RF Power Solutions for Digital Broadcasting
The TV broadcasting industry is undergoing a significant period of change. Across Europe, the rollout of transmitters for second-generation terrestrial digital video broadcasting, designated DVB-T2, continues. In the U.S., to address the looming spectrum crisis caused by 5G, the FCC initiated a spectrum “repack,” making the UHF frequency band above 600 MHz available for mobile use and forcing some 1200 broadcasters to change frequency channels. The overall repacking plan is based on 10 phases (see Figure 1), with staggered completion dates designed to minimize interruption to broadcasters during the transition. A large proportion of these broadcasters will upgrade their transmitters to the new ATSC-3.0 standard, so their viewers will not have to rescan channels multiple times. With the 5G rollout, other countries will also face spectrum reallocation, which will extend the demand for new, higher power and more efficient transmitters.
The RF power amplifier (PA) is a crucial component in the transmitter, as a broadcast operator’s costs depend on the PA operating efficiently over the broadcast spectrum. The performance requirements for the PA are driven by the DVB-T2 and ATSC-3.0 standards.
DVB-T2 - The European Telecommunications Standards Institute adopted the Digital Video Broadcast–Terrestrial (DVB-T) specification in 1997, which was widely deployed and led to the end of analog TV in many countries. With spectrum increasingly scarce in Europe, DVB-T was updated to the more spectrum-efficient DVB-T2 standard in 2009. Using orthogonal frequency division multiplexing (OFDM) modulation with many subcarriers, DVB-T2 is very flexible, with the advantage of enabling re-use of existing antennas. By 2014, DVB-T2 had been deployed in more than 12 countries, and the market research firm Dataxis forecasts 72 percent of European households will have access to DVB-T2 transmissions by 2022.
ATSC-3.0 - In January 2018, the Advanced Television Systems Committee (ATSC) released the ATSC 3.0 standard to support newer video technologies, such as HEVC for video channels up to 2160 pixel 4K resolution at 120 frames per second and high dynamic range Dolby AC-4 and MPEG-H 3D audio. ATSC 3.0 and DVB-T2 have many similarities, both using OFDM and offering similar performance and flexibility. While DVB-T2 is widely used, ATSC-3.0 is emerging, and the first TVs capable of receiving ATSC-3.0 transmissions are expected this year.
A typical digital TV transmission chain includes a transmitter with two components: the exciter and the PA (see Figure 2). The input to the exciter is a baseband signal, which modulates the RF carrier before it is amplified by the RF PA. Unlike the time-domain signal envelope for a modulated signal, which shows great variability in envelope peaks, the average power level of the carrier is constant, and the average transmitter output power drives the performance requirements of the TV transmitter. A typical TV station delivers 25 kW average RF power by combining multiple parallel PAs, each with a line-up comprising a pre-driver, a balanced driver and a final stage of four or more PA pallets (see Figure 3).
Since they were first introduced, LDMOS transistors have been the mainstay device technology for the PAs in the broadcast industry. LDMOS has remained the incumbent because it provides high efficiency and high power at an attractive “dollars per watt.” The output power performance of LDMOS has improved over the years, from the early days of a few hundred watts to devices capable of delivering greater than 1.5 kW.
As the DVB-T2 and ATSC-3.0 standards define OFDM signals with a high peak-to-average ratio of ∼8 dB, the PA must be designed to prevent saturation, which would cause subcarrier intermodulation and out-of-band interference. While the linearity can be achieved by backing off the power of the PA, this approach is not ideal, as it reduces PA efficiency, increasing power consumption and operating cost. Spectrum repacking adds a further design challenge for the PA: during the repacking process, a new transmitter may have to operate at a broadcaster’s old frequency before moving to the new one, so the transmitter must operate efficiently across the full UHF broadcast spectrum, from 470 to 806 MHz. The PA designer must maximize the power and efficiency across the full band.
PA design has been facilitated by two relatively recent developments in PA technology: 1) a new generation of rugged LDMOS transistors with higher power, gain and efficiency; and 2) high efficiency, ultra-wideband Doherty (UWD) circuit architectures, both symmetrical and asymmetrical.
Ampleon has invested significant resources to develop UWD reference designs for the UHF TV broadcast market (see Figure 4). Designs using Ampleon’s BLF888 LDMOS transistor, for example, deliver 150 W average DVB-T power from 470 to 700 MHz. The BLF888 family has proven successful addressing both the requirements of ATSC-3.0 and U.S. channel repacking, with many broadcast equipment manufacturers using the devices in their TV transmitters.
Anticipating future demands for more power and efficiency, Ampleon has built upon the success of the BLF888 to introduce its next-generation broadcasting PAs, the BLF989 and the BLF989E. With a DVB-T 8K OFDM signal, the BLF989 achieves the highest narrowband efficiency of 55 percent at an average power level of 200 W (950 W peak) per transistor from 470 to 494 MHz. The BLF989E achieves 180 W average power with a typical efficiency of 50 percent and covers the ultra-wide frequency band from 470 to 620 MHz (see Figure 5). The BLF989E efficiency and power demonstrate how high UWD efficiency can be achieved using an asymmetrical Doherty PA design.
DVB-T2, ATSC-3.0 and spectrum repacking provide opportunities for manufacturers of broadcast transmitters to meet the needs of TV broadcasters upgrading their systems. OFDM modulation, used by DVB-T2 and ATSC-3.0, challenge the design of the PAs in these transmitters, as higher power must be delivered with high efficiency across the entire UHF broadcast spectrum. Recent advances in LDMOS transistors and UWD architectures are addressing these needs, and Ampleon’s UWD high efficiency amplifier solutions are the most cost-effective broadcast PAs available, reflecting innovative engineering to provide the best overall efficiency, bandwidth and reliability.
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