Over the last decade, the desire to contain capital expenditure has driven decision-making on network investments, and this has led to the deployment of many inefficient networks in Europe. But in recent years, environmental requirements and the cost of energy have become key factors in the operational expenditures and total cost of ownership for broadcast operators. In some networks, operating expenditures can be up to 85 percent of the total cost of network ownership. Meanwhile, energy prices continue to rise due to mounting pressure on energy sources. The result is that the electricity bill is now by far the largest and fastest-growing component of the operating expenditure for a transmitter network.
Historically, transmitter efficiency has relied on transistor technology, which has enabled small incremental gains in efficiency, rising from 15 percent in the 1990s to 24 percent today. Transmitter manufacturers have driven the market for higher and higher power density without considering the total reliability of the amplification system, which is mostly achieved by controlling the operating temperature of the transistor junction. But what if an amplification technology could save a great deal more energy while at the same time improving the total reliability of the transmission system? This would definitely provide an answer to broadcasters' concern in their effort to reduce the total cost of ownership of their system. Broadcasters equipped with this technology would have clear advantage in the fierce business competition between broadcasters and telecommunications operators.
A drain modulation or envelope-tracking power amplification system is a solution to the problem. Thomson Broadcast announced its Futhura Plus transmitter, which is based on drain modulation technology, at IBC2011. The underlying principle of drain modulation has been well known in the radio industry for shortwave transmission since the 1980s. The drain modulation principle uses the envelope signal to dynamically control the power supply of the transistor. (See Figure 1.) This avoids the efficiency penalty that arises with Class AB amplifiers at low amplitudes. The drain modulation technique is wideband and is applicable to TV broadcast, as well — a clear advantage over other high-efficiency techniques, such as narrowband designs that combine a Class AB stage with a Class C stage.
The biggest challenge of the drain modulation technique is to tightly control at high speeds the power supply of the transistor in order to make it suitable for wideband application. The basic principle is to adapt the power delivered by the drain modulator to the needs of the power amplifier. (See Figure 2.) When the envelope modulation signal of the power supply provided by the drain modulator corresponds to the signal envelope, it saves the energy that would have been dissipated by the power amplifier without the drain modulation. (See Figure 3.)
The best gain in efficiency with drain modulation is obtained when applied to OFDM transmission systems (e.g., DVB-T/T2, ISDB-T, DAB). In this case, drain modulation enables a tight envelope control for signals such as OFDM having a larger peak to average ratio than single-carrier transmissions (e.g., ATSC).
Beyond efficiency, this technique adds high-speed electronics in the power amplifier design that help to control envelope overshoot at the transistor level that arises in any OFDM transmission, and improves OFDM transmission reliability. The other benefit is to reduce the operating temperature of the LDMOS transistor by 50° C. As a consequence, the lifespan of the transistor is multiplied tenfold. (See Figure 4.) Moreover, reducing the operating temperature of the RF transistor also reduces the burden on the cooling system. As less effort is required for cooling the power amplifier, the cooling system can be scaled down in terms of footprint and liquid cooling pressure for the same output power. In addition, with fewer hot spots in the design, the temperature is more continuously spread over the amplifier sink. The cooling system's power consumption represents roughly 5 percent of the entire transmitter's power consumption, and this can be reduced by 20 percent, providing another 0.4 percent in the total system efficiency.
Class AB with pre-correction remains the state-of-the-art modulation choice for digital V/UHF TV transmitters today. Although feed-forward represents an alternative with an overall efficiency of 10 percent to 15 percent, digital pre-distortion remains the most efficient solution, showing figures between 20 percent to 25 percent.
Drain modulation makes it possible to improve transmitter efficiency by 50 percent compared to traditional Class AB fixed-drain amplifier technology. The total transmitter efficiency including ancillaries rises to 35 percent power efficiency, excluding any other gains from advanced signal processing techniques.
Drain modulation is completed by digital adaptive pre-correction techniques and signal processing techniques called peak to average power reduction techniques, bringing additional efficiency to the system. For instance, the usage of tone reserved carriers in DVB-T2 transmission system could help to obtain an additional 2 percent efficiency over the total system.
If we consider the total system efficiency, including savings, in the liquid-cooled system and signal processing, the overall efficiency could go up to 37 percent.
A reliable system
In the industry, RF specialists did not envision drain modulation being used by high-power amplifiers before 2013 due to the complexity of high-speed electronics near the power section located in the power amplifier. The Futhura Plus is one of the first commercial products integrating drain modulation associated with a standard 50V LDMOS transistor to operate in drain modulation mode as well as fixed drain.
To achieve the optimum transistor capabilities in drain modulation mode, it is necessary to adopt a special doping process in order to guarantee stable transistor operation for the range of nominal power in the amplification chain. The transmitter and the forthcoming family of products sharing the technology are the result of an extensive R&D effort to make a reliable system and provide an innovative solution to operate a drain modulation system for a parallel amplifier design.
To achieve this level of efficiency, the latest transistor 50V LDMOS technology was used in combination with drain modulation techniques and a strong partnership with a transistor manufacturer. All this combined work has produced a standard transistor, which is available on the open market for use by other transmitter designers.
In the transmitter's design, the pallet is standard and wideband, enabling a quick and easy spares policy. Beyond drain modulation, the transmitter implements processing techniques related to the DVB-T2 standard such as peak-to-average power ratio (PAPR) Tone reserved techniques, enabling an additional 2 percent efficiency improvement on top of the 35 percent system efficiency gain.
Advanced transmitter techniques such as these enable low maintenance, while efficiency techniques reduce the energy bill over the transmitter life cycle. Besides helping to reduce the carbon footprint of the transmitter network, the annual savings per 5kW rms digital DVB transmitter ranges from E3,500 to E12,000 per year based on current energy costs.
Jerome David is strategic marketing manager for Thomson Broadcast.
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