50-V LDMOS Portfolio Expands To Ease Transition to Digital Broadcasting
By Barry Manz, Contributing Editor

Amid the press coverage about RF power transistors and amplifiers, devices destined for the broadcast market, which increasingly relies on LDMOS FETs to power transmitters from AM through UHF TV, don’t get much “play”. That’s not because there are no advances taking place, but more likely because it is out of the mainstream wireless market. Freescale Semiconductor has been rolling out more and more devices that are giving broadcast transmitter manufacturers greater ability to meet market needs while controlling costs. The MRF6VP3450H is the latest of these devices, delivering 450 W peak power at P1dB with 50% efficiency throughout the UHF frequency band. Its output power is 50% higher than competing UHF TV broadcast solutions.

The MRF6VP3450H is based on the company’s 50 V sixth-generation very-high-voltage (VHV6) process, which has achieved some impressive milestones since Freescale’s first 50 V devices were introduced, including the first 1 kW LDMOS FET and the first 300 W VHF RF power transistor for broadcast applications designed in over-molded plastic packaging. By virtue of its high efficiency and RF output power, the MRF6VP3450H delivers significant benefits to transmitter manufacturers (see “450-W RF Power Transistor Sets Benchmark for UHF Broadcast Applications).

The Broadcast Market
The FM and VHF TV portions of the broadcast transmitter market continue to use about the same number of RF power transistors every year, with only slight year-on-year increases. The segment expected to experience significant growth in the next few years is the UHF band, where over-the-air broadcasters in the U.S. will by law replace analog modulation schemes with digital techniques and cease analog TV broadcasting altogether by February 17, 2009. Other countries, most notably South Korea and Japan, already have digital television (DTV) systems in widespread operation, and many other countries are either conducting DTV trials or beginning to deploy DTV nationwide.

At the highest power levels, broadcast transmitters rely almost exclusively on vacuum tubes such as the inductive output tube (IOT) as the final-stage amplification device. The IOT combines features of both klystrons and triodes and can generate tens of kilowatts of RF power. Broadcast applications, along with radar and satellite communication systems, remain some of the last bastions of vacuum tubes thanks to relentless advances in RF power transistor technology that are allowing them to encroach on their domain. While LDMOS FETs have long been used as the driver stages of tube-based amplifiers, their increased RF output power, reliability, efficiency, and ruggedness has resulted in their extensive use in the final-stage amplifiers of transmitters for FM through UHF TV, at power levels up to about 16 kW. They offer all of the usual benefits of solid-state devices over their electron tube counterparts, the most appealing of which is longevity, because when operated properly they can outlive a succession of tubes that must be replaced regularly and cost thousands to tens of thousands of dollars apiece.

A Widening Portfolio
Based on the success of its 28 V LDMOS technology in applications ranging from wireless infrastructure (where the company is the dominant supplier) to industrial applications, Freescale has built a large family of 32 V and 50 V LDMOS devices for the broadcast market. The devices are available in bands from 10 MHz to 1 GHz, which covers shortwave through UHF TV, and power levels ranging from 10 W to more than 1 kW. At power levels up to 300 W they are housed in the company’s over-molded plastic packaging, which have the industry’s lowest junction thermal resistance that helps reduce both the amount of copper heat sinking, as well as the transmitter’s overall cooling budget. Above this power level, the LDMOS FETs rely on air-cavity ceramic packages. The complete portfolio is shown in Table 1.

Three examples illustrate the versatility of the 50-V product line. For FM and analog and digital VHF TV:

• MRF6V2300N: 300 W CW. Well suited for both high-power amplification and as a driver up to 450 MHz. Typical CW performance at 220 MHz is 300 W with gain of 25.5 dB and drain efficiency of 68%.

• MRF6VP2600H: 600 W CW. Designed for pulsed broadcast applications at up to 250 MHz, the device delivers typical DVB-T OFDM performance at 225 MHz of 125 W, with gain of 25 dB, efficiency of 28.5%, and ACPR at a 4 MHz offset of -61 dBc in a 4 kHz bandwidth.

For digital VHF TV:

• MRF6VP21KH: 1 kW peak. For pulsed signals service at 225 MHz. The device produces 1 kW peak power with a pulse width of 100 µs and a duty cycle of 20%, gain of 24 dB, and efficiency of 67.5%.

Their output power and efficiency allow transmitters to be constructed with fewer RF power devices and amplifier stages. For example, in Figure 1, using the MRF6VP2600H, 1.1-kW amplifier for analog and digital FM or VHF TV can be constructed using only an MRF6V2010N driver stage and two MRF6VP2600H devices in a second stage to produce 50 dB of gain with only 0.2 dB in combining losses. In each case, the high efficiency and output power of the LDMOS FETs significantly reduce amplifier complexity, cooling requirements, and the overall bill of materials, while increasing reliability.

Summary
LDMOS RF power transistors will play an increasingly important role in advancing the performance and lowering the cost of amplifiers powering transmitters for FM as well as analog and DTV broadcast. Devices such as Freescale’s MRF6VP3450H, for example, have the potential to shave thousands of dollars from amplifier cost, reduce transmitter annual energy consumption, and increase reliability. These benefits continue to accrue year after year in terms of lower operating cost and solid reliability, which are highly appealing to transmitter manufacturers and broadcasters alike.

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