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Analog renders ultrasizzling 3G phone

Posted: 15 Nov 2007 ?? ?Print Version ?Bookmark and Share

Keywords:3G phone? N95 teardown? analog devices? GSM? W-CDMA?

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.

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.

Wireless bag of tricks
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. 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.

Along with 3G cellular, distinguishing wireless attributes extend to the inclusion of Bluetooth for personal area networks, along with 802.11b/g WLANs, 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 8.5mm x 8mm BGA 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.


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.
(Click to view teardown.)

In fact, the GPS chip shares the same mixed-signal look as TI's single-chip phone!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.

The NXP part used for the FM radio (TEA5761) is an almost purely analog chip, with only a bit of I2C control logic intruding on the otherwise digital-free audio path. Selling for maybe a buck in volume, devices such as this from NXP and competitor Silicon Labs have shrunk to truly minuscule size. To that point, Broadcom, ST, TI, Marvell and CSR are quickly merging the FM radio into their own variations of Bluetooth and/or WLAN devices with seemingly modest impact on die size.

As seen in Apple's iPhone, the N95 is orientation-sensitive, with the screen layout changing depending on the position of the phone. In fact, a common accelerometer part from ST powers both designs, now using a similar multichip package to the Wi-Fi device described earlier. The LIS302 contains two analog chips to sense "which way is up" and drive display reorientation: a sealed MEMs chip for the three-axis accelerometer proper, and an analog processor used for interface to the digital world in which it communicates as a system peripheral.

Analog is king
As can be seen in the device callouts, the N95 makes significant use of localized supply regulation. Step-down DC/DC converters from National take care of business near the RF power section, the latter implemented with multichip packages combining GaAs amplifiers with CMOS power controllers and antenna switches for keeping external complexity in check. Smaller-scale regulators are also used elsewhere, and LED drivers from TI and National join up for lighting management.

Despite some dispersed regulation and conversion, dedicated analog ASICs for power management run the big show, with the N95 employing a mix of custom devices made by ST and a dedicated merchant market chip from TI (TWL92230) used for powering the N95's applications processor. All in all, this collection of analog components can be thanked for orchestrating miserly power consumption and delivering workable talk and standby times in the face of a rich product feature set and sub-4W-hr battery.

Much of the system audio is believed to reside within one of the ST custom analog power parts (4396299), which also employs a second, simple, in-package silicon chip for implementing area-consuming passives to keep the die size of the more-complex core device as small as possible. Separate audio amplifiers from Texas Instruments round out the analog audio chain; both are small in size but large in impact, considering the import of quality audio in the consumer experience.

Other specialty analog chips!such as Hall-effect sensors (for de- tecting product articulation in the dual-slider N95 design) and an Analog Devices piezo motor driver (for controlling motion in the N95's autofocus camera lens!are also found, joining a bucket of other quietly contributing mixed-signal parts too numerous to describe in this brief article.

All those details and a few omissions aside, the message is quite simple: Once you look past the low-power processors and dense memory chips of the N95, analog rules.

- David Carey
President, Portelligent

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