Aug. 11, 2025
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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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We start with a little bit of background and where we came from.
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.
Now to put things in perspective, we start with a brief overview of 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.
Some of the most popular SDRs today are as follows.
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.
In terms of cost, USRP B210 lies on the expensive side with a price range of US$ +.
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.
In terms of cost, the LimeSDR lies in the upper middle part of the price range at US$ 700+.
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.
In terms of cost, the HackRF lies in the middle of the price range at US$ 300+.
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.
In terms of cost, the ADALM-Pluto lies towards the lower middle of the price range at US$ 250+.
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.
Some of its important features are as follows.
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.
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.
Further reading:Contact us to discuss your requirements of USRP For Sale. Our experienced sales team can help you identify the options that best suit your needs.
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.
Hi guys, I'm curious about the capabilities of the NI USRP . My goal is to transmit an OFDM signal of 56MHz in RF using 61.44 MS/s (which is maximum bandwidth and sampling rate that the datasheet says it can handle).
However, I check similar equipment (e.g. B-205 mini) in which its mentioned that they have problems with the clock that connects to the FPGA and it can compromise the performance of the equipment. Does this equipment have similar problems? I would like to know this before purchasing the equipment.
I really appreciate any help/information you can give me.
Cheers
Jorge Gomez
Hi Jorge,
What are some of the key clocking and synchronization requirements for your application? And how do you intend to program the clock in FPGA? There are different synchronization methods available on different USRP devices, as was discussed on this page on the forums. Beyond this, what are some of the performance needs you have for this project that informed this concern? By getting more specific context on what you're aiming to do, we can better help identify the right hardware.
Otherwise, the sales engineering team at National Instruments can have a conversation with you about the overall application, additional requirements, and other considerations you may have to help you in the selection process, which is recommended for new projects.
Thank you and have a great rest of your day!
Cheers,
Paige
Hi, thank you for your reply. My goal is to put the NI USRP in a drone and set it to transmit an OFDM signal of 56 MHz bandwidth in RF @ 3.5 GHz continuously and receive it using a NI PXI- connected to a switch with multiple antennas for doing channel sounding. A key factor for this project is to have both Tx and Rx synchronized and for this purpose, we use the REF in and PPS in signal for the NI- and the PXIe-, we have a GPSDO for the PXIe and we have purchased a GPSDO for the USRP . Even though the GPSDO we have is good if the PLL and/or the clock circuit in the USRP is not good the synchronization will be compromised in our system.
The frequency accuracy I'm looking for is 2 ppm at most (maybe 3 ppm) when locked to an external clock signal the accuracy would be less than 30 ppb, stable in temperature and a transmit power of +20 dBm.
Cheers
Jorge Gomez
Hi Jorge,
What's the GPSDO you are hoping to use with the USRP (ex. part number)? The reason I ask this is that there is a limitation related to the GPSDOs for this device, explained in this forum.
The USRP-293X series has an integrated GPS-disciplined reference clock, and the 292X series requires an externally sourced clock. More info on the GPSDO support can be found in the Getting Started Guides for the USRP-293x and USRP-292x series. What review have you done of these devices, and would you be looking for a more inherent ability for GPS-disciplining?
Cheers,
paigec
Hi, I'm looking in some options from Jackson Labs, specifically the Firefly 1A http://www.jackson-labs.com/index.php/products/firefly_1a.
I also checked other options for USRP but one limitation we have for our project is the weight because we're going to put the USRP in a Drone for transmitting our sounding signal. We also looked for 2 options from Ettus Software: the TCXO and the OCXO.
https://www.ettus.com/all-products/GPSDO-TCXO-MODULE/
https://www.ettus.com/all-products/GPSDO-MINI/
Cheers
Jorge Gomez
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