Like almost everything else in electronics, radios are becoming processors with software that communicate via a small amount of RF I/O circuitry. Surely, then, the rise of software-defined radio (SDR) should come as no surprise.
Digital signal processing (DSP) lies at the heart of SDR. Add to that the arrival of faster analog-to-digital and digital-to-analog converters (ADCs and DACs) and processors, and SDR becomes more viable for a wider range of applications. Simply put, software continues to push hardware aside, assuming more and more processing functions.
SDR first showed up in military equipment, but it’s now used in most cell phones. It also is ideal for future public-safety communications by providing a way to deal with the myriad air interfaces and frequency spectra used by first responders in disaster situations. SDR techniques have even joined the mainstream, with services like ham radio adopting them as prices permit.
SDR DEFINED
According to the SDR Forum and the IEEE, “A software-defined radio is any radio, transmitter or receiver, in which some or all of the physical layer functions are software defined.” That means the core hardware is a processor running software that can emulate hardware functions. As a result, the signals must be digital.
The receiver must first digitize the radio signals in an ADC. In most cases, a downconverter is needed to translate the very high radio frequencies, often in the microwave region, down to an intermediate frequency (IF) that’s within the range of a decent ADC.
Today, many SDR receivers convert directly to baseband. Once the ADC converts the signals into digital form, the processor and software can take over. DSP software routinely implements receiver functions like filtering, noise suppression, and demodulation.
The digital signal processor develops the signals to be transmitted, along with any modulation. A fast DAC then converts these signals into analog form. Next, an upconverter stage translates the signal to its final higher operating frequency before it’s applied to a power amplifier and the antenna. The processor uses DSP to perform the modulation, filtering, and other functions previously implemented with analog circuits.
The most common reason for using SDR is flexibility, or the ability to change or adapt to varying radio situations. With SDR, you can accommodate virtually any modulation scheme in the same radio without adding any hardware. All you have to do is download a new software module, and you have a new radio.
Multiple modulation subroutines can exist within the code and allow an operator to change on the fly. A flexible air interface and a wide frequency range make the radio applicable for many different jobs. That’s why the military is so enamored with SDR. One radio can communicate with many different sources and terminals, lowering costs and reducing the number of radios needed.
Furthermore, SDR improves radio performance. For instance, DSP filters can make selectivity many times better than what’s achievable with inductor-capacitor (LC) or crystal filters. Brick-wall filters become a reality. Intermodulation problems can be significantly reduced. Features like automatic gain control (AGC) and noise suppression can also be improved many times over the performance produced by analog circuits.
In amateur radio, hams want the best performance with the most flexibility, and SDR provides it. Hams use multiple communications modes in multiple bands. Continuous wave or CW (Morse code), AM, FM, single side-band (SSB), double side-band (DSB), radioteletype (RTTY), and packet data are just a few of the schemes used in bands from 1800 kHz to 10 GHz.
Since the ham bands aren’t channelized, any station can operate on any frequency, making interference and closely spaced stations a challenge to overcome. Superior DSP filtering is a real blessing in most ham communications. While SDR usually costs more, at least today, it’s chosen for one or more of these benefits.
It’s important to distinguish between SDR and a softwarecontrolled radio, though. A software-controlled radio may not involve common SDR methods, except perhaps for limited use of DSP IF filtering. Instead, it’s typically a computercontrolled receiver.
With a number of models on the market, software-controlled radios use a PC to control all or most receiver functions via a graphical user interface (GUI) that emulates the receiver front panel with its tuning dial, S-meter, knobs, and switches. By pointing and clicking with a mouse, users can change frequency, select band and mode, increase volume, and perform other operations usually actuated with a frontpanel button or knob.
With PCs so common today, it’s an easy transition from a conventional knob, switch, and LCD readout front panel to the virtual front panel with its point-and-click approach. Ten-Tec’s RX-320D and AOR’s SR2200 are just two of the software-controlled radios now available.
SDR AMATEUR RADIO EQUIPMENTM
Hams have dabbled with SDR for more than a decade. These inveterate experimenters have been home-brewing hardware of all kinds since the early 20th century, and that includes some SDR in the late 1990s. SDR is very complex and expensive, though, so it’s been limited to those hams equipped with the knowledge and money.
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