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Zarlink TV Tuner Delivers Performance, Cost, Flexibility Advantages for Terrestrial Set-Top Boxes
Industry's highest performing single-conversion tuner chip cuts tuner costs by up to 25%,and board space by up to 33%
| The manufacturer says . . . | ChipCenter's Paul O'Shea says . . . |
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Zarlink Semiconductor announced immediate availability of a flexible RF silicon tuner chip that delivers very high levels of performance and reliability to both analog and digital terrestrial television STBs, while significantly reducing cost and board space. Zarlink's innovative SL2610 is the industry's first tuner chip that allows low-cost, compact, single-conversion tuner designs to achieve the same high levels of performance as larger, more expensive double-conversion tuners. The SL2610 was designed in consultation with a major U.S. television and STB manufacturer to deliver quality RF tuner performance in the most challenging reception conditions. Designers of mass market terrestrial STBs are demanding flexible, low-cost tuner chips that can reliably convert analog and digital terrestrial TV (DTTV) signals into clear, crisp pictures. Until now, designers have used double-conversion tuners ý which use two steps to convert incoming RF signals to lower-frequency, single-channel outputs ý to achieve high-quality reception of DTTV channels. Zarlink's SL2610 allows STB designers to achieve high-performance, reliable analog and digital TV reception using a single frequency conversion step. The SL2610 is also the industry's first single-conversion tuner device with on-chip image channel reject to eliminate unwanted "image channels." This image reject feature, coupled with the SL2610's sophisticated handling of high-amplitude input signals, eliminates the need for costly, complex, specially aligned front-end tracking filters in many applications. As a result, customers can integrate their tuner designs directly on STB motherboards, instead of placing them on dedicated RF boards. "With the SL2610 we're delivering what designers truly need ý a tuner chip that delivers excellent digital and analog TV reception while lowering the cost and shrinking the size of STBs," said Nick Cowley, RF systems specialist, Zarlink Semiconductor. "Tuners built with the single-conversion SL2610 cost about 25% less than double-conversion designs and use about 33% less board space." Image reject is required to eliminate undesired image channels that are picked up by tuners while receiving desired channels. The unique image reject circuitry in Zarlink's SL2610 delivers more than 30 dB (decibels) of image suppression. When coupled with a simple, non-aligned tracking filter, the chip easily delivers the performance required for operation in Europe and other countries that have adopted ETSI's (European Telecommunications Standards Institute) DVB-T (Digital Video Broadcast-Terrestrial) standard for DTTV. The SL2610 also delivers high-performance intermodulation intercept. This enables the device to efficiently tune to the desired channel in the presence of higher amplitude interferers or wide-bandwidth composite input signals. The SL2610 is a triple-band, low-noise RF mixer oscillator (MO) that operates across the full 50-900 MHz analog and DTTV frequency range. Each MO band consists of a low-noise preamplifier/mixer and local oscillator, supported by an external, varactor-tuned tank. The MO outputs share a low-impedance SAWF (surface acoustic wave filter) driver stage. The SL2610 also integrates an I2C bus-controlled PLL (phase-locked loop) synthesizer designed for low phase-noise performance and compatible with all common frequency offset requirements. At 10 kHz (kilohertz), the free running phase noise of the SL2610 is typically less than -90 dBc/Hz (decibels relative to carrier power per hertz). The device can be programmed to generate all output IFs (intermediate frequencies) commonly used in analog and digital receiver systems. |
Zarlink decided when they started designing this product that it was not going to be another me too product. It was a good business decision because there are several mixer oscillator PLL circuits on the market. The company decided to design a product that offers significant performance advantages so they collaborated with a leading STB and TV manufacturer, and specifically targeted the emerging digital terrestrial market in the US. They chose the US market because manufacturers were having problems with some installations. There were problems with high amplitude reflections of analog channels with the digital channels. Additionally, the transmitter power in the US market is not well regulated so it can be a very hostile environment for the tuner. The SL2610 was made using a SiGe process, not CMOS. Zarlink put a lot of money in the different processes and from their observations and the papers published they were not convinced that CMOS was currently the right process for the very high performance RF analog functions. This was especially true for designs that need a very wide dynamic range. Currently, Zarlink doesnýt think RF CMOS is at the stage where itýs appropriate for the high-end tuner IC. For them, SiGe offers the best performance for the high-end tuner application. As far as Zarlink knows this is the first single conversion tuner on the market with on-chip image channel reject. In fact there may not be any mixer oscillators that have image rejection built into them either off-chip or on-chip. Zarlink designed the SL2610 to integrate the image rejection, which gives better protection to the image channel, and also improve the signal handling. That means that in some of these environments where there is a very hostile analog interferer present, the chip can handle signals and cope with a very high offset interferer. The company designed the SL2610 to meet challenging requirements such as delivering an acceptable digital channel with an N+1 channel interferer or a +70dBc signal. For example, a high interferer arises when you are watching a digital channel from a relatively distant transmitter and an analog channel is transmitted from much closer proximity, causing interference. The transmitters in the area are supposed to be aligned so they don't cause a problem but if you get local reflection you can get a lot of power reflected into the receive path of your tuner. This is particularly bad in urban areas with many high-rise buildings. The SL2610 single conversion tuner provides a lower cost conversion than the typical double conversion types. In the double conversion you have a broadband input to the mixer oscillator that is converted to a high IF, and the high IF uses some type of surface acoustic wave (SAW) filter to perform the channel selection, and then it is down converted to the IF. There are two mixer stages ý the first being broadband and the second, narrow-band followed by a fixed SAW filter. In the single conversion the RF input goes through a series of tracking filters to the mixer section. That tracking filter is a very narrow-band filter centered on the desired channel. The function of the narrow-band filter is to suppress the image channel present in a single conversion system, and reduce the total width of the composite signal presented to the mixer. That prevents intermodulation distortion in the mixer. Because there is a tracking filter in the single conversion tuner, the filter tracks with the local oscillator, so as you tune to a channel at 50 MHz, for example, the local oscillator may be at 90 MHz, and the filter is centered on 50 MHz. If you tune to a channel at 500 MHz, the local oscillator will shift to 540 MHz and the tracking filter will shift to 500 MHz. So they have to track with the local oscillator and the center frequency across the whole band. To achieve that you need alignment, which consists basically of a manual or robotic line with a tracking filter that consists of coils and diodes. The function of the tracking filter is to get rid of the image channel and get rid of the composite power that hits the mixer oscillator.
The free running phase noise of the SL2610 is typically less than ý90 dBc/Hz (decibels relative to carrier power per Hz). That compares very favorably to the typical phase noise of ý86 dBc/Hz for the lower performance local oscillator PLLs on the market. The SL2610 synthesizer was integrated with the mixer oscillator designed for low phase-noise operation making it is a low phase-noise PLL. The noise sources generated in the PLL loop are very low, and when coupled with the design it allows a relatively high comparison frequency for a given step size. That means you can use a wider loop bandwidth with the SL2610, compared to other products, and improve the phase noise within that loop bandwidth. Another important feature about the tuner is that it can be programmed to generate all the output IFs. The reference divider of the PLL enables Zarlink to cover all the different offsets required by countries. The SL2610 is programmable and the software drivers in the STBs contain the appropriate register maps to select the required step size. The software control built into the PLL allows you to dynamically change the loop parameters as you tune across the band, so you can maintain a fixed PLL loop bandwidth. You can use the PLL to improve the in-close phase noise allowing you to keep a PLL loop bandwidth constant as you tune across the frequency range. With most LO designs that is difficult to achieve because local oscillators are varactor tuned and the characteristic of the varactor diode means that the gain of the oscillator will change, which means that the loop bandwidth also changes. The SL2610 is now in volume production. The chip is designed into SiGe and packaged in a 6 mm square MLP (miniature leadless plastic) flat pack. Evaluation boards are available. In high volumes, the SL2610 is priced at US$1.94.
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