When I put the RTL-SDR at the top of my Christmas wish list last year, a few eyebrows were raised. Why would a guy who has played with some of the best radios available want a little trinket that looked like a USB flash drive and cost about the same? Surely that was a mistake. It wasn’t.
According to Wikipedia, a dongle is “a small piece of hardware that attaches to a computer, TV, or other electronic device in order to enable additional functions…” The RTL-SDR is a dongle that plugs into the USB port and enables your computer to be a software defined radio. You just need to add software (which can be free) and an antenna and voila – a complete reasonably high performance radio for under $20.
The price-performance value of the RTL-SDR “system” deserves explanation. How can something so good cost so little? The answer lies at the heart of some new ways we create value with electronics and software. Here are the key points.
- Digital signal processing. If you can get electromagnetic signals into properly formatted digital samples, then you can use mathematics to replace hardware. Algorithms can manipulate data to do almost everything that used to require expensive hardware, e.g. transistors, coils, capacitors, and so on. This reduces cost and lets a general purpose laptop to behave like a sophisticated and specialized radio receiver. Most of you are familiar with using your computer for audio and video. This is also digital signal processing and software radio is the same idea.
- Low cost manufacturing of highly integrated modules, not just circuits, which can be repurposed through software changes. The RTL-SDR came to life as a consumer product for watching television and listening to FM radio on a computer. The RTL-SDR dongle was originally a device using digital audio and video broadcasting standards adopted in Europe and parts of Asia Pacific. Every time you plug a USB device into your computer, your operating system finds or loads a “driver”. A driver is a piece of software needed for the USB device and computer to work together. A number of hackers discovered they could modify the RTL drivers to adapt the dongle as the front end of a software defined radio receiver. By writing new driver software to take advantage of the already cheap consumer electronics dongles, the cost of a low-end SDR was reduced by an order of magnitude (10 times).
- Global open-source communities of enthusiasts who are prepared to contribute their time and ideas have provided the development and implementation ecosystem for RTL-SDR. There is a large open-source community supporting software defined radio. All they had to do was modify their code so that their programs could read the I/Q data from the RTL-SDR dongle.
So, the combination of DSP, open-source and ingenuity brings you software defined radio for under $20. This is systems integration at its best.
RTL-SDR Test Drive
After using the mid-range priced Perseus as my SDR for a number of years, I wondered how good a $20 radio could be by comparison? It turns out, pretty good. The main limitation of the RTL-SDR is 8-bit data, rather than the 12-16 bit data used by costlier SDR. This limits dynamic range and filtering performance, regardless of the quality or speed of the computer the RTL-SDR is connected to. Side-by-side with my Perseus versus RTL-SDR, can I tell the difference? Sure. With its 14 bit samples, the Perseus provides much better signal-to-noise, especially on weaker signals. But on a price-performance basis, the RTL-SDR is a great deal and a very usable receiver.
The other limitation is that out of the box, the RTL-SDR does not cover HF. Since I am an HF guy, my Christmas request also included the Ham It Up RF Converter. Combined with the dongle, this system provides complete DC-to-daylight receiving system: from 100 KHz to 2GHz. My Christmas gift-giver got these as a bundle from NooElec for under $100.
There are many sources of information and no-cost software for the RTL-SDR on the internet. For information, try RTL-SDR.COM and RTLSDR.ORG. For software, many people are using SDR# and HDSDR. SDR# is fairly simple and has lots of plug-ins for extra features such as recording. HDSDR is more advanced but takes longer to learn how to use well. Both have tons of features and reasonable performance to get you started. For more advanced users, there are various programs that you can use to “serve” your dongle to other SDR software, including remote operation. Currently, I use the RTL-SDR in my workshop for casual listening, for automated recording, and as piece of test equipment, e.g. as a spectrum analyzer for looking at the output of oscillators.
The only warning that I would provide about the RTL-SDR is to make sure you get the right connectors or adapters for interfacing the dongle with your antennas. You probably won’t have any MCX, F or SMA type adapters lying around in your junk drawer.