L'SDR amatoriale sulla stampa di settore

Sulla prestigiosa testata online Electronic Design è apparso un lungo articolo sull'SDR in ambiente radioamatoriale, definito come una vera e propria fucina di progetti e come ideale fonte di approfondimento per chi volesse saperne di più sul software defined radio. Un articolo molto completo che tuttavia è stato accolto con qualche perplessità proprio dai gruppi di radioamatori che animano i principali progetti in corso, come Perseus, SDR-x, Quicksilver e altri. In effetti l'autore del pezzo ha commesso parecchi errori nel riportare le caratteristiche e le modalità di funzionamento di alcuni dispositivi, come Perseus (la cui parte software viene declassata a semplice tool di visualizzazione di un lavoro di trattamento e demodulazione svolto interamente in FPGA, cosa che non è affatto vera).
In ogni caso è un buon riassunto dei principali concetti da conoscere.

SDR Transforms Amateur Radio

Software is changing the way hams operate, catapulting a classic hobby into the 21st century.

Louis E. Frenzel
ED Online ID #19439
August 14, 2008

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.
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