Delivering High-performance Car Radio Designs At Low Cost With Digital Low-if Cmos Amfm Automotive

High-growth economies such as Brazil, Indonesia, India and China have an emerging middle class and rapidly growing automotive markets. The vehicle retail price in these markets must be relatively low, putting cost pressure on components in these vehicles. Additionally, automotive markets in developed economies such as the United States, European nations and Japan are also seeing an increased demand for cost-sensitive solutions after the global financial crisis that began in 2008.

Traditional technologies used in automotive AM/FM tuners are typically BiCMOS tuners with a 10.7 MHz IF tuner architecture. This technology/architecture delivers on radio reception performance, but the process and architecture are cost prohibitive for mixed-signal architectures where RF circuits are integrated with digital processing engines such as DSPs and microcontrollers in a monolithic circuit to deliver fully processed audio outputs.

Digital low-IF CMOS AM/FM receivers very much fit the bill due to the much lower cost points of CMOS foundries and the use of a digital low-IF tuner architecture. Digital low-IF AM/FM receivers have been in use in the handset and portable media player markets for a number of years.

RF Dynamic Range
In crowded FM spectrums that are very common in urban settings, the abundance of radio stations can overwhelm the radio receiver tuned to a desired station whose signal strength is relatively weak. The most severe and common non-linear distortion resulting from strong interferers is third order inter-modulation (IMD3) distortion resulting from two strong close-in blockers that show up on-channel on a weak desired station tuned to.

In many applications an external tracking filter is used on the front end of the receiver to attenuate the blockers. This technique, however, is costly in terms of components and PCB real estate.To minimize IMD3 distortion products, the dynamic range of the receiver front end must be adequate.

Selectivity and Dynamic Bandwidth Control
Selectivity refers to a tuner’s ability to receive a weak station in the presence of stronger stations at small frequency offsets. This is especially important for receivers in crowded FM spectrums in urban environments. An important performance requirement, especially for European markets where the channel spacing is 100 kHz for FM broadcast, is adjacent channel selectivity, which refers to suppression of signals 100kHz away from the tuned frequency and which is usually no better than 30dB in many tuners found in the low-cost category.

While 30 dB selectivity performance may be adequate for many field conditions, there are many situations in the world in which a listener is tuned to a station broadcast from a distant location while driving past towers broadcasting multiple stations in the local region. In this scenario, much higher selectivity is required.

Sensitivity refers to a tuners ability to pull in weak stations. If you live in a rural area away from the transmitters of most radio stations, the sensitivity of a tuner will be of great importance. With current car radios, the performance requirements are typically microvolt level sensitivity, allowing the car radio to pull in an extremely weak signal and extending its ability to tune into a station dozens of miles away.

Alternative frequency (AF) Check
In Europe, alternative frequency (AF) technology is an optimal choice that allows the car radio tuner to tune to a different frequency that provides the same station when the first signal becomes too weak when moving out of range. This technology is often used in European car radio systems, enabled by radio data system (RDS) technology where the AF list is transmitted via RDS data.

In summary, continuing cost pressures in the global automotive market are driving the development and deployment of highly integrated and innovative CMOS radio receiver IC solutions that will provide significant cost savings in transitioning from traditional architectures and analog-centric semiconductor processes typically employed for car radio tuners.