At this year's NAB show, the most noticeable feature of the new television transmitters was their close resemblance to last year's models. There were differences, to be sure, but they were not earthshaking. Instead, they were smaller, sophisticated changes in design and operation.
Low-power DTV transmitters were abundant. Many stations have started broadcasting their DTV signals with low-power systems to provide immediate service to the city of license with the intention of upgrading to higher power to cover a larger area at a later date. Some good low-power transmitters have been introduced as a result. For example, Harris' Ranger series of DTV transmitters offer power up to 1kW. Their power level is low, but the quality and features on these transmitters are right up there with their big brothers. They use the same CD-1A exciter, produce the same signal as high-power systems, and have full adaptive correction.
Nearly all manufacturers now offer solid-state transmitters, most with power levels up to 10kW. They do this simply by combining multiple plug-in amplifier modules. Some manufacturers, such as Harris and Thales, offer higher-power transmitters by providing higher power from each module. But, in attempting to increase the power of solid-state power modules, they run into the same old problems — primarily, cooling. The smaller the module, the harder it is to cool. Some manufacturers have solved this problem by liquid cooling each individual module.
As with solid-state systems, cooling continues to be a headache for makers of IOT transmitters. For a better understanding of transmitter cooling, you can find an technical paper at www.axcera.com that covers the subject in great detail. Axcera uses both liquid and air cooling in its transmitters, but the technical paper also briefly discusses the use of oil. Most other manufacturers primarily use air cooling in their solid-state transmitters and liquid cooling in their IOT systems. Furthermore, most use what can be described as a single-loop cooling system. This uses a single liquid to cool the amplifiers. The system routes the liquid (usually a glycol-water mix) through an external heat exchanger to lose the heat to the outside air.
Thales now uses an oil-based cooling system for its MSDC-IOT devices. This application uses a relatively small amount (about 10 gallons) of synthetic oil similar to that used for cooling transformers. After flowing past the MSDC IOT, the oil passes through a heat exchanger that transfers the heat to a glycol-water mixture, which is cooled in the conventional manner through an external heat exchanger. This type of system offers several advantages over other methods. First, the oil is an efficient coolant, so only a small amount can be used to easily transfer the heat to the glycol and water. Second, the small amount of oil involved is easy to filter. Third, unlike de-ionized water, the oil does not permit electrolytic corrosion. In addition, many broadcasters are purchasing new transmitters to replace existing systems. Those existing systems usually have heat exchangers in place to deal with water-glycol coolants. Those same heat exchangers can be used with the new transmitter, saving the station a considerable amount of money.
So, which is the best cooling system? There is no single answer. Transmitter designers pick a cooling system and include the requirements of the system in their overall transmitter design. Is one better than the rest? Probably not. At least, there's probably not enough difference in performance from one cooling system to the next to make it the deciding factor when you are selecting a transmitter, but it is one factor to consider.
The only setback that comes to mind is that inevitable day when the broadcaster will have to replace the coolant. Dumping many gallons of water into the transmitter building would be a real mess. Mixing that water with glycol would be even less pleasant. But 10 gallons of transformer oil running across the floor would be more than one could tolerate.
Adaptive correction, remote control
Another transmitter advancement is adaptive correction, which nearly all transmitters now use. The miracle of microprocessors has completely changed exciter design. The new exciters can correct almost any fault, nonlinearity or distortion in the amplifiers. The better systems also provide extensive displays, available either by modem or the Internet, to advise operators of the system's performance and allow them to adjust it extensively from the comfort and convenience of their family room. Even more significantly, the networking systems currently available allow the director of engineering to check and adjust all the transmitters in the system across the country in the same fashion. Raise/lower arrows on the keyboard have replaced the old knobs and cranks. At least the station staff usually knew what was going on when they turned a crank. When the computer does it, it's more like some magical incantation.
The other area of real change is in the development of DTV systems for television boosters or translators. Digital television offers some new possibilities for translators. Again, the reader is referred to the Axcera Web page for more information on this subject. The company's engineers have done some interesting work in the translator area. Perhaps the most significant point is that digital translators can be chained together for 40 or more consecutive hops. Analog systems degenerate to noise with bad audio after three or four hops. But digital systems clean up the received signal with error-correction circuitry at each receiver. The signal that is sent forward is then as clean as the original.
Don Markley is president of D.L. Markley and Associates, Peoria, IL.
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