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SDRs Everywhere

Feb. 22, 2021
It seems pretty much any wireless device these days is a software-defined radio. Lou Frenzel discusses the reasons behind this overwhelming trend.

This article appeared in Electronic Design and has been published here with permission.

Are you following this? My sense of the direction of wireless products is that practically everything is becoming a software-defined radio (SDR). The highest-volume wireless products, cell phones, are all SDR. And looking at most other wireless products, they’re all SDRs. Virtually all new RF ICs are applicable to SDR. Perhaps you’ve already recognized and experienced this trend that further increases the software component in electronic devices.

An SDR is a radio in which some of the functions are performed by software rather the traditional hardware-only designs. In a receiver, for example, the signal is digitized as soon as possible, and the digitized version is subjected to digital processing.

Some of the main functions, previously delegated to hardware like filtering, mixing, and demodulation, are now handled by digital signal processing. That means an MCU with DSP-friendly instructions or a DSP chip. Many SDRs now use FPGAs to handle the processing, especially in faster high-frequency designs.

What’s made all of this possible is the technology that’s given us the analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) capable of sampling at megabit and gigabit rates. It’s also made processors fast enough to keep up with the barrage of data produced by those fast devices.

I was trying to think of a non-SDR product today. I’m drawing a blank. TV sets are now SDRs. Most communications gear like amateur radio equipment is now in SDR format.

I admit to having a general-purpose SDR for several years now. It’s the SDR-IQ manufactured by RFSpace. That company makes several general-purpose SDR models. The receiver covers 500 kHz to 30 MHz.

The SDR-IQ is of the direct-sampling type that takes the raw input signal from the antenna, provides some initial filtering, then sends the signal directly to a fast 14-bit ADC and subsequently to a DSP. The software runs on a PC and provides the control panel as well as spectrum-analyzer and waterfall displays. It works great, but it takes some getting used to. I still prefer a big knob to tune with rather than a mouse.

SDR Advances

This trend has produced some interesting new developments in low-cost SDR devices. There’s major interest in experimenting with what’s called the RTL-SDR. It’s a low-cost (<$25) SDR dongle based on an IC originally developed for DVB-TV, a digital TV standard.

Called the RTL283U, this IC and its offshoots have most of what you need to make an SDR radio that can receive almost any signal in roughly the 50 MHz to 1.7 GHz range. It uses an 8-bit ADC that can sample in the gigabit-per-second range depending on the version and model. Most of the dongles come in a USB drive format with a 75-Ω RF input. Not 50 Ω, as this was designed for TV sets, most of which have a 75-Ω input.

Of course, the big issue is the software. One seemingly popular product is SDR Sharp, which seems to enable almost any receiver format or standard. There are others, but none seem to be full commercial products. There is also GNU Radio, which is an open-source software-development environment available for free download. Reading the blogs, I see that having a good antenna at VHF and UHF frequencies is also an issue.

Most of the activity in RTL-SDR seems to be at the advanced hobbyist level. Plenty of experimentation. If you’re interested in tinkering with SDR, this movement may interest you. I just got one of the dongles and some software, but I haven’t had time to play with it. I will let you know how it goes.

So, does anyone still make a non-SDR radio these days?

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