Electronic Design

2.5G Cell Phones Gain An EDGE With Chip Set

Most modern cell phones use the GSM or CDMA standard. CDMA phones using the cdma2000 1xRTT standard can accommodate data at rates up to 300 kbits/s. Even higher rates are possible with the 3xRTT version. GSM phones can handle data if they use the General Packet Radio Service (GPRS), which is available on most systems today. The packet speed of GPRS is limited to about 40 to 60 kbits/s, though.

Today, an upgrade option for most GPRS systems is EDGE (Enhanced Data Rates for GSM Evolution). With this system, data rates up to about 300 kbits/s are possible thanks to 8PSK modulation. Adding EDGE is basically an upgrade to existing basestations, but a new cell phone is typically needed to use it.

Few EDGE cell phones are available now, but with Skyworks' new chip set, EDGE phones should begin to proliferate. Skyworks offers a three-chip solution to create a quad-band (850/900/1800/1900-MHz) phone that supports GSM, GPRS, and EDGE. The SKY74045 is a single-chip direct conversion transceiver. The fractional-N synthesizer with a voltage-controlled oscillator is built in. The SKY74046 power-amplifier (PA) controller/modulator works with the transceiver. A complete control loop maintains maximum output power and efficiency. Both the 4045 and 4046 are made with biCMOS.

The SKY77316 chip uses heterojunction bipolar transistors (HBTs) made with gallium arsenide. It's divided into separate 850/900- and 1800/1900-MHz paths with full impedance matching networks and bias control blocks.

The chip set's patent-pending Polar Loop modulation technology enables the transmission of both constant and non-constant envelope signals. The GMSK (Gaussian minimum shift keying) modulation used in GSM and GPRS is a form of frequency-shift modulation, so it has a constant amplitude or envelope. As a result, it can be amplified with efficient saturated class C or even class D or E amplifiers, giving efficiencies over 80%.

However, the 3π/8-8PSK modulation implemented in EDGE causes amplitude as well as phase variations that must be amplified in a linear PA. This reduces efficiency to no more than 60% at best. The Skyworks system allows a saturated nonlinear GSM PA to be used thanks to the feedback control system, which also permits a voltage standing-wave ratio up to 6:1 without damage to the PA or having to rely on an external isolator.

The chip set, sampling now with full production slated for the fourth quarter of 2003, costs $8.50 in 10,000-unit quantities.

Skyworks Solutions Inc.
www.skyworksinc.com
(781) 376-3000

See associated figure.

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