With an ESP32 and a car radio chip you can build the ultimate SDR receiver

At a time when we all have smartphones in our hands that allow us to stream high-definition movies, the romance of listening to old-school analog radio still remains. For some, it’s a break from the cookies, contracts and terms of service that lurk behind every online activity. For people like me, though, a big part of the appeal is the thrill that comes from catching a signal from thousands of miles away—and doing it the old-fashioned way: with an understanding of atmospheric conditions, antennas, and electronics.

This pastime of using diverse knowledge and skills to attract distant stations is called DXing. Today, digital signal processing makes it possible to package extremely powerful receiver electronics in a cost-effective and highly portable package. So there’s never been a better time to be a DXer. And among these high-performance electronics, there is probably no better example than this TEF6686 chipintroduced in 2013 by NXP Semiconductors and has been revised several times since then.

The chip has proven very successful in car radios, partly because of its low cost and high fidelity, but mostly because of its astonishingly high sensitivity and selectivity to radio signals. The TEF6686 can receive both FM and AM and can be configured to take into account the different bandwidths used by broadcasters in different countries. It can also be the digital RDS (Radio data system) feed, which, if available, contains constantly updated information such as the title of a currently playing song.

The chip’s extreme selectivity and sensitivity results from the clever use of software-defined radio and digital signal processing (DSP) technologies to filter out neighboring frequencies. This allows reception of very weak signals that would otherwise be drowned out by nearby transmitters. The chip has proven irresistible to radio enthusiasts, attracted by the chip’s features that go far beyond the requirements of a car radio. It can receive not only the commercial broadcast bands, but also the shortwave and longwave bands. The chip can also provide instant signal strength information.

The TEF6686 chip is housed in a handy module (top left) that provides electromagnetic shielding and a through-hole interface. An ESP32-based development board (top middle) controls the module and performs signal processing. An LCD screen displays the user interface, controlled by buttons and dials (bottom left). James Provost

As an active radio amateur (PE5PVB) in the Netherlands, I was intrigued by the glowing reviews I saw about the TEF6686. During the COVID lockdown in 2020, I began developing a completely open source tuner that would get the highest possible performance out of the chip for FM-DXers. My excitement grew when I discovered TEF6686 tuner modules on AliExpress. These contain a TEF6686 chip in a DIY-friendly package that is suitable for through-soldering (the TEF6686 itself is a surface-mount chip) and features high-frequency shielding to minimize interference. These modules are inexpensive – typically available for around $25.

I soon settled on a configuration consisting of two circuit boards connected via a ribbon cable. There is a main board that contains the TEF6686 module and microcontroller, and a display board with a small OLED display and the switches and encoders that control the radio. I tested different versions of Arduino compatible microcontrollers and found that most of them were too slow and didn’t have enough flash memory.

The microcontroller requires a lot of flash because not only does it need to store all the firmware to operate the radio, which is sent to the TEF6686 after each boot, but it also needs to store the various fonts for display, various images, etc. in a database of North American callsigns and Canadian provinces and US states (this information, in conjunction with other features, allows the user to immediately see the call sign and state where the station they are receiving is located). Finally I decided to do it a variant of The ESP32 Module that had the speed and memory capacity I needed, but could still be programmed with the popular Arduino IDE.

At the beginning of 2021 I have published on GitHub a first version of the firmware and circuit diagrams for other DIY enthusiasts. I was also in contact with a DIY webshop in the Netherlands, Team AmateurRadioShop.nlthat still sells a kit an earlier version of the radio. In fall 2021 I created a second version with a so-called human-machine interface (HMI) displayNextion. This display has an integrated processor, so I was able to take over more user interface tasks from the ESP32. This sped up the radio significantly and also opened up some new graphical possibilities. In this version I also added Wi-Fi, which allows connection to online resources such as XDR GTK, a user interface that allows fine computer control of the radio tuner, and RDS Spy, which decodes the RDS data into usable information in real time. If you want to build this version yourself, you will have to have your circuit boards assembled yourself. But Gerber files, a parts list and construction tips are available on my website at www.pe5pvb.nl. Soldering together takes about 2 hours.

The display and controls are connected directly to the microcontroller via general-purpose input/output pins, while the TEF6686 receiver chip is controlled via commands sent via an I2C serial interface. The left and right channels are amplified and routed to standard RCA audio jacks.James Provost

For those who don’t feel like creating a kit, there is also an option. In early 2022, a Chinese hobbyist, Justin Peng, contacted me to let me know that he had built a portable version based on my design. This summer the first versions based on his design were created Appeared on AliExpress. In the months that followed, interest in my open source project exploded and the radio was adopted by FM-DXers worldwide.

In 2023 I was contacted by a Czech hobbyist, Marek Farkaswho invited me to a group he was starting Social platform Discord with other hobbyists dedicated to working on and improving the software for this radio. In this team we now have a graphic designer, some people specialized in RDS, others who are very experienced programmers and a Chinese who has written a version suitable for hardware designers who are designing a radio for the AliExpress – want to develop the market. Together we fully reviewed the code and added smoother graphical design, more connectivity, selectable languages ​​and other improvements.

I’m very grateful to this team for all their hard work in making this open source radio perhaps the most powerful radio of its kind anywhere. I hope you give it a try, and even if you don’t spend your time searching for distant stations, you rediscover the joy of high-quality FM radio broadcasts!

This article appears in the December 2024 print edition as “The Ultimate SDR Receiver.”