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Silicon TV tuners clearing hurdles

Posted: 04 Jan 2008 ?? ?Print Version ?Bookmark and Share

Keywords:WLAN transceivers? solid-state TV tuners? digital TV? MediaFLO?

Although mobile phone and WLAN transceivers made the transition to silicon ICs years ago, solid-state TV tuners have been slower off the starting block. Recently, several companies have started to offer IC-based TV receivers that will replace the traditional "can tuner."

The past decade has witnessed significant technological innovation in TV products: high-definition DTV, flat-panel displays and stereo audio. In addition, Microsoft Windows XP Media Center has created a booming market for TV reception on the PC. What's more, there is a growing demand for multimedia capabilities on mobile handheld devices to allow a consumer to watch live TV or download video. Added to all these innovations is the imminent worldwide transition in TV broadcasting from analog to digital modulation.

The DTV revolution is developing rapidly; indeed, in some jurisdictions, analog broadcasts have already ceased. However, to minimize the impact on viewers and to facilitate a smooth transition, many governments have mandated the coexistence of legacy analog (i.e., NTSC, PAL and Secam) and digital (i.e., DVB-T, ISDB-T and ATSC) signals until Feb. 17, 2009. Thus, in many regions, DTV conversion will span many years, and hybrid broadcast systems represent a significant market.

Interestingly, almost all TV sets and digital-to-analog set-top converter boxes for legacy analog TVs still use low-tech, power-hungry "can tuner" modules, which consist of hundreds of discrete components. The "can tuner" is so called for a good reason: The discrete fixed and tunable inductors, along with individual transistors, are housed in large (often 2-inch x 4-inch) metal enclosures to minimize RF interference. The manual intervention required to "tune" all these separate components leads to time-consuming manufacture and test processes.

Obviously, solid-state TV tuners can help resolve these and other issues, including multiple standards, integration, size and power consumption. The fact that we are still at the initial stages of IC TV tuner entry into consumer products suggests that TV tuner design adaptation faces greater challenges than do cellular transceivers, for instance. Then, why can't we just use the cellular network to bring TV-type applications to a mobile device?

Tuner technical issues
The primary technical differences in tuner design considerations between mobile phone applications and broadcast TV are bandwidth, interference and dynamic range. A cellular receiver has to function over a very narrow range of frequencies. For instance, a typical GSM receiver needs to cover only 25 MHz or 30MHz, whereas a TV tuner must receive more than 800MHz of bandwidth (from 42-864MHz).

This broad frequency response affects three major elements of the receiver design: the frequency synthesizer, the low-noise amplification blocks and the filter. The main issues for the synthesizer are its ability to generate the required frequencies and to meet the lock time requirements while maintaining a small form factor and low power consumption. The major design concern regarding amplification blocks relates to their susceptibility to a broader range of interfering signals, which can result in blocky images, artifacts and signal loss. These interfering signals may come from broadcast TV stations, cellular transmissions and even automobile ignitions. Unlike cellphone receivers, which use a fixed filter in front of their receivers to effectively eliminate potential interference across a broad spectrum (due to the narrow frequency bands involved), a TV receiver cannot use a single fixed filter to reject interfering signals effectively, because of the large broadcast bandwidth involved. As a result, TV tuners require very specialized active channel filtering techniques.

Another design obstacle is accommodating the large dynamic range associated with broadcast TV signals. The receiver's arbitrary, variable distance from the transmitter leads to wide variations in channel signal strength. The need to handle these potentially large differences in dynamic range calls for very complex IC designs. The fact that TV signals carried over cable have fairly uniform signal strength from channel to channel allowed early ICs to be deployed in cable TV receivers.

Figure 1: Annotated die image of the XC3028 single-chip analog and digital TV tuner showing the fully integrated RF-to-baseband functional blocks.

Because of the dynamic-range problem, tuners must be sensitive to very weak signals, including incoming TV signals close to the receiver noise floor, even in the presence of strong interference. For instance, the undesired signal strength can exceed that of the desired signal by as much as 55dB, requiring the receiver to amplify the desired signal in the presence of a much stronger interfering signal. Again, this is accomplished by advanced channel filtering techniques that block out interference.

An example of a silicon IC TV tuner for set-top box applications is the Xceive XC3028. It measures 4.2mm x 4.3mm and is fabricated by Jazz Semiconductor in a five-metal-layer, 0.18?m BiCMOS process. It supports the NTSC, PAL and Secam analog standards. It also supports the ATSC, DVB-C, DVBT and ISDB-T digital standards by generating a digital demodulator interface signal that is then processed by additional digital demodulation circuitry within a TV set design. Xceive also produces a low-power receiver IC especially suited for USB-based designs that add TV capability to desktop or laptop PCs.

TV tuners for mobile devices
Multimedia capability in mobile handsets now includes data, audio, video and real-time DTV programming. Delivering TV service to mobile handsets is done in two ways. The first uses the existing cellular network, the second a dedicated one-way broadcast TV network.

However, the TV-over-cellular approach is limited by low resolution and low frame rates, which can result in jerky, stop-and-go motion. Even more significant, the high-quality TV service requires 10x the data rate of voice service. Hence, if many users attempted to use TV services simultaneously, the demand would exceed the cellular network's capacity, resulting in dropped calls or "network busy" error messages.

Figure 2: The highly integrated MaxLinear MxL5005S analog and digital TV tuner provides a low-cost solution for reception of multiple TV standards.

Thus, cellular networks are unable to provide a range of TV services to all users, which means TV-over-broadcast holds greater promise of high-performance, real-time broadcast TV capability for mobile handsets. There are several competing standards, including Digital Video Broadcasting-Handheld (DVB-H), Digital Multimedia Broadcasting (DMB), Integrated Services Digital Broadcasting (ISDB-T) and MediaFLO.

An ultralow-power TV tuner for STB and mobile applications is the MaxLinear MxL5005S Global Standards silicon IC tuner. It measures 3.7mm x 3.7mm and is fabricated in a five-metal-layer, 0.18?m CMOS process. MaxLinear states that the device consumes only 300mW of power and is suitable for both analog and digital TV reception.

As integration inches forward, the next step for silicon tuners will likely be the incorporation of the TV demodulator either into handheld applications processors or, with the RF TV tuner, into systems-in-package. Some predict that the next step will be the integration of the demodulator onto one die with the RF tuner. Nevertheless, multimode and multiband tuners will continue to evolve along with the broadcast standards.

- Michael Keller
Product Technology Analyst for RF, Semiconductor Insights

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