Dual-mode MSDC IOTs

The future of broadcast TV is digital, but the future isn’t yet here. There are a considerable number of digital TV stations operating in the United States, but the broadcasters who operate them rely almost exclusively on their analog signal for revenue.

Ai’s e2v ESCIOT five-stage depressed-collector IOT installed in a high-power amplifier cabinet. This dual-use tube is designed to maximize efficiency and minimize costs in both analog and digital operating modes.

It’s no surprise that many broadcasters are taking another look at their analog transmission plant to ensure that the engine for their major revenue stream will remain viable for what now appears to be an indeterminate number of years. In many cases, that analog transmission plant uses aging technology that is expensive to operate and maintain. In addition, spare parts and replacement tubes are becoming increasingly difficult and expensive to obtain. Older transmitters also lack modern safety features, perhaps exposing station personnel to unsafe conditions, and exposing the station itself to considerable liability. Replacing an aging analog plant with a modern inductive output tube (IOT) transmitter using the latest multi-stage depressed-collector (MSDC) technology could prove to be a wise investment for many broadcasters, particularly if the new transmitter can be converted easily to digital service in the future.

Recent developments
In the last 10 years, developers have made significant advances in transmitter technology in the areas of pre-correction, control and monitoring, solid-state amplification and high-power linear amplification, to name a few. There is no fundamental reason why broadcasters cannot apply these advances to analog transmission. This would offer immediate benefits in terms of reduced power and maintenance costs, improved reliability, increased available building space and enhanced safety. In some cases, the resulting power savings alone in a new plant can pay back the investment in less than five years. For the several hundred stations that still need to build out full-power digital facilities, new analog transmitters offer the added benefit of being readily convertible to digital service at the appropriate time.

Table 1. Electrode currents in a five-stage depressed-collector IOT during digital and analog operation. Transmitter designers must consider these differences when designing dual-use transmitters. Click here to see an enlarged diagram.

The advent of IOT technology in the early 1990s truly enabled the adoption of common amplification as the dominant amplification mode in broadcast transmitters. This technology allowed transmitter designers to eliminate major differences between equipment optimized for analog service and that optimized for digital service, giving the broadcaster maximum flexibility to accommodate future requirements.

The design of a high-power UHF TV transmitter is so strongly influenced by the choice of the final amplifier device that selecting that device can become the determining factor in achieving the goal of dual use. For the past 10 years or so, the IOT has been the dominant choice as the final amplifier in high-power UHF transmitters. It is ideally suited to amplify signals of any TV format. The newest range of high-power IOT transmitters adds the field-proven energy-saving feature of collector depression to the already efficient IOT. Major tube suppliers offer several multi-stage depressed-collector (MSDC) IOTs, with differing numbers of depressed stages and cooling methods. It appears that the best compromise between efficiency enhancement and practical collector manufacture results from collectors with five individual stages, as was the case with the MSDC klystron.

Most developers of MSDC IOT transmitters have focused their efforts on DTV performance. But it has become clear that analog TV still has a primary role to play in maintaining station profits, so developers have re-focused their efforts to enhancing the performance of these new devices in analog service.

Table 2. In a typical dual-use transmitter configuration (i.e., two IOT high-power amplifiers in parallel), an ESCIOT uses less power than a standard IOT, in both digital and analog operation. Click here to see an enlarged diagram.

Dual-use concerns
The typical high-power IOT amplifier can provide a power level of 25- to 30kW average in DTV service and 60- to 70kW peak sync in analog service. Conventional IOT devices with these power levels have been available for several years. In fact, the physical structure of the IOT and elements such as the electron gun were primarily designed for the demands of analog service, before they proved to be more than adequate for digital applications. But reversing this approach and optimizing the IOT elements for digital service can compromise the tube’s analog power-handling capability.

The defining difference between conventional and depressed-collector IOTs is the collector structure. The principle function of any collector, depressed or otherwise, is to dissipate the “spent” energy of the electron beam efficiently and safely under all signal conditions. Since the spent electron beam has a distribution of energies, the MSDC’s multiple collector stages, which are set at discrete and different voltages with respect to ground, can “sort” the electrons by energy and recover a significant percentage of the electron beam’s energy, thus reducing power consumption. A DTV signal distributes the electrons in a fairly well-defined and effectively static manner over the internal surfaces of the collector. This predictable electron distribution allows designers to select the collector stage voltages to distribute the spent beam relatively evenly, in terms of electron current, thus reducing the average power density and making cooling the tube easier. But a collector designed for these static DTV currents is not suited for full-power analog service. The NTSC signal is amplitude-modulated, and the energy spread in the spent beam is significantly different at different picture levels. Amplifier designers must account for this effect when designing the collector, the cooling system and the high-voltage power supply for dual-use tubes.

Figure 1. A simplified schematic of the high-voltage connections for the five-stage ESCIOT shows that it uses only two additional outputs from the power supply. Click here to see an enlarged diagram.

Table 1 shows the currents at each electrode in a five-stage depressed-collector IOT under digital and analog signal conditions. (The stage-1 electrode is at ground potential and the other electrodes are increasingly more negative with respect to ground.) APL is average picture level.

Consider the consequences of a transmitter designed with maximum ratings based on the collector electrode current in the “Digital – 30kW” column in Table 1. To ensure that the tube’s ratings are not exceeded in analog service, the maximum analog power rating of the IOT and transmitter would need to be reduced to well below that of an equivalent non-depressed-collector IOT.

Therefore, amplifier designers should follow the proven principle of designing for the worst case — in this case analog — when designing a transmitter for true dual-use applications.

MSDC IOT economics
The ultimate purpose of the depressed collector is to reduce the broadcaster’s power costs and thus enhance profits. Table 2 compares the performance of a standard IOT with that of a particular type of five-stage depressed-collector IOT called an energy-saving-collector IOT, or ESCIOT, in a typical transmitter configuration (i.e., two IOT high-power amplifiers in parallel).

Two things are worth noting from Table 2. First, the ESCIOT tube is capable of full-power operation in both digital and analog modes. Second, contrary to some manufacturers’ recent claims, this depressed collector reduces power consumption by the same percentage whether operating in digital or analog mode. In other words, this tube can help broadcasters minimize product differences for analog and digital operation. Of added interest to the transmitter designer and the end user, these improvements are obtained by using only two additional high-voltage outputs from the high-voltage power supply, since two of the five ESCIOT collector stages are actually connected to other stages. Figure 1 shows a simplified schematic of the tube’s high-voltage connections.

Broadcasters can achieve efficiencies equal or close to those in Table 2 at lower output powers by matching the tube’s beam voltage to the required power, in accordance with the tube manufacturer’s recommendations.

Table 3. Using a five-stage ESCIOT in an analog transmitter, as opposed to a standard klystron can provide significant savings. Click here to see an enlarged diagram.

Analog performance
Although an ESCIOT-equipped analog transmitter offers 30 percent lower power consumption than the standard IOT transmitter, it is unlikely that this would be considered sufficient savings to justify the replacement of relatively new NTSC IOT equipment, since most IOT transmitters are less than 10 years old. But there are a significant number of aging klystron transmitters still in use, and replacing these transmitters with a new ESCIOT-equipped product is a viable option. Table 3 compares the costs incurred when using a standard klystron with those incurred when using a five-stage ESCIOT in an analog transmitter.

Table 3 assumes that the klystron transmitter operates in a non-pulsed mode. If the transmitter operates in a pulsed mode, the savings would be less than $198,747, but would still exceed $125,000 per year.

Other dual-use transmitter components
So far, we have discussed the IOT — perhaps the most costly component of the high-power transmitter — in some detail, particularly in relation to minimizing the difference between analog and digital service. But what of the other critical components? Exciters that require only the exchange of one module to convert from analog to digital service are now available. Furthermore, at least one exciter has a front-panel interface that allows the user to set it to any UHF channel without having to change any components or tune RF circuits. The IOT driver amplifiers for this exciter are fully broadband, allowing operation on any UHF channel.

The NTSC signal will probably remain a TV station’s main source of revenue for several years to come, so replacing aging high-power UHF transmitters now might be a prudent move for some broadcasters. Recent changes in the tax law pertaining to accelerated depreciation may provide further financial incentive to make this move, increasing power savings and enhancing bottom-line benefits for the station. Modern, water-cooled MSDC IOT-equipped transmitters offer many broadcasters immediate benefits in either digital or analog service and provide maximum flexibility for future conversion to DTV service.

Andy Whiteside and Ray Kiesel are engineering vice presidents at Ai.

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