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Silicon-Germanium: The superior semiconductor technology for solid-state TV tuners

Because of the ubiquity of television in the modern world, practically everyone is aware of the rapid and stunning advances in TV technology over the last decade. During The last 10 years, common television sizes have evolved from 27-to-32 inches in the 90s to 37-inch and larger (much larger). At the same time, the once-universal Cathode Ray Tube (CRT) has been dying a fairly quick but silent death. The dim, murky pictures seen on analog big screen TVs a decade ago have been completely supplanted by bright, saturated color displayed in photographic-like detail on LCD, Plasma, and DLP screens of huge dimensions.

What is less apparent to the consumer is the tremendous challenge that has faced designers of solid-state tuners. The majority of televisions produced today still use “CAN-type” tuners, little different than those used when transistors replaced vacuum tubes in TV design in the late 1960s. That, however, is on the precipice of change.

The Solid-State Tuner Challenge
The CAN tuner, so called because it is housed in metal enclosures to eliminate crosstalk and stray radiation, is as mature as any technology can get. Manufacturing processes are standard and efficient. Size and signal capability is consistent from manufacturer to manufactureras are the shortcomings.

CAN tuners are large and bulky. The required tunable and fixed coils in the tuned circuits require discrete transistors, concomitant with higher voltage and current demands than those of most other digital and analog circuitry in the television set. Worst of all, the coils require tuning in the manufacturing process that is both time consuming and expensive and are prone to losing alignment over time. Relying on discrete components, CAN tuners' parameters are not very well controlled and vary significantly from unit to unit. Typically, CANs can handle only one standard from one region of the world, requiring many different parts be stocked and to cover today's global market.

Clearly, though, CAN tuners have jealously held on to advantages over available solid-state designs that have ensured a long life. On one end of the spectrum, low-end CAN tuners offer mediocre quality but optimized cost; while, on the other end, high-end devices offer good performance at a heightened complexity and cost, so covering the complete spectrum of applications. Enclosing all RF components in a standalone unit facilitates the design of the final product.

Early silicon designs could not compete with the superb signal-to-noise (SNR) capability and high dynamic range of high-end CANs and were not competitive enough on price at the low-end. With the advent of digital TV, however, hybrid CANs have proven difficult to design in, are complex, and more expensive than traditional solutions. Meanwhile, silicon tuners have proven its ability handle multiple standards with a single chip. Still, to supersede CAN tuners, solid-state tuners must compete in both signal-handling ability and cost.

Television Signal Fundamentals
Television signals have tremendous signal-strength variability. Not only does the signal strength vary from channel to channel based on the distance from the transmitter (for direct reception) or distance from the last cable repeater (for CATV systems) but the signal strength also varies in the time domain due to environmental factors, number of receivers on the cable and other factors. The tuner must be able to accommodate this signal strength variability and still produce a solid, linear output to the television circuitry.

Further, a television tuner must tune over an enormous bandwidth. Unlike, say, a wireless router that tunes a few MHz on either side of its center frequency, a television tuner must be able to tune over nearly 1 GHz of bandwidth.

Clearly, solid-state tuners have advantages over CAN tuners. They are much smaller and require significantly less power. The small size means that multiple tuners can be housed in even less space than a CAN tuner, allowing picture-in-picture and multiple program recording. More importantly, they don't require tuning and can work with any transmission standard.

So, what is the holdup? The answer to that question is in the semiconductor technology itself. Early tuners have attempted to use a CMOS process, which has well served the digital realm, but has problems with the very heart of the TV tuner: dynamic range and SNR. In addition, voltage-frequency tradeoffs have not allowed CMOS to operate from the lower voltages desired in the television circuit.

New Process Technology Solves the Problem
The answer to the problem lies in the physics of the semiconductor itself. As has been demonstrated in microprocessor technology, shrinking the geometry of the CMOS transistor allows for greater speed. The most modern CMOS geometry has allowed impressive switching speed. But, like every physical process, there is a limit. Realization of the smaller geometry requires, in part, thinning of the oxide layer of the transistor. The gate of an MOS transistor comprises a metal layer separated by a very thin oxide layer (Figure 1).

Figure 1: The Gate is insulated from the Source and Drain by a thin oxide layer

Increased speed has led to the gate oxide being as small as 1.2 nm, which is just five atoms thick. Physics dictates that the thinner the layer of oxide, the lower maximum voltage the transistor can tolerate without tunneling leakage and, ultimately, failure. Thus as speeds have increased due to decreased geometry, operating voltage has decreased. The fastest CMOS chips operate at supply voltage close to one volt.

Therein lies the problem, when the application is a television tuner, where SNR is king, a high SNR first requires a large enough signal to produce the desired SNR. At 1.8 volts or below, there just isn't enough signal to produce the requisite SNR. To operate at a high enough voltage to produce acceptable SNR, CMOS speed is not great enough to allow high-speed linearization techniques required to produce stable, constant, and noise-free output signals.
It isn't a question of “waiting for a better CMOS;” the physical constraints do not allow for the requisite speed with acceptable SNR, dynamic range and linearity. The answer lies in a different process.

Silicon-Germanium BiCMOS
Germanium transistors? That sounds like a step backwards. Since the advent of digital integrated circuit technology, the vast majority of transistors have been produced from silicon substrates. However, the venerable germanium transistor has some distinct advantages over silicon, chief among them is low noise at low operating voltage. By exploiting the advantages of bipolar germanium and silicon transistors, and CMOS, researchers arrived at a semiconductor technology that is perfectly suited to the demands of RF mixed-signal applications, especially television tuners.

Silicon-germanium (SiGe) transistors exhibit numerous advantages over standard CMOS for television tuner applications. Chief among those advantages is a substantially higher operating voltage for a given operating speed. This increased operating voltage allows for larger magnitude input signals and a corresponding significant increase in SNR. Consider that 10 mV of noise impressed on a 1.0 V signal yields 1 % noise, but that same 10 mV of noise on a 3.3 V signal is a full order of magnitude better SNR. In practice it is much worse than this simple example demonstrates because the complete supply voltage range cannot be used by the signal. There is a significant voltage overhead required to properly bias the active transistors, and it typically does not scale with the process. Reducing this overhead has a high price in term of linearity and noise performance, again for fundamental physical reasons.

Figure 2 shows the Frequency vs. Voltage plot for CMOS and SiGe transistors. At 3.3 V, SiGe has a fourfold frequency advantage. To realize the CMOS geometry capable of the same 78 MHz 3.3 V SiGe, the CMOS supply voltage must be reduced to 1.2 V, thereby handicapping its dynamic range handling capability.

Figure 2: Frequency vs. Voltage plot for CMOS (triangles) and SiGe (diamonds)

SiGe transistors also have a superior dynamic range when compared with CMOS, again, largely due to the increased operating voltage for a giving frequency. Wide dynamic range and high SNR are absolutely critical to extracting digital-quality television signals.

A third critical advantage is the increased linearity possible with bipolar transistors. While MOS transistors certainly can operate in their linear region, the reduced operating voltage renders the linear portion significantly smaller than that of bipolar transistors. Germanium transistors have long been the most linear of the three major transistor types (silicon bipolar, germanium bipolar, and MOS), as well as the quietest (least amount of thermal and Schottky noise, almost negligible flicker noise). Bipolar devices are also far superior in terms of matching accuracy that is critical to many designs.

Integrating the superior capabilities of SiGe bipolar transistors with CMOS provides the perfect architecture for mixed analog and digital signals on a single integrated circuit.

Silicon-Germanium Tuner Design
The accumulation of the SiGe advantages positions this technology as the best for applications such as television tuners. Other wide-bandwidth RF applications are also turning to SiGe processes, but the bandwidth, noise figure, dynamic range, and adjacent-channel rejection requirement of television tuners place the greatest demands on the technology.

Ideal amplifiers would amplify the input signal without inducing any distortion of any kind onto the output signal. Of course, there are no 'ideal transistors' so there are no ideal amplifiers. However, SiGe operating at up to 200 GHz provides opportunities to use feedback techniques to increase linearity, thus approaching an ideal tuner stage. Greater linearity allows the tuner to accept much larger signals without distortion, increasing both SNR and dynamic range.

Xceive SiGe Tuners Match CAN Performance
Currently, all solid-state television tuners use SiGe process technology. However, Xceive tuners are the only solution on the market using the most advanced 0.18um BiCMOS process; all competitors are using the older 0.35um geometry. This advanced process enables Xceive to integrate a complete DSP on the same silicon than the RF tuner and therefore is the only company providing a single-chip RF-to-baseband tuner, and as a result, significantly more integration than the competition. Using SiGe devices that are one generation more advanced, Xceive products enables higher integration and better performance, clearly putting Xceive tuners in front of the competition. Xceive tuners have increased sensitivity, ISF certified and are the only tuners that truly compete with CAN tuners. Indeed, Xceive's tuners have a number of advantages over CAN solutions.

Xceive's SiGe tuners are more power efficient than conventional solutions. The low-power SiGe BiCMOS technology has lower total power requirement than the discrete technology used in CANs and operates with typical voltage available in today's applications (3.3V and 1.8V) instead of the older 5V standard and of the requirement for non-standard 30V supply. Part of this results from the Xceive tuner being significantly smaller, which is another large advantage over CANs. In fact, Xceive's tuners are so small, that several tuners can be housed in the same space as a CAN, allowing simultaneous tuning of several frequencies for multiple picture-in-picture capability.

Finally, Xceive's solid-state tuners lock onto a channel significantly faster than CAN tuners. This presents a much better human interface, especially in High Definition where current tuners can take seconds to lock onto a new channel.

Summary
CAN tuners have held a virtual lock on both analog and digital television tuners for a long time. Until now, solid-state tuners could not compete, primarily because of the unusually high requirements for low noise figure, high dynamic range and signal linearity.

The development of the SiGe BiCMOS process has broken the CAN tuner's lock and will certainly propel the process to the forefront, especially in digital HD applications. Not only can the SiGe BiCMOS produce equivalent or superior signal handling than previous tuner technology, but it can do so with less power, a smaller footprint, and significantly faster tuning lock-on.

The demands of the ever-evolving TV industry present clear signs that CAN tuners are not compatible with the design and efficiency required by today's modern consumer. Further more, manufactures can offer consumers functionality previously unavailable with CAN tuners, including advanced multi-picture and rapid tuning capabilities. With just a single tuner meeting global standards manufacturers can reduce inventory costs, and as the cost of silicon tuners drops below the cost of CANs, additionally costs savings can be realized. The SiGe BiCMOS process imbedded within Xceive tuners allow all manufacturers to advance to the next phase in the TV industry, allowing TV to be available virtually everywhere.

About the Authors:
Alain Serge Porret is the co-founder and VP of Engineering for Xceive Corporation. Before co-founding Xceive, he was the principal engineer of a Silicon Valley start up company in charge of the system-level analysis of their VHF/UHF tuner chips. He received his BSEE from the HES-SO (University of Applied Science – Western Switzerland) and his MSEE and Ph.D. degrees from EPFL. He can be reached at wong@xceive.com.

Alvin Wong is the VP of Marketing for Xceive Corporation. He joined Xceive in December 2004, bringing 16 years of management experience in the semiductor industry. Most recently at Infineon Technologies, Wong was the VP of Marketing, responsible for building strategies and running operations in North America for the Wireless Division. He can be reached at wong@xceive.com.

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