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DTV combined approach

Real-world digital television (DTV) deployment solutions aim to make the best use of existing broadcast site RF hardware, such as towers, antennas and buildings. This ensures that the new DTV services can be overlaid with existing analog services in the timeliest and most cost-efficient manner.

An important tool that can help achieve this is the RF combiner. The balanced combiner has wide applications because of its modular construction and minimal interaction between inputs. (See Figure 1.) Its purpose is to combine multiple transmitter signals into a single antenna system, while keeping the transmitter isolated and properly matched. Forming an operational duo with the broadband panel array, RF combining technology has taken center stage in realizing such combined DTV solutions.

For North American DTV deployment, combiners fall into one of three site power-based categories:

  • high-power sites (2kW to 100kW), which provide coverage to cities and larger centers;
  • medium-power sites (20W to 5kW), which address regional center coverage, gap-filling and transposer applications; and
  • lower power sites (less than 250W), which are used for small area coverage, gap-filling and translator applications.

Adjacent channel challenge

Many of the advanced performance characteristics required of DTV combiners directly result from the allocation of adjacent channels for DTV broadcast. Located between existing analog channels, these gaps are the channels preferred by spectrum authorities around the world for DTV, in a quest to optimize spectrum use. In conventional analog broadcast applications, interference problems previously rendered these adjacent channels unusable. The advent of digital broadcast has liberated adjacent channels, but at a price. To realize contiguous channels, complex masking filters — essentially brick wall filters — are required to minimize out-of-band products.

The issue of site space constraints, particularly in high-density urban areas, is another practical problem confronting broadcasters in many parts of the world. Most DTV retrofits aim to realize the extra channels within the space constraints of existing building leases. The pace of the worldwide DTV rollout demands higher levels of flexibility and modularity in filter/combiner application. Combiner filters that are tunable — ideally across the entire UHF band — have proven to be a major step in this direction.

Early work focused on the high-power areas (2kW to 100kW). Waveguide coupling technology was obviously the preferred method to accommodate such power levels. Conventional waveguide UHF combiner systems posed limitations because they were large and bulky, and capable of handling less than 20 percent frequency span, which limited the scope for future channel additions outside this span.

Also, adjacent channel combining demanded waveguide filters as complex as eight-cavity (eight-pole). These could be as tall as 13ft, which is often too large for many rooms. To overcome these problems, a directional waveguide combiner — a waveguide filter technology often used in multichannel multipoint distribution systems (MMDS) — can be used. Several innovations were required to convert the directional waveguide combiner for UHF use, and many of these are patent-protected. In this arrangement, the two perfectly matched filters in a balanced combiner are effectively realized in a single circular waveguide assembly. Similarly, careful design of the wideband waveguide path (the spine) has provided dramatic improvements in frequency span.

Cross-coupling between cavities has helped realize eight-pole performance while using the shorter six-pole assembly. The result is a particularly compact waveguide filter/combiner, which is about half the size of conventional waveguide combiners, yet offers a frequency span of around 46 percent. This allows two models of the directional waveguide filter to accommodate the entire UHF frequency range.

This compact waveguide filter/combiner technology is now being used across North America, Europe and Australia, where larger power broadcast sites are more common. The most notable application of this technology is the combining of 14 high-power channels for three antenna systems on the Sears Tower in Chicago. Using directional waveguide combiners, this was achieved in a room with a footprint of 23ft × 23ft.

Medium-power focus

The medium power (20W to 5kW) combining applications have been an area of important development. Such powers are the regime of the more compact all-coaxial filter solution. As with the waveguide combiner, the challenge is to realize a higher order coaxial filter in the most compact total package.

External cross-coupling is an important design element. By applying a network of cross-coupling paths, it is possible to create an elliptical function filter in six- and eight-pole options for adjacent channel applications, plus three- and five-pole Chebychev variants for conventional wide and semiadjacent channel combining.

The cross-coupling paths are carefully tuned to produce notches or cross-coupling zeros in the filter characteristic. (See Figure 2 on page 62.) These provide the sharp masking filter response required for DTV semiadjacent and adjacent channel applications.

External cross-coupling is preferred over conventional folded configuration. This permits the development of a purely in-line configuration, comprising long slender filter assemblies, with the input coupler at the base and the output coupler at the top. This in-line configuration permits a single 34in × 43in rack (see photo on page 62), to accommodate up to six channels, in a mix of three-, five-, six- and eight-poles. Today, filters are often ordered in advance of final frequency assignments. This allows channels to be added in after design completion. Filters are retired at one site and redeployed at another.

Combining for LPTV

Recent advances have grown out of the need for innovative combining solutions for low-power site applications (less than 250W). At such sites, the combining equipment must be much smaller and correspondingly lower in cost, while still retaining maximum performance. These technologies will play an important role in the imminent LPTV digital conversions.

By applying advanced manufacturing techniques, founded on an integrated coupler architecture, a new generation of highly compact low-power combiner systems are now available. Tuneable across the entire UHF band, these low-power combiners offer cost-effective and readily-customized products optimized for the 20W to 250W power range. They typically have a footprint of 5in × 12in and allow up to 12 channels to be accommodated in a single 19in rack. They come in a range of mounting options, including rack, wall and ceiling mounts.

Solutions addressing the technical, economic and practical limits of DTV are evolving quickly and will continue to challenge the bounds of RF combining know-how. These developments will be driven by broadcasters' demands for higher performance, more compact and lower cost combining technology. BE

Mick Bennett is the global product manager, broadcast and defense systems, for Radio Frequency Systems (RFS).