SDR Tuners and the Enigma of Digital Radio Broadcast

September 3, 2019 Majeed Ahmad

In 2017, Norway vowed to switch off analog broadcasts in favor of digital radio that offered more stations and greater coverage compared to AM/FM radio. The digital radio broadcast also dramatically improved the reception while facilitating CD-quality sound without noise.

However, digital radio encompassed different broadcast standards — HD Radio, Digital Audio Broadcasting (DAB) and Digital Radio Mondiale (DRM) — that also occupied regional footprints. And the hardware implementation of each standard could be expensive and added to the board real estate and power consumption.

Enter software-defined radio (SDR) solutions with their ability to support several digital radio standards with a single chip through tightly coupled RF front-end and baseband software. SDR, one of the most important technologies in the modern wireless arena, can tune to any frequency band and implement different modulation and demodulation schemes. In other words, it can facilitate multiple standards in the same device by using reconfigurable hardware and powerful software.

Figure 1: An RF front-end paired with a software demodulator to create an SDR solution. Image: Mirics Semiconductor

On the receive side, a software-defined radio generally feeds the radio signals directly into an analog-to-digital converter (ADC) to digitize the analog signal in the receiver as close to the antenna as practical. The signals are then filtered, demodulated, and separated into individual channels. Finally, the signal is connected to a processor to tune the signal and extract the modulated audio or data.

On the transmit side, a software-defined radio performs coding, modulation, etc. in the digital domain and hands over the signals to a digital-to-analog converter (DAC) to turn the signal back to analog format and pass it on to an RF power amplifier for transmission.

In short, an SDR-enabled RF-CMOS chip can be reprogrammed or reconfigured with different waveforms and protocols via dynamic loading of new waveforms and protocols. And different functions like modulation techniques and performance characteristics are defined in software as part of the waveform.

Silicon Labs’ SDR Journey

The SDR technology, commonly associated with wireless communications, is making steady headway in the automotive radio designs where the SDR-enabled tuner chips can upgrade frequency band, air interface and other aspects of radio functionality via software downloads. And it’s apparent from chipmakers’ design roadmaps.

For instance, in 2017, when Norway announced the digital radio switch, Renesas turned its R-Car system-on-chip (SoC) for in-vehicle infotainment into an SDR-enabled platform by incorporating Silicon Labs’ Dual Eagle Si47961/Si47962 analog AM/FM radio receivers and digital radio tuners on the hardware front. On the software side, the R-Car infotainment SoC integrated digital radio demodulation software developed by M's Style Technology.

The software demodulator running on radio tuner IC’s embedded DSP allowed the infotainment SoC to support analog and digital radio specifications. Fast forward to 2019, Silicon Labs has incorporated the hybrid SDR tuners in its Dual Eagle family of AM/FM receivers and digital radio tuners to support global digital radio standards on a common platform.

Figure 2: The DRM-capable Si4796x hybrid SDR chips combine AM/FM radio receiver with HD-Radio™/DAB/DAB+/DMB/DRM tuner. Image: Silicon Labs

When a single platform can demodulate and decode worldwide digital radio standards, it simplifies car radio design and lowers cost. Silicon Labs’ hybrid SDR technology offers DSP-based features such as maximal ratio combining (MRC), digital automatic gain control (AGC), digital radio fast detect, and dynamic zero-IF (ZIF) I/Q.

The company’s Si479x7 tuners supporting the DRM standard are an extension of the Dual Eagle AM/FM receivers and digital radio tuners earlier used in the R-Car infotainment design from Renesas. It’s the company’s first automotive radio tuner that supports the DRM standard, a high-quality digital replacement for the current analog radio standards operating in the AM and FM/VHF bands.

Here, it’s worth noting that a single digital radio platform can be achieved either with an SDR-based design or by using a tuner-plus-coprocessor approach. However, in multi-channel RF systems, the latter SoC-centric approach still requires a significant amount of analog signal processing. That, in turn, can lead to RF interference susceptibility and an increase in board size and analog design complexity.

More Digital Radio Switchovers

The SDR-based solutions, usually known for providing a single wireless communications platform for multiple connectivity technologies like Bluetooth, Wi-Fi and GPS, have become a key element in the design recipe of digital radio broadcast. The SDR-capable tuners help automotive OEMs and Tier-1s in creating a single platform to demodulate and decode worldwide digital radio standards.

Apart from facilitating a high-quality digital replacement for current analog radio standards operating in the AM and FM/VHF bands, the SDR solutions also ensure an easy adaptation to regional variations via software downloads a.k.a. over-the-air (OTA) upgrades while the car radio is still in service.

Following Norway, Denmark has been scheduled to turn off analog FM broadcast in 2018 and U.K. in 2019. That means more SDR tuner solutions are on their way to cater to the swelling demand for DAB services. And these software-based programmable chips are likely to replace eight to 10 discrete chips in the digital car radios.

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