as well as bandpass filters. From there, different varieties of SDR technology are implemented to handle the analog signal differently.  With bleeding edge devices, Direct Digital Sampling (DDS) technology takes the entire spectrum of the RF band received and digitizes it by a high-end analog-to-digital converter (ADC) from there the resulting bits of data are manipulated by powerful algorithms running on heavy duty silicon devices.  Sometimes this work is offloaded to the Central Processing Unit (CPU) of our Personal Computers.  Recently this job is increasingly going to something called a Field Programmable Gate Array (FPGA) – it’s like a Lego “build it yourself CPU” where, via software, you configure its architecture to optimize its ability to carry out the computational tasks you need.  Done right, it provides significantly more computing power than any CPU built.  If you know anything about computers, a state of the art FPGA will soon be capable of 10 TeraFLOPS (keep in mind that the Cray-2 Supercomputer was the fastest computer until 1990 and it was only able to operate at 2 GigaFLOPS – that’s 1/10,000 slower). In addition, Digital Sound Processors (DSP) are also part of the hardware design all operating within the digital domain. Only at the very end is the digital audio signal converted back into analog with a complementary ADC and amplified for your listening pleasure.

Earlier generations of Anan (the 100D) and Flex Radios (1500, 3000, 5000) don’t have silicon quite powerful enough to handle the direct bandwidth of the RF antenna feed.  These earlier devices use something called a quadrature detector that implements a down sampling Intermediate Frequency (IF) prior to being fed to a less powerful Analog-to-Digital Converter (ADC).

Transmission requires far less manipulation.  Your transmission audio is amplified and converted into digital by an ADC, sent through a DSP to be equalized, compressed, expanded, band passed, and then digitally modulated.  The digital results are back converted into analog RF and put through a chain of RF amplification circuits until it leaves your radio through the feedline to your transmission antenna.

The above is the most bleeding edge and you pay for it with more than your blood.  Some SDRs use two FPGAs each, which were priced in the $3,000 single unit price when they were first manufactured a few years ago!

At the other end of the price range is an amazing little gadget.  Someone found out that the European digital TV converter dongles possessed a chip (the RTL820T  and RTL2832U as well as similar chips) capable of much much more (with the proper software).  Just “Google RTL Dongle” – they sell for a little over $20 and can receive anywhere from 25MHz to 1.7GHz capable of 3.2MHz continuous bandwidth!  Of course for that price you have to supply a computer to manipulate all those bits.  Its your computer’s job to massage the data and demodulate it with the help of your computer’s CPU and soundboard.  The software is freeware, but requires a degree of tinkering. Is it the same as the high end toys?  Sorry, there just ain’t no free lunch, but it will certainly give you a good idea what SDR is all about.

The biggest additional cost depends on whether the SDR does its own computations. All SDRs accept an RF antenna input and reduce it down to a manageable series of 1’s and 0’s.  Less expensive SDRs use something called a Quatrature Detector and supply a binary stream of down converted IF data in two channels – one in phase, the other 90 degrees out of phase (otherwise known as quadrature phase).  Using a Universal Serial Bus (USB) or the quickly disappearing “Firewire” port to transfer these pairs of bitstreams a “good” computer with the right software can select a portion of the bandwidth, demodulate it, and do a degree of digital signal processing on it so that the experimenter can listen to radio signals very inexpensively.  This is the least expensive approach, but may require a degree of jury rigging to get things working and some experimenters find that some of their computers can handle it but other computers can’t, with no obvious reason!

TAPR (Tucson Amateur Packet Radio) (, is an amateur radio organization that has shifted its focus from packet radio into SDR technology.  They have developed circuit boards which handle different stages of an SDR Transceiver and software that could put it through its paces, squeezing the most out of its hardware. They use Direct Down Conversion (DDC) architecture. You can buy it in kit form, or if you would prefer to distance yourself from the hassle of kit building and testing, a company in  India, Apache Labs ( have taken TAPR’s designs and packed it into a small case in their ANAN Series of transceivers.

SDR - 6


The TAPR design, available in the Anan 200D Transceiver, as well as the Flex Radio System’s 6000 Signature line (which uses Direct Digital Sampling, DDS architecture) uses FPGA’s to do all the heavy lifting of figuring out what their data streams of bits mean and turns it into the audio signals that we have all been used to hearing from our radios since we were mere SWLs. Flex Radio’s less expensive, older models, their 1500 model uses USB and their 3000 model uses Firewire to connect to a reasonably powerful computer to do the heavy data manipulation of their SDRs.

Just as an aside, the possible “philosophical” differences in using Flex 6000 Series vs. Anan/TAPR (in addition to the fact that APR’s software is older and thereby more “mature”) is similar to the difference between Apple’s OS and Linux. The TAPR/Anan software, like Linux lets you get under the hood and tweak just about everything.  If you know more than the experts with this software you might be able to squeeze a little bit more power under the hood, if you aren’t that much of an expert, I assume you could make a mess out of everything.  The Flex/SmartSDR approach is much more like Apple’s paternal attitude that they know what’s best for you.  The kernel for OSX (an outgrowth of  Berkely’s version of BSD Unix) and Linux are very close, yet Apple has welded the top down, and you can’t break  too much, even if you try!  So, if you are an intrepid software tinkerer that thinks he knows much more than the software designers, TAPR/Anan may be your cup of tea, otherwise you might be better suited with Flex/SmartSDR where most of the serious settings are hidden from the users.  You can’t screw up SmartSDR, but neither can you tweak it all, day after day, in search of the perfect settings.  Whatever floats your boat!

It has not been unusual to hear Anan owners tweaking all manner of obscure settings on the air in search of fixing some minor source of nuisance or tweaking some spec that may require a ‘scope to have any idea it exists.  Flex 6000 owners are either thrilled with their new toy or less than happy that it’s software upgrades are taking so long to fashion into a form suitable for the public. It’s a different mindset (in my humble opinion).

SDR - 7SDR - 8

Flex 1500                                                                     Flex 6300


There are many varieties of SDR Receivers available.  This is a very short list, for a bigger list check out

SDR - 9SDR - 10

RTL Dongles                                                                         ELAD SDR

SDR - 11SDR - 12

Perseus SDR                                                       FUNCube Dongle


Just as important as the hardware is the software (that’s why it’s called Software Defined Radio).  The software raises the hardware from an expensive paperweight into the next age of Amateur Radio!

As an example of what’s becoming technologically available is a little device coming from Great Britain, the Watson W-DRX1.  This little beauty is an SDR receiver that can handle anything from 100KHz to 2GHz without a gap!  It has two switchable antenna ports and can display (by freeware SDR software) a spectrum from 96KHz to 1 MHz, depending on how powerful a computer you have attached to its USB port!  His device goes for the equivalent of $155 in pounds (£99.95)!

SDR - 13

Watson W-SDRX1

Even those conservative guys over in Japan are starting to get on the bandwagon! Icom has recently announced a new HF Transceiver, the IC-7300 Which is reported to have a Direct Conversion Radio Receiver, with an FPGA and a DAC/ADC under the hood, that might be selling for something like $1,200 by early 2016. It’s built to look like a legacy radio with a very nice panadapter. Details are sketchy, but I doubt that this will be the last Direct Conversion Radio planned from Japanese manufacturers!

SDR - 14

The very recently announced Icom IC-7300 SDR HF Transceiver


But hardware is useless without decent software! One of the oldest open source programs is PowerSDR.  Originally cobbled together by Gerald Youngblood, K5SDR for his Flex Radios, he released the source code and others have tweaked it to run with many other SDRs, but there are others – here’s just a sampling:

SDR - 15


SDR - 16


SDR - 17

Perseus SDR Software

SDR - 18

ELAD SDR Software


SDR - 19


SDR - 20


SDR - 21



A quick look at all this software shows that many features are equivalent or even identical.  Visual Panadapter Screens showing wide bandwidth of the received signal with the ability to “click” directly to an active frequency, “clickable” designing of passband filters that DO NOT have “ringing artifact” that sharp analog filters often possess, “waterfall” screens showing the recent history of spectral signals received, a large display of tuned frequency with multiple methods to change reception frequency and large easily readable S-Meters.  Some have more “buttons” than pop-out menus or vice versa.  There are no knobs on these devices, but third parties have been trying to come up with “legacy” knobs and dials that will interface with the software – some people love them, others do not…

SDR - 22SDR - 23

FlexControl                                                               Tmate2


Some hobbyists are trying to jury rig small DJ interfaces to operate as the controls of SDR devices.

Is this technology for everyone?  No, probably not – if you MUST have rows and rows of knobs and dials, and mechanical buttons and lit up meters, then you probably have your eye on one of those high end old style rigs from the usual Japanese Manufacturers (but it seems like even they will be modernizing soon, although most of it will be well hidden “under the hood”).  I believe that SDR technology is the future, what the final “control surface” for such technology will be, has yet to be determined, but as far as I can see, the “future” is here and it’s called SDR.

SDR - 24

Sticking with legacy radios…

 – The Editor –