In this article, I describe 5 of the most popular SDRs available for RF experimentation today. As a 6th member of this list, I include a surprisingly common and free SDR that can be used for your fun radio projects.

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Table of Contents

We start with a little bit of background and where we came from.

Background

Software Defined Radio (SDR) has revolutionized wireless communication in the same way Microsoft revolutionized the scope of personal computing in its early days. Today the SDRs are used by electronics and DSP engineers, amateur radio hobbyists, physicists, cybersecurity professionals, financial service providers and countless others who come from a diverse set of civilian, military, industrial and academic backgrounds. Some of its major applications are in the following areas.

• Point-to-point wireless links
• 2G, 3G, 4G, 5G handsets and base stations
• Aircraft tracking
• Satellite imagery
• Drone and UAV control and networking
• Smart grid
• Radar
• Gateway for industrial Internet of Things (IoT)
• Experimentation
• Amateur radio
• Emergency responders and public safety communication
• Radio astronomy

Now to put things in perspective, we start with a brief overview of where we came from.

Where We Came From

In the old days, a conventional workflow in designing a radio went as shown in the figure below. The arriving signal was captured from the air and electronic circuits were used for modification and extraction of the desired signal. These were the days of analog modulation like Amplitude Modulation (AM) and Frequency Modulation (FM).

In the hindsight, all these circuits were carrying out mathematical operations. And what can carry out mathematical operations better than a computer? In particular, after the publication of Shannon's landmark paper A mathematical theory of communication, the era of digital communication truly set in. This led to a change in radio design where analog signal processing was still dominant but demodulation was performed on a digital machine, as illustrated in the figure below. This demodulation was little more than making symbol decisions based on predefined decision regions.

The idea of Software Defined Radio (SDR) first appeared in s but it became more popular after a publication by J. Mitola. It was not conceived out of thin air as the era of digital electronics was changing the landscape of audio, photography and almost every other technology at the same time. Thanks to Moore's Law that predicted a doubling of transistor density every 18 to 24 months, the analog signal processing part of the digital receiver started shrinking, while the digital signal processing part started expanding. Many of the functions that were thought impossible to implement in digital domain could easily be carried out in digital machines, not only at a better cost but also with surprisingly ingenious techniques.

While the exact hardware vs software partitioning depends on the radio architecture, an illustrative block diagram is drawn in the figure below.

Since a digital signal processor, a microcontroller, or an FPGA, is programmed through software, we can say that the software portion in a receiver determines whether the radio is an SDR or not. An ideal software radio is the one in which there is exactly zero percentage of analog signal processing. The signal is directly sampled at the antenna and all subsequent functions are carried out in a digital machine. A direct sampling receiver architecture comes as close to realizing this goal as possible.

Top SDRs

Some of the most popular SDRs today are as follows.

5. Universal Software Radio Peripheral (USRP)

USRP was developed by Ettus Research and has been quite popular in the wireless community, particularly among the academic researchers. For them, connecting your device to a host computer using a high-speed link, enabling control of the hardware and data transmission/reception through host-based software was a dream come true. This enabled them to implement and verify the results of their innovative ideas, which could only be done in simulations before and real wireless transmission was considered an effort of industrial proportion. For independent standalone operations, a class of USRP models also features an embedded processor thus eliminating the need for a general-purpose processor.

Some of the salient features of USRP B210 as a reference are as follows.

• Frequency range: 70 MHz ' 6 GHz (based on Analog Devices AD)
• ADC and DAC: 12 bits wide
• Channel bandwidth: Up to 61.44 MHz
• MIMO support: 2×2
• FPGA: Xilinx Spartan-6

In terms of cost, USRP B210 lies on the expensive side with a price range of US$ +.

4. LimeSDR

Like other SDRs, LimeSDR is used by RF experts to build any of the applications mentioned before. In addition to these primary customers, it addresses a much wider audience through Snappy Ubuntu Core that is similar to an app store concept. The idea is to connect producers and consumers of the software where the users can easily download new apps developed by individuals around the globe and run them on the LimeSDR hardware.

Some of its main features are as follows.

• Frequency range: 100 kHz ' 3.8 GHz
• ADC and DAC: 12 bits wide
• Channel bandwidth: Up to 61.44 MHz
• MIMO support: 2×2
• FPGA: Altera Cyclone IV

In terms of cost, the LimeSDR lies in the upper middle part of the price range at US$ 700+.

3. HackRF One

The HackRF One was developed and produced by Michael Ossmann from Great Scott Gadgets. Its main appeal lies in its open-source nature, attracting the interest of not only amateur radio enthusiasts but also from hackers and RF security practitioners. It has been regularly used for demonstrations of RF hacking in cybersecurity conferences.

Some of its important features are as follows.

• Frequency range: 1 MHz ' 6 GHz
• ADC and DAC: 8 bits wide
• Channel bandwidth: Up to 20 MHz
• MIMO support: None
• FPGA: None

In terms of cost, the HackRF lies in the middle of the price range at US$ 300+.

2. ADALM-Pluto

ADALM-Pluto is one of my favorite SDRs due to its ease of use and level of integration. The acronym ADALM stands for Analog Devices Active Learning Module, as the Analog Devices projects this device as a portable and self-contained RF lab that is specifically designed to cater to students, faculty and self-learners from all levels and backgrounds. Due to a variety of resources available online, ADALM-Pluto is my first recommendation if you are looking for the fastest route to building a project of your interest.

Some of its salient features are as follows.

• Frequency range: 325 MHz ' 3.8 GHz (based on Analog Devices AD). Nevertheless, this frequency range can be extended to Tx/Rx up to 6 GHz with a little tweaking.
• ADC and DAC: 12 bits wide
• Channel bandwidth: Up to 20 MHz
• MIMO support: There is no MIMO in older versions. However, new Rev. D features include second Tx and Rx channels routed out to U.FL connectors in the PCB that enable a MIMO operation with some modifications. According to Analog Wiki, this second Tx/Rx is not tested during production and hence ADALM-Pluto is not an officially MIMO device.
• FPGA: Xilinx Zynq Z-

In terms of cost, the ADALM-Pluto lies towards the lower middle of the price range at US$ 250+.

1. RTL-SDR

RTL-SDR is the most popular option in this list. The inception of RTL-SDR can be traced back to the wide usage of mass-produced DVB-T TV tuner dongles, which relied on the RTLU chipset. It was discovered that the chip captures the raw I/Q samples and allows to transfer them to the host computer. This discovery along with its affordability led to RTL-SDR transform from a simple DVB-T tuner to the most widely used SDR in the world.

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Some of its important features are as follows.

• Frequency range: 500 kHz ' 1.766 GHz
• ADC and DAC: 8 bits wide
• Channel bandwidth: Up to 2.4 MHz
• MIMO support: None
• FPGA: None

In terms of cost, the RTL-SDR is the cheapest option at US$ 30+. Below is a summary of what we have discussed so far.

We now turn towards our final SDR.

0. Your PC's Soundcard

The 5 SDRs described above are commonly known but what if you do not want to use any of them and still desire to send your signal over the air to demodulate it yourself? There is a lesser-known SDR that is free and easier to use for RF experimentation. And that is your PC's soundcard that comes with an ADC and a DAC. In the audio band, the speakers act as wireless transmitters and the microphone acts as a wireless receiver, see the figure below.

This is how learners in my SDR course implement a final project on a single PC in which a file is sent and received over the air using its soundcard.

If you already own one of the above mentioned SDRs, it is still possible to complete the course project for over-the-air transmission. You will simply have to replace the Audio block in the GNU Radio Companion file with the corresponding block that interfaces with your SDR, and that's it. Good luck for the fun experimentation.

Companies are increasingly replacing their older serial-only or analog radios with software defined radios (SDRs). The ability to reconfigure transmission protocols, irrespective of hardware, offers tremendous flexibility to businesses to redeploy devices and extend their investment. It also opens up a range of new applications for their SDR transmitters and receivers, helping them scale more easily.

As processing power becomes more affordable, this transition is likely to gather even more pace until SDRs become the new standard for wireless industrial data transmission. Here's a quick primer on software defined radios and their applications for businesses.

How Do Traditional Radios Work?

In a traditional radio, the hardware components are responsible for most of the signal processing. This means that you have limited ability to reconfigure or reprogram the device to transmit new waveforms. If there is an error in the hardware, there is no way to correct the problem. What's more, it also prevents you from redeploying the radio for a different use. If your company scales or you'd like to transition to a different niche, your radio devices can't necessarily scale with you. Expand your reach and the longevity of your technology with XetaWave legacy migration solutions and versatile SDR products.

What Is a Software Defined Radio?

A software defined radio is one where all of the communication is done through software. The radio frequency (RF) signal is converted to a digital bit stream and all the necessary modulation and demodulation is done via digital signal processors (DSPs). Essentially, if you'd like to transmit a new form of radio protocol, you can do that just by reformatting the device. This opens up an SDR to a host of new applications.

An SDR comprises two main functional blocks ' a radio frontend (RFE) and a digital backend or the DSP. The frontend acts as the receiver and transmitter for the SDR, tuning into radio signals across a wide frequency band, ranging from 150 MHz to 2.4 GHz. 

The digital backend functions as the 'brain' of the radio, performing modulation, demodulation and reconfiguration functions. Together, this architecture allows SDRs to support multiple devices and transmit simultaneously across channels, giving industrial users the flexibility they need.

Applications of Software Defined Radios

SDRs are highly versatile devices; practically every industry has a use case for them. Here are some of the most popular uses for SDRs.

1. Testing and Measurement

Test and measurement (T&M) systems are crucial for a range of industrial functions, from system calibration to performance evaluation, and ensuring compliance with regulatory standards. Various industries such as aerospace, medical devices, additive manufacturing and more require T&M sensors in place. 

However, conventional T&M methods depend on outdated technologies that aren't able to handle the volume of data at the pace required. This makes it an ideal application for software defined radios with efficient transmitters and receivers that can meet the demands of modern data management. The flexibility offered by SDRs means they can easily be reconfigured to a different T&M function easily and cheaply.

2. SCADA Systems

Supervisory Control and Data (SCADA) systems are used for remotely monitoring, controlling, and analyzing industrial devices and processes. A SCADA system typically combines software and hardware components that facilitate remote and on-site data collection and computing. 

SCADA systems are ubiquitous across a range of industries, such as water and wastewater plants, oil and gas wells, agriculture, utilities, and more. SCADA is a great application for SDRs, given the latter are purpose-built to enable rapid data transfer and analysis. Versatile systems, such as those provided by XetaWave, tend to have built-in diagnostic tools and provide robust integration with third-party apps for powerful intelligence gathering. 

3. On-Site Edge Computing

With the right SDR technology, you can move beyond reporting and controlling to powerful on-site computing. One of the primary benefits of SDRs is that they facilitate data networks in spots where commercial cell service is hard to come by, such as a remote oil rig or mining operation. 

In these spots, edge computing systems are an excellent SDR application that help you collect and process equipment data on-site rather than have it relayed to off-site computing centers and back. This offers site managers the flexibility to pull insights, preempt events such as machinery breakdown, and proactively respond to maintain uptime.

4. Satellite Navigation

Satellite navigation is one of the most useful modern-day applications of SDRs. Global Navigation Satellite Systems (GNSSs), such as GPS, GALILEO, and BeiDou operate at different frequencies. SDRs are built to be flexible and can tune into any of these frequency bands without having to modify the hardware. What's more, an SDR transmitter and receiver can operate on multiple channels and tune into all these bands on a single device. 

5. Remote Command & Control

SDR technology has proven transformative for remote command and control situations, such as for civilian and military drones. Often, drones need to be operated in remote spots with patchy cell networks for aerial photography or surveillance. SDRs can help bridge this gap by providing a robust data network, allowing operators to seamlessly complete their missions. 

6. Medical Devices

SDRs are very useful for medical applications, particularly devices that work off wireless radio technology. Devices such as CT scanners, MRI systems, minimally invasive energy devices, IoT-embedded devices and transmitters, and more can all function using SDRs. The technology is also a tremendous help when designing and prototyping medical instruments. The fact that SDRs can be reconfigured to different frequency bands means innovators can repurpose SDR hardware at minimal cost for subsequent iterations. 

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XetaWave is an industry leader in the SDR space and the largest independent U.S. manufacturer of such devices. Our catalog features a near-comprehensive selection of high-performance SDR solutions across a global frequency of 10 MHz to 2.4 GHz that can be fully customized to any use case. Benefit from the expertise of a team of highly technical professionals with over 35 years of experience in the wireless communications industry. See outstanding results and robust support with XetaWave. Schedule a demo or speak to an expert today.

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