Electronic Design

Analog/Mixed-Signal: More

RF And Interfaces Gain Prestige In The Analog Mix

Amplifiers and data converters dominate the analog and mixed-signal markets, but lots of other functions are crucial within this industry segment. Space constraints prevent addressing all of them, so we'll focus on two of the fastest growing areas: RF and interfaces. The main drivers are for lower cost and higher performance in both areas. However, unlike the other analog and mixed-signal circuits, size and power consumption are important but not critical issues.

RF is an enabler for growth. End applications include cell phones, wireless local-area networks (LANs), and personal-area networks (PANs). The key issue for RF circuits is to get transmitter-receiver sets to the lowest power and costs possible. While the physics of the situation say that a transmitter's output is a function of the power amplifier's efficiency, a change in the basic architecture can significantly change the system power consumption.

By going to a differential zero-intermediate frequency architecture, IC vendors have arrived at a one-chip transceiver solution, which includes everything except the power amplifier. Infrastructure and radio link manufacturers also desire higher integration, although they're much less willing to sacrifice performance.

The need for higher performance continues in terms of linearity and distortion, as the demand for greater sensitivity remains an important design constraint. To address the performance issue, differential signal topologies are emerging as newer equipment gets produced.

Most new RF products are offered in some type of chip-scale packaging. Even though the packages are getting smaller, internal component counts are increasing. Now RF components include almost everything except the power amplifier. The increasing levels of integration help the systems design team by reducing the design of the building blocks and their interconnections.

In the interface realm, interface parts connect the computer data streams to the rest of the world, even though the parts aren't digital. For data communications, interface parts must handle the analog signal characteristics of the transport media, from simple twisted-wire pairs for an RS-232 to multiwire functions like USB or an orthogonal frequency division 10-Gbit/s Ethernet. The number of interface standards continues to escalate to address the many changing data-transmission requirements.

Communications circuits need high-linearity and low-distortion specifications to meet the communications systems requirements and because the DSP and its algorithms can't correct all of the nonlinearities and distortion. The drivers in communications systems usually reduce the quality of the signals, rather than just act as a buffer and transmitter. The distortion specifications for high-speed communications systems signals are even higher than those of high-end stereo equipment.

Interface circuits can also transform a signal from one standard to another, such as low-voltage differential signaling (LVDS) to PECL. The level and signal transforms allow signals in one part of the system to communicate with other sections without direct digital interconnections across relatively short distances.

From a cost perspective, it's better to keep circuits like interfaces separate from other chips. The standard interfaces are catalog parts that offer high flexibility and availability over an integrated solution.

See associated figure.

>EXPECT MORE SILICON-GERMANIUM (SiGe) PROCESSES for both the higher speeds and other specialized capabilities, due to the engineered band-gap voltage in the transistors. The requirements for lower noise and higher bandwidth are not possible in most other processes. The SiGe may be used for characteristics other than the speed of the process, because bandwidth is not currently the most limiting parameter.

>HIGHER LEVELS OF INTEGRATION ARE COMING in RF components. Building blocks of medium-level integration for radios include a combination of low-noise amplifier, mixer, and so forth at supply voltages as low as 1.8 V.

>MORE DIGITAL modulation and demodulation components seem to be the trend. High-speed converters will eventually change radio architectures to a zero intermediate frequency structure to take advantage of the improvements in converters and high-speed amplifiers.

>RADIOS WILL SEE A WIDER RANGE of differential topologies. The continuing decrease in supply voltage requires differential signals for the dynamic range, lower distortion, and greater linearity.

>EXPECT GREATER EX-POSURE of biCMOS and SiGe as an increasing amount of parts are developed. The CMOS parts have the lowest cost, but the other technologies offer higher performance.

>HYBRID PREDISTORTION ARCHITECTURES will be interim solutions for digital modulation architectures. This will allow for migration to the zero-IF structures with good signal fidelity and acceptable costs.

>INTERFACES WILL PROLIFERATE with more translator functions from one standard to another. The discrete interface functions provide the essential bridge functions to standard-based interconnections, even as the number of interface standards keeps increasing. Multivoltage components will enable easier interconnections between different sections of systems. Some of the interfaces will span the latest digital supplies and legacy systems, as the new systems must still work with the existing equipment and older standards.

>10-GBIT/S ETHERNET COMPONENTS in CMOS are finding their way into today's 130-nm processes. Meanwhile, 40-Gbit/s components in CMOS will settle into the 90-nm process node. Further speed increases are possible in CMOS with more efforts in exploration into newer architectures that can run at higher speeds on lower power. Speeds continue to increase, yet CMOS processes can still meet the requirements.

>GREATER LEVELS OF INTEGRATION will help to reduce board space. The high levels of integration reduce the challenges of designing the interfaces between functions and provide a prebuilt platform in a chip. The higher speeds and higher loading on the interfaces will trigger the need for more reference designs and applications help, as the design of high-speed pc boards and interconnections is not a trivial design exercise.

>FASTER DATA SPEEDS IMPOSE MUCH TIGHTER requirements for signal fidelity. Therefore, high-speed interfaces will need tighter specifications and have to include compensation capabilities to keep the signals clean on both the end and receive sides of the interface.

>UBIQUITOUS WIRELESS LANs (WiFi or 802.11b) are popping up as local hot spots all over the country. Currently, all laptop computers plus all of the Apple computer line include the adapters in new shipments. The local hot spots are starting to displace the 2.5 and 3G cell phones' data capabilities and are moving toward partial compatibility with Bluetooth.

TAGS: Digital ICs
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