In an age once we all carry smartphones in our hands that allow us to stream high-definition movies, the romanticism of listening to old-style analog radio still persists. For some, it’s a break from the cookies, contracts and terms of service that lie behind every online activity. But for people like me, a big a part of the appeal is the thrill of receiving a signal from 1000’s of miles away – and doing it the time-honored way: with an understanding of atmospheric conditions, antennas and electronics.
This pastime of using a variety of information and skills to attract very distant stations known as DXing. Today, digital signal processing makes it possible to place extremely powerful receiver electronics in a cost-effective and highly portable package, so there has never been a higher time to change into a DXer. Among these high-performance electronic devices, there’s probably no higher example than TEF6686 chipintroduced in 2013 by NXP semiconductors and since then it has been modified over and over.
The system was very successful in car radios, partly due to its low price and high audio fidelity, but mainly due to the astonishingly high sensitivity and selectivity of radio signals. The TEF6686 can receive each FM and AM and might be configured to accommodate the different bandwidths utilized by stations in several countries. It may decode the digital RDS signal of a broadcast station (Radio data system) channel which, if present, comprises repeatedly updated information, comparable to the title of the currently playing song.
The chip’s extreme selectivity and sensitivity result from the skillful use of software-defined radio technology and digital signal processing (DSP) to filter out adjoining frequencies. This allows the reception of very weak signals that might otherwise be drowned out by nearby programs. The chip has proven irresistible to radio enthusiasts, attracted by its features that go far beyond the requirements of a car radio. It can receive not only business broadcast bands, but in addition short and long wave bands. The chip may provide fast signal strength information.
The TEF6686 chip might be present in the handy module [top left] providing electromagnetic shielding and pass-through interface. ESP32 based development board [top middle] controls the module and processes the signal. The LCD screen displays the user interface, which is controlled using buttons and knobs [bottom left]. James Provost
As an lively radio amateur (PE5PVB) in the Netherlands, I used to be intrigued by the rave reviews I began seeing about the TEF6686. During the COVID lockdown of 2020, I started designing a completely open source tuner that might squeeze the highest possible performance out of the chip for FM DX users. My enthusiasm increased once I found TEF6686 tuner modules on AliExpress. They contain the TEF6686 chip in a DIY-friendly, through-soldering package (the TEF6686 itself is a surface mount chip) and with radio frequency shielding to help minimize interference. These modules are low-cost – you can often find them for around $25.
I soon settled on a setup consisting of two circuit boards connected by a ribbon cable. There is a motherboard that houses the TEF6686 module and the microcontroller, and a display board that houses a small OLED display and the switches and encoders that control the radio. I evaluated various versions of Arduino-compatible microcontrollers and found that almost all of them were too slow and had insufficient flash memory.
The microcontroller needs a lot of flash memory since it must store not only all the radio-enabled software that is distributed to the TEF6686 after each boot, but in addition must store various display fonts, various images, in addition to a database of North American call signs and the Canadian provinces and US states (this information, combined with other capabilities, allows the user to immediately see the call sign and determine where the receiving transmitter is positioned). I finally made up my mind variant the ESP32 a module that had the speed and memory capability I needed, but which could still be programmed using popular ones ArduinoIDE.
At the starting of 2021 I released on GitHub preliminary version of software and diagrams for other DIY enthusiasts. I also contacted a DIY online store in the Netherlands, Team AmateurRadioShop.nlwhich still sells set older version of the radio. In the fall of 2021, I created the second version with the so-called the company’s human-machine interface (HMI) displayNext. This display has a built-in processor, so I used to be able to outsource more UI tasks to the ESP32. This significantly accelerated the operation of the radio and also opened up latest graphic possibilities. In this version I’ve also added Wi-Fi which allows you to connect to online resources comparable to XDR GTK, a user interface that permits precise computer control of the radio tuner, and RDS Spy which decodes RDS data into usable information in real time. If you want to build this version yourself, you will need to prepare your personal circuit boards. But the Gerber files, bill of materials and construction suggestions can be found at my website at www.pe5pvb.nl. Soldering will take roughly 2 hours.
The display and controls are directly connected to the microcontroller via universal input/output pins, while the TEF6686 receiver chip is controlled via commands sent via the I2C serial interface. The left and right channels are amplified and output to standard RCA audio jacks.James Provost
For those that do not feel like constructing a set, there’s also an option for you. In early 2022, a Chinese hobbyist named Justin Peng contacted me and said he had built a portable version with my design. The first versions based on this design of his appeared this summer appeared on AliExpress. Within a few months, interest in my open source project exploded, and the radio was adopted by FM DX users around the world.
In 2023, a Czech hobbyist contacted me, Marek Farkaszwho invited me to the group he founded on Discord social platform with other hobbyists working on and improving the software for this radio. On this team we now have a graphic designer, several people specializing in RDS, other very experienced programmers, and a Chinese guy who has written a version suitable to be used by hardware designers looking to create a radio for the AliExpress market. Together, we completely reviewed the code and added a smoother graphical design, greater connectivity, language selection, and other improvements.
I’m very grateful to this team for all their labor that made this open source radio probably the most successful radio of its kind ever. I hope you will try it and even when you don’t waste time trying to find distant stations, you will rediscover the joy of high-quality FM radio broadcasts!
