Hacking A Car-Radio Chip Into The Ultimate SDR Receiver

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At a time when we all carry smartphones that can stream high-definition movies into our hands, the romance of listening to old-school analog broadcast radio nevertheless endures. For some it’s a break from the cookies, contracts, and terms of service that lurk behind every online activity. For folks like me though, a big part of the charm is the thrill that comes from pulling in a signal from thousands of kilometers away—and doing it the time-honored way: with an understanding of atmospheric conditions, antennas, and electronics.

This pastime of using varied knowledge and skills to pull in very distant stations is called DXing. Today, digital signal processing makes it possible to put stupendously capable receiver electronics into an economical and very portable package, so there’s never been a better time to be a DXer. And among these high-performance electronics, there’s arguably no better example than the TEF6686 chip , introduced in 2013 by NXP Semiconductors and revised multiple times since then.

The chip has been very successful in car radios, in part because of its low cost and high audio fidelity, but especially because of its astoundingly high sensitivity and selectivity to radio signals. The TEF6686 can receive both FM and AM, and can be configured to accommodate the different bandwidths used by stations in different countries. It can also decode a broadcast station’s digital RDS ( Radio Data System ) feed, which when present contains continuously updated information such as the title of a song currently playing.

The chip’s extreme selectivity and sensitivity results from its adept use of software-defined-radio and digital-signal-processing (DSP) technologies to filter out adjacent frequencies. This enables reception of very weak signals that would otherwise be drowned out by nearby broadcasts. The chip has proven irresistible to radio enthusiasts, attracted by features of the chip that go far beyond what is needed by a car radio. It can receive not just the commercial broadcast bands, but also the shortwave and long-wave bands. The chip can also provide instantaneous signal-strength information.

The TEF6686 chip can be found 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, which is controlled using buttons and rotary controls [bottom left]. James Provost … Read More