LDMOS RF Power Transistors Serve Defense Applications
By John Powell, Freescale Semiconductor
Defense systems, from communications to avionics, radar, missile guidance, electronic warfare, and identification friend or foe (IFF), all have a well-earned reputation for demanding the highest levels of performance, ruggedness, and reliability from every one of their constituent components. This applies especially to RF power transistors that form their amplifiers and transmitters, which as critical system elements must meet stringent requirements for a broad array of performance metrics. Driven by continuing demands for “more”, RF power transistors have made dramatic advances in reliability, ruggedness and virtually every other performance metric. LDMOS RF power transistors, for example, today deliver levels of RF power output, gain, efficiency, and ruggedness (among others) that make their predecessors pale by comparison.
The LDMOS devices in Freescale’s Airfast family provide 5% greater efficiency and more than 20% high power density than previous generations along with higher linearity, broad instantaneous bandwidth, and advanced overmolded plastic packaging. “High-ruggedness” LDMOS models designed for the most demanding operational environments today can withstand extreme load mismatches with VSWR greater than 65:1, deliver RF power outputs greater than 1 kW CW, and have efficiency in some cases reaching 80%. Many devices in both families are housed in overmolded plastic packages, which Freescale first introduced to the industry in the mid 1980s, first for automotive and industrial application and in 1997 as the first high-power, high-frequency RF devices in plastic packages for wireless infrastructure.
By 2002, this packaging technology had advanced to the point where plastic-packaged 2 GHz RF power transistors could reach the frequency, thermal performance, maximum junction temperature, standards compliance, and reliability previously only achievable with metal-ceramic packages. The OMNI package introduced in 2009 extended the technology to the most rugged high-power RF devices. In addition to its performance advantages, overmolded plastic packaging fits neatly within DoD’s drive to use COTS devices whenever possible to save cost while maintaining high performance, and reducing weight.
Freescale’s LDMOS RF power transistors have been employed in defense systems of many types for decades, and this month the company created a business unit dedicated exclusively to defense customers (see “Freescale RF Business Creates Defense Business Unit”)
More than just a name, the RF business new defense sector brings with it a team dedicated to defense and a commitment to long-term product obtainability. Freescale’s LDMOS devices span a frequency range from HF through 3 GHz with RF power outputs to 1250 W and are complemented by GaAs MMIC gain block amplifiers, power amplifiers (up to 4 W), and low-noise amplifiers with noise figures as low as 0.35 dB that cover a frequency range of 500 MHz to 5 GHz.
Defense radar systems operate over a wide range of frequencies, principally from L-band through X-band with some systems operating at both S-and X-band. However, over-the-horizon radars that operate in the HF region are a mainstay of U.S. ballistic missile defense, generating the massive effective radiated power required to track missile launches through their various stages of flight from many thousands of miles away. These systems, many of which are either being upgraded or will be, benefit from the high RF output power, efficiency, and ruggedness of today’s LDMOS RF power transistors.
For example, Freescale’s MRFE6VP61K25H (Figure 1) delivers up to 1250 W CW or peak RF output power from 1.8 to 600 MHz with typical power-added efficiency of 74% and large-signal gain of 24 dB at 230 MHz. The device operates from a wide range of supply voltages from 30 to 50 VDC. In order to assure optimal ruggedness, the MRFE6VP61K25H—and other high-ruggedness devices—are designed to operate under conditions well in excess of anticipated operating conditions, such as twice their rated RF input power into a 65:1 impedance mismatch. The MRFE6VP61K25H will deliver 1500 W peak power with no device degradation when driven by a pulsed input signal (100 µs, 20 ms duty cycle) 3 dB above its rated level at 50 VDC and a VSWR greater than 65:1 at all phase angles.
This level of ruggedness, which has proven itself in other demanding applications such as CO2 laser and plasma exciters, allows the transistor to not only function but deliver very high power under the most hostile conditions ever likely to be experienced in service. Its broad bandwidth also makes it suited for applications beyond radar, including battlefield communications, IED jammers, and other systems that require high RF power outputs. With high gain and high efficiency, the device makes it possible to build amplifiers using fewer RF power transistors to deliver a specific RF output level, with a commensurate reduction in bill of materials and cooling overhead. This is especially valuable in a radar environment in which hundreds or even thousands of devices are employed.
Freescale’s MRFE6VP100H is also designed for extreme ruggedness, and has a broad operating frequency range of 1.8 to 2000 MHz, RF power output of 100 W CW, pulsed, or PEP, with efficiency of 70% and large-signal gain of 26 dB. Its broad bandwidth allows designers to use a single amplifier to cover multiple bands, such as 1.8 to 54 MHz, 30 to 512 MHz, and 400 to 1000 MHz, providing even greater effectiveness. Both devices have internal networks that enhance circuit stability over a wide range of operating conditions and are externally matched so their performance can be optimized for specific frequency bands.
Freescale has a pallet of devices that can be employed in S-band applications. These devices employ the MRF8P29300H LDMOS RF power transistor that has an operating frequency of 2.7 to 2.9 GHz and delivers 320 W peak output power (100 µs pulse, 10% duty cycle) with 49.6% efficiency and gain of 14 dB. The transistor will operate without degradation into a 10:1 VSWR at 32 VDC with pulse duration of 300 µs and input drive 3 dB greater than rated. The MRF8P29300H is internally matched, designed for push-pull operation, and has enhanced ESD protection and a wide negative gate-source voltage range for improved Class C performance. Freescale offers a compact test fixture for device evaluation as well. The performance characteristics of the MRF8P29300H make it an appealing choice for S-band radars.
Serving Vehicle-Mounted and Handheld Radios
LDMOS RF power transistors in the Airfast family are a good fit for the type of service experienced in hostile environments and thus can be used in applications such as battlefield radios, whether handheld or vehicle-mounted. Three examples, the AFT05MP075 (Figure 2) and AFT05MS031 that operate from 136 to 520 MHz and the AFT09MS031 that operates from 764 to 941 MHz, deliver RF output power of 70, 31, and 31 W respectively. Like their higher-power companions, they provide ruggedness and stability that have previously been unattainable by LDMOS or other RF power transistor technologies at these frequencies.
In addition, they are also evaluated under an additional set of conditions specific to their applications, with a typical test combining operation at -40º C with 20% under-voltage and an RF drive level twice their rated input power into a VSWR of 3:1. Under these conditions, the devices deliver their rated output power and other performance characteristics over their entire operating frequency ranges.
The AFT05MP075 has a rated RF output power of 70 W, efficiency of 68 to 72%, gain of greater than 18 dB, and produces its rated output power with only 1 W of drive. The AFT05MS031 delivers 31 W P1dB RF output power with efficiency of 62 to 71% and gain up to 23 dB with only 500 mW of drive. The AFT09MS031 operates from 764 to 941 MHz with 31 W P1dB RF output power, efficiency of 68 to 74% and gain up to 18 dB, and requires only 600 mW of drive. All three devices can operate into a 65:1 VSWR at 17 VDC with 3-dB input overdrive with no damage or degradation in performance. They also have integrated ESD protection.
Battlefield radios pose challenges for RF power transistors and amplifiers that are analogous to vehicle power systems, such as the “semi-regulated” DC power supplies. While this might at first appear to be a benign environment, in operation their nominal 12 VDC can drop to 10 VDC and rise to 17 VDC depending on environmental and other conditions. In addition, most transceivers have power monitoring circuits that maintain a stable RF output power under varying power supply voltages.
However, when voltage drops the circuit boosts the RF power delivered by the driver amplifier to the final amplifier, sometimes well in excess of the rated RF input power of the amplifier’s transistors. To compensate for these conditions, the Airfast devices internally incorporate most of the circuitry needed to maintain stability, which reduces transceiver circuit complexity while providing the required stability under a wider range of conditions. Typical RF power management circuits are made from discrete components on the circuit board, but as this function is integrated within the Airfast devices, most external components for RF power management are eliminated, which has obvious benefits in reduced cost and complexity.
The RF power transistors described in this article are just some of the devices that are not only well suited for defense applications but are part of the longevity program. Other devices from Freescale’s commercial RF product portfolio will be added in the future as well. More information about Freescale’s defense products is available at freescale.com/rfmilitary.com.
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