Our love of the automobile is matched only by our love of music. Put them together and you get the rise of satellite digital audio radio service (SDARS), which breaks down the traditional commercial-driven radio model and opens up mobile audio options well beyond the trunk CD changer.
The Sirius S50 receiver, Figure 1, takes the mobility idea to heart in the form of a two-piece design.
Figure 1: Outside the Sirius S50 Satellite Radio.
In essence, the design splits the overall SDARS receiver into the RF tuner and a mating handheld, which contains the digital processing portion of the receiver along with audio storage and playback circuits. The RF tuner portion is intended to reside in the car, while the handheld unit can be undocked to let you take prerecorded SDARS content with you.
We'll focus here on the RF receiver portion of the design, the most analog of the two halves and the key ingredient to pulling in usable signals, Figure 2.
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Figure 2: Inside the Sirius S50 Satellite Radio
While the antenna for the receiver is a modest affair, its capabilities are amazing. A roughly 50-mm discoidal patch antenna made of low-loss Teflon dielectric and a simple, round plate of metal are all that are required to pick up signals transmitted in the 2320- to 2332.5-MHz band.
While SDARS implies a purely satellite-based solution, however, the broader system works by use of terrestrial repeaters that rebroadcast signals in areas where obstructions-buildings in urban areas, for example-make the required line of sight with the satellite problematic. The antenna module (which also contains a ceramic filter and some simple gain block circuits) is listening in for both the satellite signal and repeater signals where and when available.
To provide this seamless coverage, whether it's by land or by space, the tuner must therefore deal with multipath signals. Rather than reject multipath (as would be the case for a traditional receiver), the Sirius design likely brings in both satellite and terrestrial signals with the ability to switch as needed. While the dual-receiver design attribute is a bit speculative, the chip design topology suggests just such an architecture.
Looking at the die photograph of the single-chip STA210 tuner from STMicroelectronics, the lower half clearly shows duplicated circuitry, likely the first-stage amplifier and downconversion twice over. Not all the critical elements of tuning can be space-efficiently handled in silicon, however. A pair of IF SAW filters for each of the two radio paths-four in total-are the only way to keep signals clean at IF before they head to the decoder chip for A/D conversion and signal processing.
SDARS is a sophisticated analog RF tuner that listens twice and cuts once to provide a relatively steady stream of tunes.
About the author
David Carey is president of Portelligent (www.teardown.com). The Austin, Texas, company produces teardown reports and related industry research on wireless, mobile and personal electronics.
