By Alan Hutton, Walter sneijers, herm titulaer and houssem schuick
The broadcasting industry is in a period of fundamental change. In Europe, digital video broadcasting, the second generation terrestrial (DVB-T2) transmitter, continues to be launched. In the United States, the Federal Communications Commission (FCC) has taken action to solve the imminent spectrum crisis caused by the arrival of 5g, and launched a spectrum “reprogram” process, forcing many broadcasters to change transmission channels. At the same time, with the emergence of the new standard atsc-3.0, most of these broadcasters are likely to upgrade their transmitters and transfer to new channels to solve the problems caused by multiple changes at one time and ensure that their customers do not have to redesign many times.
With the continuous introduction of 5g, other regions will have to face the problem of spectrum allocation. These changes will drive the demand for new higher power and higher efficiency transmitters for many years to come. Since the cost of broadcast operators depends on high-power amplifiers that can operate efficiently in the whole UHF broadcast spectrum, radio frequency power amplifiers (RFPA) are a key part of signal transmission system.
This paper will discuss the current market trends in detail, discuss the technical impact of these changes, and analyze some challenges and available solutions for designers engaged in high-power and high-efficiency RF power amplifier solutions for digital broadcasting.
Current development trend of broadcasting industry
At present, the three main factors driving the continuous transformation of the global broadcasting industry include the continuous introduction of DVB-T2, the emergence of atsc-3.0 and the “re planning” of TV spectrum in the United States and other regions.
The European Telecommunications Standards Institute has adopted a set of digital broadcasting standards. The digital video broadcast terrestrial (DVB-T) specification released in 1997 has been widely deployed all over the world, and has promoted the closure of analog broadcasting in many countries. With the increasing scarcity of European spectrum, DVB released an updated DVB-T2 standard with higher spectral efficiency. By using orthogonal frequency division multiplexing (OFDM) modulation with a large number of subcarriers, DVB-T2 is a very flexible standard with additional advantages such as reusing existing antennas. DVB-T2 was originally released in 2009 and has been deployed in more than 12 countries by 2014. Market research firm dataxis predicts that 72% of European households will be able to transmit TV signals using DVB-T2 standard by 2022.
The ATSC series marked another important milestone in the development of ATSC broadcasting system in January 2018. ATSC 3.0 contains about 20 standards and aims to support some new technologies such as video channel hevc with 120 frames per second and up to 2160p 4K resolution, high dynamic range, Dolby AC-4 and mpeg-h 3D audio. ATSC 3.0 and DVB-T2 have many similarities. They both use OFDM modulation and provide similar performance and flexibility. However, DVB-T2 has been widely used, while atsc-3.0 has just appeared. The first batch of TVs that can receive atsc-3.0 TV signal transmission are expected to appear in 2020.
Spectrum re planning
In 2012, because it is expected that valuable radio spectrum will be scarce in the future, the U.S. government authorized the Federal Communications Commission to encourage radio and television companies to abandon some spectrum. In the initial 470MHz ~ 860MHz broadcast spectrum, the high-frequency band above 600MHz has been available for mobile wireless operators. At the same time, the launcher has been designed for the new atsc3 0 standard is ready. In order to start this process, the Federal Communications Commission conducted the first auction and ended in March 2017. As a result, 84 MHz spectrum was vacated from channel 38 to channel 51, and then 70 MHz was sold to wireless operators in 10 MHz frequency bands. With the end of the auction, the Federal Communications Commission estimated that about 1200 television stations would be affected by the process, and it would take about three years for each station to enter a new lower frequency band. The overall spectrum re planning is roughly divided into 10 stages (see Figure 1), with staggered completion dates, in order to minimize the interference to broadcasters during the transition period.
Figure 1: FCC’s spectrum rescheduling agenda. (source: FCC)
Spectrum re planning will require many TV stations to change their transmission frequencies, which requires careful planning. If the TV station needs to transmit with two frequencies at the same time for a period of time, it needs a second transmitting antenna, which will have a chain effect on the signal transmission tower, other Co located antennas, HVAC and other systems. In many cases, broadcasters may find it more economical to replace existing transmitters, especially if the original transmitters are old and cannot support atsc-3.0 due to power or other limitations.
Because operators have exhausted their frequency resources, the transition from the current situation to DVB-T2 and atsc-3.0 and the re planning of spectrum will lead to the extensive replacement of TV transmission equipment in Europe, the United States and the rest of the world.
Key considerations of power amplifier in TV transmitter
A typical digital TV transmission chain includes a transmitter, which includes two basic components, exciter and RF power amplifier (PA) (see Figure 2). The input of the system is a baseband signal. Through the baseband signal, the RF carrier is modulated in the exciter, and then amplified by the RF power amplifier (PA) unit. In contrast, the average power level is constant. The average output power (TPO) of the transmitter determines the performance of the TV transmitter system.
Figure 2: a typical digital TV transmission device. (source: ampleon)
RF Transistor Based on LDMOS (transverse diffusion metal oxide semiconductor) has become the main solution of various power amplifiers since it was first introduced, especially in the broadcasting industry. There are two different explanations for this fact. One is considering the high efficiency and high power advantages of LDMOS process, and the second is the costs per watt. LDMOS based transistors can provide a cost-effective solution.
A typical transmitting station can provide an average RF power of 25kW. This is achieved by integrating four or more amplifier trays (including multiple amplifiers), balanced drive stage and front stage amplifiers and RF power in the transmission link unit (see Figure 3). In the past few years, the emergence of such high-power LDMOS RF transistors has promoted the paradigm shift of high-power amplifier performance. From the early hundreds of watts, we can now see RF components that can handle more than 1.5KW power. In fact, these transistors are rapidly becoming leaders who can meet high-power and efficient transmission, as well as extreme durability standards in the RF industry.
Figure 3: a typical power amplifier unit. (source: ampleon)
The advanced DVB-T2 and atsc-3.0 standards use OFDM modulation signal, which will affect every part of the transmission chain, especially the RF power amplifier, because it requires a peak to average ratio (PAR) of about 8dB to prevent saturation in the power amplifier, otherwise subcarrier mutual harmonic and out of band interference will occur. The above problems can be solved by reducing the backing off power in the power amplifier, but this will reduce the efficiency, affect the power consumption, directly affect the power consumption and increase the operation cost.
Therefore, the challenge for RF power amplifier designers is to find the best balance between power and efficiency, and design an amplifier with high efficiency under various working conditions. Since many new transmitters deployed during spectrum re planning need to operate on the previous frequency of the broadcaster before they can be transferred to the newly designated frequency, the RF power amplifier must be able to work efficiently in the whole UHF broadcast spectrum from 470 MHz to 806 MHz.
Current and future RF power amplifier solutions for DVB-T2 and atsc-3.0
Two relatively new advances in RF power amplifier technology can help designers complete design tasks more easily: proven successful
High efficiency ultra wideband Doherty (UWd) architecture, including symmetric and asymmetric, and a new generation of high-power LDMOS transistors, provides unprecedented durability and optimal gain and efficiency.
As an important global provider of power solutions for the broadcasting industry, ampleon has invested a lot of resources in developing UWd reference designs. These solutions show how to use blf888 (see Figure 4) to provide 150W average DVB-T power in the whole spectrum range of 470 ~ 700 MHz. Blf888 transistor series has been very successful in the market. In order to prepare to meet the needs of the US “spectrum re planning” program and the launch of atsc-3.0 standard, many broadcast equipment manufacturers use these devices in their TV transmitter solutions.
Figure 4: ultra wideband Doherty designed with blf888e. (source: ampleon)
Based on the market success of blf888 and the market expectation of higher power and higher efficiency in the future, ampleon recently launched the next generation of broadcast amplifier products blf989 and blf989e RF power amplifier s. Blf989 can achieve the highest narrow-band efficiency of 55% under dvb-t8k OFDM signal. The extreme average power level of each transistor is 200W (950 W Peak), covering the range of 470MHz ~ 494mhz. Blf989e (see Figure 5) can provide an average power of 180W, with a typical efficiency of 50%, and can cover the ultra wideband of 470MHz ~ 620mhz.
Figure 5: UWB Doherty with blf989e (source: ampleon)
The blf989e efficiency and power versus frequency curve (see Figure 6) demonstrates how to achieve the highest UWd efficiency through a special asymmetric Doherty structure design. These new UWd high efficiency amplifier solutions represent the most cost-effective broadcast RF power amplifier in the market today. Through unique innovation, the highest efficiency, bandwidth and reliability can be ensured.
Figure 6: blf989e efficiency and power versus frequency curve. (source: ampleon)
The launch of DVB-T2, the emergence of atsc-3.0 and the re planning of spectrum require broadcasters to upgrade the equipment system, which provides huge business opportunities for TV signal transmission equipment manufacturers. This market will continue to grow as all regions of the world have to solve the problem of spectrum scarcity. Because higher power and higher efficiency must be balanced in the whole UHF broadcast spectrum, the advanced modulation schemes (such as OFDM) used by DVB-T2 and atsc-3.0 pose special challenges to the design of RF power amplifier, which are being met by the latest progress of high-power LDMOS transistor and UWd architecture. The market demand for higher power and efficiency is not expected to decrease soon. The subversive development trend of the broadcasting market will continue to promote the LDMOS price / performance curve, but without sacrificing quality. As the world’s leading manufacturer of power transistors in the broadcasting industry, ampleon is fully ready to support its partners in the broadcasting industry ecosystem. Ampleon’s flagship product blf888 series transistors have played an important role in providing a wide range of solutions for the industry. In the face of the market’s growing demand for higher power and higher efficiency, blf989 and blf989e represent the future development roadmap.