Tuning in performance in the latest CATV STBs

As cable television (CATV) operators expand their service offerings to include video on demand (VOD), pay per view (PPV), and digital content, the CATV set-top box (STB) is undergoing a dramatic evolution. In addition to providing an analog video picture, it is now common for STBs to support multiple functions, including personal video recording (PVR) / digital video recording (DVR), picture in picture, broadband VOD, telephony, digital video, and high-definition (HDTV) content.

The multimedia nature of today's cable service offerings requires STBs to be equipped with more than one tuner. According to InStat, 38% of set-top boxes in 2005 provided PVR/DVR functionality. This means that many STBs shipping today have multiple tuners; typically this includes one or two analog video tuners and a digital data tuner as shown in the block diagram in Figure 1. Ultimately, tuner manufacturers are benefiting from this increased demand, but older technologies will not stand up to the job. In fact, new market needs are driving tuner performance specifications to improve, sending designers back to the drawing board to develop highly integrated tuners using novel approaches and advanced process technologies. In the end, OEMs and subsystem manufacturers should not consider all tuners equivalent, and should carefully select the tuners, and associated reference designs, that are right for their applications.

Figure 1. Set-top boxes contain multiple tuners for both analog and digital applications.

Packing the Cable Spectrum
In pursuit of becoming the market leader in triple play (voice, data, and video), cable providers are packing the spectrum with more services than ever before (Figure 2). Traditional tuners operate across a frequency range of 50 to 860 MHz, but plans are underway to expand some CATV systems out to 1 GHz. As a result, tuners now need to have unprecedented performance, especially in terms of linearity, dynamic range, and noise. But that's not all: as OEMs increase the functionality of their STBs, they need simpler, smaller designs that streamline manufacturing while offering high reliability. The onus is on the tuner manufacturer, then, to find the best combination of technology, design, and level of integration to satisfy OEM needs.

Figure 2. The TV spectrum requires tuners with channel selectivity and linearity to produce crisp, sharp video.

Integration: From Chipset to Single Package
Just a few years ago, the best tuner offerings were in the form of a double-conversion tuner chipset. These upconverter and downconverter chip combinations offered low noise, low distortion, and integrated synthesizers. Measuring about 6 mm by 10 mm per chip, they led the industry for years, shipping in the hundreds of millions. Recently, tuner manufacturers have greatly streamlined these designs, into a single package with a smaller footprint than even one of the chips in the older chipset. Typical single package tuners measure 8 mm by 8 mm, but some are as small as 6 mm by 6 mm. As shown by the example in figure 3, these devices combine the up/down conversion and frequency synthesizing in a single package and incorporate previously off-chip functions, such as tuning varactors and gain control.

The integration level and design complexity of the tuning function are key differentiators among tuner options. OEMs need to understand the full bill of materials (BOM), because tuners vary in their level of integration, ease of manufacture, and need for additional off-chip circuitry. For instance, a tuner with a single-ended RF input eliminates the need for an expensive off-chip wideband balun. Before selecting a tuner, OEMs should ask questions like these: Is an RF input filter necessary? Is the post-downconverter IF amplifier integrated? Is there on-chip RF and IF gain control, and is it adequate to eliminate the need for external gain control circuitry? What will my complete BOM cost be with this tuner?