One Exciter For All Standards The proliferation of digital standards continues to be challenging — not only for the consumer and receiver manufacturers, but also for the transmission equipment manufacturer. Indeed, at the time of writing, there are at least 11 known digital modulation standards worldwide that are either in use or soon will be. These include systems specifically engineered for fixed and mobile applications. Such standards include ATSC, DVB-T, DMB-T, T-DMB (Korea) CMMB (China), ISDB-T (Japan), ISDB-TB (Brazil), Qualcomm’s MediaFLO™, MPH, DVB-H and, on the near horizon, the DVB-T2 system. Along with the multitude of digital modulation variants, the transmitter designer must not forget that most of the world is still broadcasting analog television formats, which include country-specific variants of PAL and NTSC systems. Previous-generation exciters were mostly standard-specific and used hardware-defined modulator circuitry or limited software/firmware techniques, usually based upon the specific standard. Recent advances in exciter design have allowed the use of integrated circuits that can use software code to develop the required modulation waveforms. Along with the capability to use software-based modulation comes some very worthwhile features: a) the ability to quickly load new software, allowing
migration from analog to any digital standard, or a change from an existing standard to a new standard (e.g., from DVB-T to DVBT2) and b) the ability to automatically and continuously adjust precorrection in the digital stages of the exciter, based on the difference between the transmitted signal and the near-perfect reference signal within the exciter. Such Real-Time Adaptive Correction (for example, Harris RTAC™) provides the transmitter network operator with a system that is simple to set up and guarantees optimal performance at all times, even with varying amplifier characteristics, fluctuating AC line voltages and thermal drift of filter characteristics. By processing information from RF samples both pre- and post-mask filter, the appropriate AM/AM or AM/PM precorrection can be applied. An algorithm in the exciter system constantly monitors the correction applied and makes small adjustments, as needed, to keep the output as close to ideal as is possible. Adaptive correction, such as RTAC, allows peace of mind for the technicians, who no longer need to make such corrections manually. A simplified block diagram of the Harris software-defined exciter, Apex™ M2X, is shown in Figure 1.
Figure 1 — Apex M2X Block Diagram
Power Level Requirements A wide range of power levels is required to fulfill the varying needs and applications of terrestrial broadcasters around the globe. A few analog TV stations have transmitters with output powers of 100 kW or higher. On the other end of the scale, there are also gap-filler, transposer and low-power transmitter requirements that can be less than 10 Watts. As you can imagine, a 10,000:1 range in power level cannot easily be accommodated with one design, or a single technology approach. At least two or three basic architectures are typically required. These may be divided up into air-cooled low power (typically 10 W to 1 kW average power),
air- or liquid-cooled medium to high power (1 kW to 20 kW average power) and liquid-cooled very high power systems, often employing efficient tube technologies. For the most frequently used power range of 1 kW to about 10 kW average digital power, there are solid-state devices that can readily fulfill the requirement, providing important benefits — safety, simplicity and stability — over earlier tube-type designs. Harris’ new Maxiva™ ULX transmitter has been engineered for power levels from approximately 1 kW average power to over 20 kW average power, using the latest LDMOS FET device technology.
Transmitter System Architecture A common approach that has been used for several years is shown below in the block diagram in Figure 2.
Figure 2 — Atlas Transmitter Architecture
This architecture uses basically two active RF components: the exciter, which generates a low-level on-channel fully processed RF signal; and an array of parallel, high-gain amplifier stages. Such an approach has been used in recent Harris designs such as DiamondCD® and the Atlas™ series of solid-state transmitters. While this architecture provides excellent parallel redundancy and on-air reliability, the drawback is that the PA stages tend to be rather heavy and complex, and servicing requires skilled factory technicians or well-trained station personnel with the proper test fixtures. The high gain needed to take the low-level signal derived from the input power divider forces the use of ALC or AGC techniques, and fine adjustment is needed for overall module phase and gain to ensure optimal module combining with least loss. While such modules can be manufactured and are reliable, they can prove to be rather daunting for the typical untrained user when servicing is required.
A different approach that has already proven the test of time is the use of lower-gain building blocks for the RF amplification. An example of such architecture is the Harris® Platinum® series of TV transmitters, developed in the late 1980s. Platinum transmitters employed this type of low gain architecture, which resulted in perhaps the most successful and most popular solid-state VHF-TV transmitter ever designed, with over 1,200 units sold worldwide. Reliability and ruggedness, along with ease of serviceability, were the key attributes of the Platinum design. A modern approach using this low gain building block approach has been successfully implemented in the Maxiva ULX transmitter architecture. Figure 3 shows the simplified block diagram of the Maxiva ULX transmitter
Figure 3 — Maxiva ULX Transmitter Architecture
So as not to compromise reliability and on-air power levels during servicing and replacement of the lower-level RF stages, parallel redundancy with automatic switching is included as a standard feature. In addition, each power module comprises smaller subassemblies, which enable the technician to replace an RF pallet, RF device or AC-to-DC converters with ease and without the use of expensive test equipment. With the use of an optional module test unit, a simple test fixture can be provided for remote diagnosis of the module away from the transmitter.. Other notable features include an all-new expandable control system with built-in “life-support” control capability, which allows basic operation if the main controller is faulty. Individual plug-in cards are used for the various functions required and can be replaced easily from the front of the unit. Standard parallel remote control and Ethernet, Web GUI control are included as standard features. In multiple-cabinet versions, individual cabinet controllers are used to control each PA cabinet separately and independently, providing even further system level reliability.
Power Density And Transmitter Footprint Recent developments in LDMOS device technology have resulted in major improvements in power density, resulting in more compact transmitter designs. Solid-state designs from a few years ago could achieve about 3.4 kW average COFDM power and 10
kW peak sync analog power per 19” rack cabinet. As newer RF device technology has emerged, several manufacturers have taken advantage of the higher per-package power levels of these devices to develop transmitter power levels up to 5 kW to 7 kW average power and up to 16 kW analog peak power. To provide even higher power density, Harris, in partnership with a major semiconductor supplier, has developed a design that provides unsurpassed RF pallet power using newly developed state-of-theart LDMOS devices. Such devices are the core of Harris’ new PowerSmart™ technology. These devices are the first UHF LDMOS design to use a 50-volt structure, which results in an immediate improvement in power per device and linearity/efficiency. The devices are rated at 450 W CW power per package, which is far superior to LDMOS devices used in previous-generation Harris transmitters and current-generation transmitters from other suppliers who use 150 W to 250 W power devices. An RF pallet using a pair of the new devices can operate at approximately 180 Watts average DVB-T power — more than a 250 percent improvement in power per pallet. In the Maxiva ULX design, Harris elected to use four identical RF pallets per plug-in PA module, resulting in a very compact and power-dense module design. The overall PA module is rated at 650 Watts average DVBT power, which is significantly above the 460 Watts obtained form the Atlas PA module that used twice the number of pallets per module.
Figure 4 â€” A Compact Liquid-Cooled PA Module Design - Isometric View
Another important feature of the new 50-volt LDMOS devices is that the gain of each device is approximately 19 dB, a large boost over standard 32-volt parts that typically offer 14 to 15 dB gain per device. This increased gain works well with the Harris architecture, due to the reduced number of driver stages required. The Maxiva ULX PA component layout is shown in Figure 4.
combinations of PA modules to provide single-cabinet power levels of up to 8.7 kW COFDM, 12.3 kW ATSC, or 26.2 kW analog, providing the highest power density design currently available from any major supplier. Multiple-cabinet configurations can be used to provide even higher power levels, if required. Figure 5 shows the simplified PA module block diagram.
The Maxiva ULX transmitter can be configured with various
Figure 5 â€” Maxiva ULX PA Module Block Diagram
Efficiency And Long-Term Cost Of Ownership While there are a multitude of factors that can affect long-term cost of ownership, perhaps the most important and most often misrepresented by suppliers is overall transmitter efficiency. For digital transmitters, the efficiency can be calculated as the power out divided by the power input. RF device data sheet efficiency at CW is of little value to the customer. Actual circuit efficiency at
average digital power levels and usable crest factors are more important. The new 50-volt devices used in the Maxiva ULX transmitter can provide over 25 percent typical PA module efficiency (AC power in versus RF power out), resulting in overall transmitter efficiencies typically in the range of 20-22 percent. When compared to previous designs, this represents an efficiency improvement of up to 10 percentage points, or an improvement of 35 percent or more from the original figure.