[Technology Report]
Dynamic Performance Is The Mantra For Analog And Mixed-Signal ICs
Third-generation communications applications are calling for improved dynamic range over a much wider bandwidth.
All anyone seems to be talking about lately is dynamic performance, dynamic performance, and... dynamic performance. It doesn't matter if it's a broadband amplifier or a high-performance, high-resolution data converter. Either way, every analog and mixed-signal data-converter designer is reciting dynamic range at higher frequencies and wider bandwidth. This specification takes on even more meaning as we migrate to 3G cellular and wireless applications, where data rates and functionalities are greater. Suppliers are determined to obtain it in miniature packages at lower voltages and prices.
Analog and data-converter vendors have been efficiently catering to the ongoing needs of the 2G cellular and wireless communications systems around the world. As OEMs cranked out compact handsets featuring more bells and whistles, base-station makers concentrated on wider bandwidth, additional channels, and higher output power. Suppliers of analog and mixed-signal ICs responded by promptly answering the call for adequate dynamic performance at the desired frequencies and bandwidth. They also put the right amount of features on board.
Now, the move toward next-generation communications standards has redoubled the pressure on suppliers of amplifiers and data-converter ICs. As we enter the next millennium, 3G communications specifications are being tweaked and finalized as standards are being put into effect. Makers of 3G cellular phones; broadband software radios; and multimode, multicarrier base-station transmitters are seeking much higher levels of performance. They want it in a minute footprint, without sacrificing power and cost. So newer products will be slimmer, as well as more elegant and powerful. They'll handle voice and data concurrently. Plus, these systems will cost less and work longer using the same battery source.
Meanwhile, direct-conversion, or no-IF, transceiver designs are winning popularity in radio receivers. Suppliers of analog and data-converter ICs are feeling pressure to significantly increase on-board features, slash the external component count, and consume a fraction of the power of previous-generation devices.
Aside from that, they must possess the processing ability to deal with IF signals closer to the antenna. Designers are exploiting state-of-the-art deep-submicron CMOS processes to meet the conflicting requirements of speed, resolution, higher on-chip density, and lower system cost. In designs where CMOS is inadequate, especially at low voltages, they're exploiting biCMOS. And when CMOS and pure silicon-based biCMOS run out of gas, some developers are going after SiGe heterojunction-bipolar-transistors (HBTs), or simply bipolars, to meet performance goals.
Advances Propel Software Radios Concurrently, advances in digital signal processing (DSPs) and high-speed analog-to-digital and digital-to-analog converters (ADCs and DACs) are pushing designers toward wideband digital receivers or universal software radios. Faster, high-resolution data converters with good linearity and wider dynamic range at higher frequencies also are emerging. When coupled with the quantum leaps in DSP technology, those converters are making wideband software radios feasible for many of the commercial, wireless-communications applications now coming out. Soon, developers will be able to deliver a single analog front end with sufficient horsepower to handle all of the channels over a wide bandwidth and pick out the right signal from the incoming frequencies (Fig. 1).
By replacing the traditional superheterodyne transceiver, universal software radios usher in an era of programmable transceivers. Multiple analog and digital cellular standards can be supported through reprogrammability. That aspect will allow users to quickly adapt their radios to upcoming standards via easy software upgrades.
As the makers of wireless handsets and infrastructure equipment prepare their transition to 3G specifications, they've turned their backs just long enough to let an interim architecture gain popularity. Enhanced data rates for GSM (EDGE), with 384-kbit/s data transport capability, are being put in place to deliver wider bandwidth and increased capacity. Labeled as a 2.5G standard, EDGE is a cooperative effort between proponents of GSM and IS-136. Its goal is to establish a common, high-speed standard that capitalizes on the established 2G infrastructure.
As National Semiconductor's John Steininger puts it, EDGE is the harbinger of how 3G applications will look. He adds that as these emerging standards employ newer modulation and demodulation schemes for faster data rates, the transmitters and receivers must be accordingly redesigned. Newer amplifiers and data converters must comply with these requirements.
For instance, EDGE implements an 8PSK modulation scheme to achieve a higher data rate of 384 kbits/s. This requires that the receivers and transmitters in the base stations be modified, as well as the transceivers in the handsets. There's a tremendous push in the analog and mixed-signal design world to achieve greater dynamic range over a substantially wider bandwidth. Designers are even talking about attaining 100-dB spurious-free dynamic range (SFDR) at a 200-MHz signal frequency with a bandwidth of 30 to 35 MHz.
This level of dynamic performance cannot be achieved without lowering the noise floor and improving the integral-linearity (INL) error. Pressure is mounting to significantly slash the noise generated by the amplifiers and the data converters employed in the new designs. Then, the signal-to-noise ratio (SNR) of the receivers and transceivers could be high enough (greater than 75 dB) to decode even a weaker signal. They would thereby be able to extract and process both strong and weak signals with the same level of integrity.
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