David Carey, Portelligent
TechOnline

Nokia's hot little number in 3G phones these days is the N95. Sitting somewhere at the top of the lineup, the N95 gets anointed in Nokia's marketing as a "multimedia computer," but in one view, it's the still the analog IC that sets it apart.

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Often lost in the shadows of processor horsepower and copious memory, the analog devices in many of today's mobile devices are still at the heart of root functionality. This teardown will focus on some of the mixed-signal wizardry that lurks in the margins to bring together a high-tier handset, with a starting nod to the radio designer's craft.

The N95 features support for both legacy GSM and newer W-CDMA networks in its UMTS 3G capability. Both communication modes are handled in a common transceiver, designed by Nokia and fabricated by STMicroelectronics (ST). Bearing the unassuming Nokia part number 4380206, the device is one of the first high-volume dual-mode cellular transceivers to be rendered in a monolithic semiconductor slice. It's worth noting that the bland external part numbering is spiced up a bit by the on-die markings Anheus--Finnish for "greed." (I wonder what the message might be there.)

GSM's time-division multiplexing has long allowed transmit and receive chains to be co-located (they're never active at the same time), but the simultaneous nature of W-CDMA's frequency-division multiple access means low-power receive signals must coexist with outgoing--and higher-powered--transmit signaling. No small feat, that, and Nokia's design is a commendably efficient implementation that others in the industry are just recently starting to match.

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Along with 3G cellular, distinguishing wireless attributes extend to the inclusion of Bluetooth for personal area networks, along with 802.11b/g wireless LANs, and here again the analog IC has made huge strides. Almost without exception across the industry, Bluetooth has been rendered in fully monolithic form, including both the radio transceiver and the digital baseband essential to a complete solution. The die photo of the N95's CSR BlueCore4 chip (BC41B143A) clearly shows a die with areas roughly split evenly among RF radio, baseband logic and on-chip memory for a truly mixed-signal device.

The 802.11 Wi-Fi device from ST takes a different approach to the challenges of cost-effective mixed-signal. The company's STLC4550 package contains three separate chips stacked neatly together--one for the radio and baseband front end, one for the primary media access control (MAC) logic and one for the analog power

management of the whole Wi-Fi solution. From the outside, the 8.5 x 8-mm ball grid array package hides an alternative approach to mixed signal that implements digital, power control and RF analog in differing devices, each in process technologies optimized for their unique functions.

The N95's other wireless tricks include a GPS and FM radio, and the approach now is back to the monolithic strategy seen in the Bluetooth solution. In the case of GPS, TI's GPS5300 bears more than a passing similarity to the topology of the CSR part, with radio, logic and memory subsections seen on the die.

In fact, the GPS chip shares the same mixed-signal look as TI's single-chip phone, covered in an earlier column (www eetimes.com, search article ID: 201805290)--no coincidence, given that both revolve around TI's digital RF processor (DRP) technology. Make no mistake, however; DRP can't get off the ground without analog headwaters and the accompanying process technology to render it into a common process with digital parts downstream.