Recent advances in LDMOS device power density and performance have contributed to significant improvements in UHF power amplifier linearity, efficiency and reliability. Such devices can be applied to broadcast transmitter designs for terrestrial transmission of traditional television and multimedia content, including analog and all digital worldwide standards. The challenge for broadcasters is to provide these services at high quality in the most cost-effective and efficient manner.
Harris has developed a solid-state transmitter based on its patented PowerSmart architecture that allows terrestrial broadcasters to transmit an array of local, over-the-air broadcast services at various power levels. The architecture leverages these new advances in LDMOS technology and other design initiatives to provide a multitude of benefits that equate to a more efficient and cost-effective operation at any power level. The new transmitter design is available to UHF broadcasters through its Maxiva range of liquid- and air-cooled transmitters.
The transmitter platform has been engineered for power levels from about 10W to more than 20kW average power. With the PowerSmart architecture as its backbone, the transmitter provides significant space savings through a compact transmitter footprint. In addition, the architecture provides a sharp reduction in operating costs through improved efficiency, a green design that is compliant to industry standards and future-proof software-defined modulation techniques to address multiple broadcast standards.
LDMOS advances
The transmitter employs a low-gain building block approach. Each power module comprises smaller sub-assemblies to replace an RF pallet, RF device or AC-to-DC converter with ease and without the use of expensive test equipment. A simple test fixture can be provided for remote diagnosis of the module.
The LDMOS device technology at the core of the architecture delivers the power density improvements in the transmitter. Recent solid-state designs could achieve about 3.4kW average COFDM power and 10kW peak sync analog power per 19in 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 5kW to 7kW average power and up to 16kW analog peak power.
To provide even higher power density, the company, in partnership with a major semiconductor supplier, has employed state-of-the-art LDMOS devices to provide additional RF pallet power. These UHF LDMOS devices use a 50V structure, which results in an immediate improvement in power per device and linearity/efficiency.
Benefits
The 50V devices are rated at 450W continuous wave (CW) power per package — superior to LDMOS devices used in previous and current generation transmitters that use 150W to 250W power devices. An RF pallet using a pair of the new devices can operate at an 180W average DVB-T power — a 250 percent improvement in power per pallet.
The transmitter uses four identical RF pallets per plug-in PA module to create a compact and power-dense module design. The overall PA module is rated at 650W average DVB-T power, which is significantly above the 460W obtained from a previous Harris PA module that used twice the number of pallets per module. Maintenance is also simplified, with field service based on maintaining individual power supplies and pallets versus entire modules. Broadcasters installing Maxiva transmitters from both the liquid-cooled (ULX) and air-cooled (UAX) series have the advantage of sharing spare parts because both designs use the same internal circuitry and technology.
Another important feature of the new 50V LDMOS devices is that the gain of each device is about 19dB, a large boost over standard 32V parts with typically 14dB to 15dB gain per device. This increased gain reduces the number of driver stages required, with power amplification handled in a single device as opposed to using a series of lower power RF devices for staged amplification.
Design features
The single-stage power amplifier minimizes parts in the transmitter, which drastically reduces the transmitter footprint — a key tenet of the PowerSmart architecture. All Maxiva transmitters also incorporate cooling designs that enhance the compact nature. The efficient, liquid-cooled design of the ULX range transfers heat generated within the transmitter to a built-in cold plate with liquid before being directed outdoors. This puts less stress on the AC system, as the heat is passed directly outside, and reduces the cost of cooling the transmitter facility.
The low-power UAX transmitters use the same RF pallets, but the pallets are mounted instead on a standard heat sink and cooled by fans providing clean filtered air. The efficient design uses fewer modules, each producing less heat than previous designs.
The architecture also enables a simple analog-to-digital upgrade path for international broadcasters through the use of Harris Apex exciter technology. All ULX transmitters integrate the Apex M2X exciter — a software-definable platform that enables analog broadcasters to easily transition to digital in the field and enables multichannel broadcasting of HDTV, DTV and mobile TV channels.
Green improvements
The transmitters incorporate several green design initiatives to minimize the environmental impact of the transmitter. The architecture addresses European regulations such as Waste Electrical and Electronic Equipment (WEEE) and RoHS. The reduction in AC power requirements reduces greenhouse gases emitted at the power generation plant.
The green design initiative carries over to the operational efficiency of the transmitter. 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 the overall transmitter efficiency. The new 50V devices provide more than 28 percent typical PA module efficiency (AC power in versus RF power out), resulting in overall transmitter efficiencies typically in the range of 22 percent to 25 percent. This represents an efficiency improvement of up to 10 percentage points compared to previous designs, or an improvement of 35 percent or more from the original figure.
The range of liquid- and air-cooled transmitters represents a shift in transmitter design for UHF broadcasters. The PowerSmart architecture is the basis for this evolution, lowering the cost of ownership for UHF broadcasters over the course of the transmitter life through a compact and energy-efficient design.
Martyn Horspool is the television product manager for Harris Broadcast Communications.
