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Going Digital: Advanced Set Top Boxes Benefit From New Class of Tuner ICs
Using SiGe technology for the tuner chips inside new cable access gateways
is boosting performance and reliability while cutting power and costs.
by Kim Beumer, IC Design Manager, Microtune, Inc.

Designers of next-generation cable set top boxes (STBs) and digital interactive access gateways are looking closely at the cost and power requirements of each component in the system. New applications such as High Definition TV programming, web pages, streaming media, electronic commerce, home banking, interactive TV services, enhanced electronic program guides, Video-on Demand, and personal video recording (PVR) in the cable STB demand unprecedented component performance.

Underlying the increasing functionality of this new breed of set-top box are high-performance tuners that integrate more functionality and support more complex features than previous versions. In addition, the broadband cable market is becoming increasingly competitive, so these solutions must also offer low-power and low costs. While it has been used in integrated circuit (IC) processing in other communications markets, silicon germanium (SiGe) is new to the cable tuner market, and it is proving to offer the kind of reliability, integration, power, and cost required to help multiple system operators (MSOs) maintain a competitive edge and bring new services on line.

In fact, an advanced SiGe BiCMOS process can be used to manufacture a new class of powerful, solid-state tuners that can support the most demanding analog and digital STB applications, making it possible to produce low-cost, low-power, advanced set top boxes.

Advanced Set Top Box Needs

Maintaining a competitive advantage in the home communication services arena is becoming increasingly difficult as voice, video, data, audio services converge, and providers compete to satisfy the maximum number of customers. One way that the cable market is addressing this concern is with next-generation cable access points, or advanced STBs. These new boxes are designed to deliver digital services and high-speed cable Internet access.

Over the past several years, cable MSOs have been upgrading their hybrid fiber coax (HFC) cable plants to offer new digital services. As a result, the technological demands on the STB have increased. For instance, they require multiple high-performance tuners that can handle different types of digital data. In addition, they must be backward compatible with existing analog services.

Essentially, each new digital capability within the STB, such as picture-in-picture, PVRs, and cable modems, requires its own tuner. This use of multiple tuners (see Figure) increases the importance of high integration, low power, and small form factors. And, the competitive nature of this market also requires low cost components.

More Integration

One way to improve power consumption, reduce size, and improve cost in high-volumes is to use a highly integrated silicon tuner solution. For instance, by integrating multiple passive capabilities that were previously realized with discrete components, STB vendors can simplify manufacturing and cut costs. As an added benefit, fewer components tend to improve reliability.

The key to developing such a highly integrated STB tuner was to find a semiconductor material well suited to integration. SiGe is known in other industries, such as wireless communications, for providing highly functional, integrated, single-chip solutions with a smaller footprint than previous silicon generations.

Key Tuner Specs

Linearity is an important specification for a tuner used in an advanced STB. In general, good linearity reduces interference and distortion, delivering a clearer picture for video applications. For digital video, data, or audio, good linearity provides sufficient signal to noise ratios, thereby improving performance by receiving signals that otherwise might have been lost. SiGe is well known for its linearity, which is another reason it is a good fit for this application.

Good performance in the presence of noise allows for the best digital demodulation, so noise figure and phase noise are also important specifications for a tuner used in an advanced STB. In addition, low noise allows the STB to handle advanced modulation schemes such as 256-quadrature amplitude modulation (QAM) or higher. Again, SiGe offers good noise performance, making it well suited for this application.

A New Process

For performance and flexibility in STB tuner applications, a major advantage of SiGe is its linearity. As developed by IBM, SiGe is a very high fT transistor process. This means that by using careful design, engineers can trade excess tuner bandwidth for improved linearity by using feedback amplifiers with high loop gains. As a result, the designs can feature more linearity versus power dissipation than any other currently available process.

Another major advantage of SiGe in this application is its ease of integration. For example, a single-chip SiGe STB tuner can fully integrate voltage controlled oscillator (VCO) functionality on chip, as opposed to previous generation solutions where the inductive resonator portion of the VCO component had to be placed on the circuit board to maximize the quality factor (Q) of the resonator. In another example, SiGe allows designers to integrate secondary functionality onto the die, such as bringing the variable gain intermediate frequency (IF) amplifier on chip. Using other processes, this functionality is handled by a second chip.

When compared to other available processes, another major benefit of SiGe in this application is the reduction in noise given a certain power consumption, bandwidth, and linearity. Once again, it is the high ft that enables this reduction in power consumption at a given level of performance. For instance, a single-chip tuner can be designed in 1.5W using SiGe, which is significant reduction in comparison to other 3W technologies.

Flexibility

Set top boxes are typically deployed within the residence, so flexibility, reliability, and long product life cycles are very beneficial attributes. Since the new boxes require a tuner for each application, it would be easier for design, manufacturing, bill of materials, and service if the same tuner could be used for every application. Most currently available tuners are application specific, such as for a cable modem only. So, using a flexible tuner that could be used for all of the current and even future advanced services could be very beneficial to the STB vendor. The new class of SiGe tuners offers this kind of application agnostic flexibility.

An Enabling Solution

Implementing high performance SiGe tuners for advanced STBs confers major advantages to the STB manufacturer as well as the consumer. These flexible, high-performance tuners deliver on demanding next generation specifications, and these highly integrated single-chip solutions can improve reliability of the final product.

Improvements in STBs and home gateways will likely continue into the future as the industry ramps up to higher modulation schemes, such as 1024 QAM. Using a BiCMOS SiGe process, tuner designers can continue to integrate functionality on the die with the performance required for these higher modulation standards. And, as consumer hunger for bandwidth continues to grow, SiGe could also offer some performance headroom.

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Figure 1: Multiple single-chip tuners in an advanced set-top box architecture.