Buying a Radio with a Car Attached

There’s some inherent quality in music – whatever your tastes may be – that resonates with the human psyche. And those feelings have gotten much deeper since the radio and the car came together to create our own private traveling concert halls. New car buyers often spend more time fiddling with the radio than checking out the engine or the transmission. And sometimes, decisions about which vehicle to purchase can hinge on the choice of audio present.

As AM gave way to FM, and FM gave way to satellite, a funny thing happened. The older technology still thrives despite its limitations. AM frequencies, which are very sensitive to impulse noise such as lightning, spark plugs and electric motors, is still very much present. The same is true with FM, which coexists with AM on different frequency bands using different modulation.

As a matter of fact, the airways have become jammed with so much energy across the spectrum that the FCC is redistributing the spectral assignments to free up even more RF space for services and data. On top of that, local and traveling-point source emitters are putting out even more RF. Cellular phones, Bluetooth headsets, Wi-Fi connections, GPS, and a multitude of other licensed and unlicensed emitters can make it harder to extract weak signals. And it is in these instances where a Low Noise Amplifier (LNA) makes a big difference, especially when trying to link to a distant satellite.

Arthur C. Clarke first proposed the idea of orbital (manned) communication satellites in 1945, and the Soviet Sputnik was the first orbital emitter that basically sent out a beacon. Telesat was the first workhorse communication satellite in 1962, and it showed the world that live sounds and images for news, sports, and entertainment were possible.  But, this was for broadcast redistribution hubs only. The equipment was too big and costly to be personalized.

LNAs used to be large cryogenically cooled linear amplifier assemblies that would take a weak signal (typically from a parabolic concentrator) and extract a clean signal out of the noise. Combined with filters and circuits to discriminate against unwanted RF energy bands and signals, LNAs better equipped satellites to use lower power transmitters in order to increase battery or solar charge effectiveness.

Larger antennas and circuits made satellite radio and tracking systems like OnStar possible for the automotive market, and semiconductor LNAs are partly to thank. Without dense and integrated semiconductors, this would not be practical.

While satellite radio has proven itself as a competitive alternative to free local airwaves, it has suffered from a major drawback. Local conditions, traffic, weather, and other alerts are not typically available from an orbiting satellite, even if it is geo-synchronous.  It is possible to use multiple channels to achieve more granularity, but that is a waste of bandwidth. Instead, Satellite Digital Audio Radio Services (SDARS) are enabling Sirius and XM subscribers to extract digital data riding on the audio signal to find information pertinent to the receiver.

However, the extraction of data in parallel with the audio for digital radio services puts increased demand on an LNA. The ability to reject out-of-band signals and coexist with Wi-Fi, Bluetooth, GPS, CDMA and 3G/4G/5G cellular services means that the LNA must perform better. If a few milliseconds of audio are missing, the listener will probably never hear it. If errors creep into the digital data stream, though, it can corrupt the entire transmission.

Fortunately, Avago has introduced the ALM-2203 miniature highly integrated LNA filter RFIC module designed to enable SDAR signals to be extracted in the presence of other higher-level signals. The ALM-2203 uses Avago’s proprietary GaAs Enhancement Mode pHEMT process to achieve low noise and high gain. A key feature is the ability to simultaneously have SDAR and GPS operations taking place. This allows an SDARS-enabled radio to extract only information pertinent to the actual location.

Housed inside the ALM-2203 are three LNAs and an internal film-buck-acoustic resonator (FBAR) that provides 38.5 dB of gain with a low-noise figure of 0.83 dB. Designed for use in the next generation of automotive radios with satellite data connectivity, the ALM-2203 measures 5 x 5 x 0.95 mm allowing it to easily fit into modern-day form factors. And, it does not require liquid nitrogen.  

Figure 1: Avago’s proprietary LNA technology can reject non-SDAR satellite signals and reduce the need for larger antennas and circuits. (Source: Wikipedia)