[Product Innovation]
Transceiver Chip Set Wrings Out GSM Phone Costs
A highly integrated CMOS three-chip set replaces many components and lets designers focus on baseband and data-transmission features.
Designing a low-cost multiband cell phone just got easier. Silicon Laboratories' Aero chip set provides a complete two- or three-band GSM transceiver using only a few external components. This three-chip set includes the Si4200 transceiver IC, the Si4201 baseband interface, and the Si4133T synthesizer IC. Silicon Labs' claim to fame is a 100% CMOS highly integrated chip set that significantly reduces the number of discrete components needed while lowering costs and greatly minimizing the number of manufacturing steps. Furthermore, the resulting circuits take up less board space and use less power.
As cell phones and the wireless infrastructure evolve from the current 2G digital systems toward the elusive 3G, both phones and basestations must support not only legacy analog systems, but also two or more digital standards and bands as well as newer 2.5G features. A typical phone might have to cover two or more of the following: the U.S. 800-MHz band, the U.S. 1900-MHz PCS band, the European 900-MHz band, and the 1800-MHz DCS band.
Newer phones and basestations are increasingly including more advanced data-transmission facilities. The most popular for TDMA systems, like GSM, is general packet radio service (GPRS). This system provides packet data transmission by stealing time slots from the eight voice channels. A data rate as high as 115 kbits/s is possible, but most units will only cover up to about 56 kbits/s.
With such data-transmission capability inside the cell phone, the emphasis in design shifts from RF to the baseband section, where the various data applications will be implemented. Luckily, designers can rely on chip sets like the Aero to take care of the RF part of the design while reducing the space on the pc board as well as the number of discrete components. Additionally, the Aero meets GPRS class 12 requirements.
The Aero chip set breaks new ground, as it greatly reduces the total number of components needed to implement the RF section (Fig. 1). Compared to today's typical handset, the reduction is dramatic, with 80% fewer components and 50% less board space required. The chip set eliminates the SAW IF filters, RF and transmit voltage-controlled-oscillator (VCO) modules, and dozens of discrete components.
According to Tyson Tuttle, Silicon Labs' RF products manager, "In one design, the RF section was reduced from 130 components in a 900-mm2 area to only 21 components in a 400 mm2 area. Over 100 components, mostly discrete, were eliminated. Just think of the savings in parts cost, procurement, inventory, board insertions, and manufacturing yield. On production runs of 10 million phones a year or more, over 1 billion parts are eliminated."
The design of the transceiver is pure CMOS versus the competition's bipolar or biCMOS approach. Using 0.18-µm processes, the die size is smaller and, therefore, less expensive. Also, the devices can be made in most digital fabs with higher yields.
The architecture includes a low-IF superheterodyne receiver with digital IF processing that interfaces to standard baseband chips (Fig. 2). The transmitter includes the upconverters, an on-chip transmit VCO with offset PLL, and drivers for the external power amplifiers (PAs).
The antenna is typically connected to a gallium-arsenide (GaAs) PIN transmit-receive switch. SAW filters provide the initial selectivity for the three GSM 900-MHz, DSC 1800-MHz, and PCS 1900-MHz bands. Discrete LC matching networks are used to connect to the differential low-noise amplifiers. Image-reject types of mixers downconvert the signal to an IF of 100 kHz.
Low IF Provides Advantages This very low IF has several advantages. First, it greatly reduces the image rejection requirements, eliminating the need for image filtering after the LNA. Second, it permits on-chip IF filtering, thereby eliminating the requirement for an expensive and space-consuming external SAW IF filter.
The low-IF approach was chosen over the direct-conversion or zero-IF design because the latter is much more difficult to realize in a single chip. Direct-conversion receivers are extremely sensitive to dc offsets and greater 1/f noise. In addition, zero-IF designs suffer from local oscillator and RF self-mixing and second-order distortion of blocking signals.
Higher linearity in the low-noise amplifiers and mixers as well as better filtering can overcome these problems. But, this usually results in higher power consumption and a larger chip. While numerous direct-conversion receivers have been developed, notably Analog Devices' Othello chip set and proprietary designs by Nokia, Ericsson, Alcatel, and others, Silicon Labs chose the low-IF approach. It completely avoids these problems while providing all of the same benefits.
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