LDMOS Devices Set New Standards for Performance for Public Safety Radios
By David Lester, Freescale Semiconductor

Public safety portable and mobile VHF and UHF transceivers must be designed to take a beating from both how they are used (or abused) and the conditions in which they must sometimes operate. To visualize one chilling scenario, consider a police officer sitting in a remote area in a patrol car whose battery cannot start the car. The outside temperature is -30º F and he or she needs to call for help. The radio at this point is the officer’s only hope. Manufacturers of VHF and UHF radios used by police, fire, EMS, and other agencies must design their equipment to be operable in these and innumerable other potentially life-threatening scenarios. Three new LDMOS devices in Freescale Semiconductor’s Airfast RF power solutions family (Figure 1) are tailored to perform flawlessly in public safety radios even in some of the most extreme operating environments. Two of the devices operate in the VHF and UHF public safety bands delivering 31 and 75 W respectively and the other delivers 31 W in the 700 and 800 MHz public safety bands. They provide levels of ruggedness and stability that have previously been unattainable by LDMOS or other RF power transistor technologies for these applications.

Automotive DC power systems are generally taken for granted as being stable sources of voltage and current. However, vehicle-mounted public safety radios are at the whim of this “semi-regulated” power system, whose supply voltages range from as low as 10 VDC to as high as 17 VDC depending on a variety of factors. In the scenario depicted earlier, if the officer turned on his headlights, voltage would drop precipitously, and if the car was somehow able to be started, the voltage would rise to the higher level to charge the battery. Most radios also employ power monitoring circuits whose job it is to maintain RF output power even when supply voltage drops, increasing RF power from the drivers to the final amplifier and resulting in an overdrive condition that severely stresses the amplifier and its RF power transistors. To achieve this, various combinations of components are cobbled together. The new Airfast devices integrate much of the circuitry required to maintain stability within the devices themselves, simplifying design and maintaining stability under a wider range of conditions.

Freescale’s new Airfast devices have been designed not only to withstand potentially damaging conditions but to deliver their rated performance and remain stable as well. Like all of Freescale’s other RF power transistors, the Airfast devices are tested under extreme conditions and rated conservatively. A typical test combines operation at a -40º C ambient temperature with 20% undervoltage and an RF drive level twice their rated input power into a VSWR of 3:1. Under these conditions, the devices are expected to deliver their rated output power over their entire operating frequency range.

Two of the new devices, are designed for operation in the VHF and UHF bands from 136 to 520 MHz, and one is optimized for operation in the 700 to 900 MHz bands:

• AFT05MP075: 136 to 520 MHz, 75 W P1dB RF output power (85 W P3dB), efficiency of 68%, gain of 19 dB. (Achieves full power with as little as 1 W of drive power). Capable of operating into a >65:1 VSWR at 17 VDC with 3-dB input overdrive with no damage or degradation in performance.

• AFT05MS031: 136 to 520 MHz, 31 W P1dB RF output power, efficiency of 71%, gain of 18 dB. (Achieves full power with as little as 500 mW of drive power). Capable of operating into a >65:1 VSWR at 17 VDC with 3-dB input overdrive with no damage or degradation in performance.

• AFT09MS031: 764 MHz, 33 W P1dB RF output power, efficiency of 71%, gain of 17dB. (Achieves full power with as little as 600 mW of drive power). Capable of operating into a >65:1 VSWR at 17 VDC with 3-dB input overdrive with no damage or degradation in performance.

The new Airfast devices operate from a nominal 12.5-VDC supply and have integrated ESD protection for increased resistance to stray voltage during assembly and better performance when operated in Class C mode. The AFT09MS031 is available in Freescale’s two-lead over-molded plastic TO-270-2 (AFT09MS031N) and TO-270-2 “gull wing” packages (AFT09MS031GN). The AFT05MP075 is available in the four-lead TO-270 WB-4 (AFT05M075N) and TO-270-WB-4 “gull wing” plastic package (AFT05MP075GN). Detailed specifications are provided in Table 1.

Amplifiers designed for public safety radios generally employ either discrete RF devices or amplifier modules. RF power amplifier modules have become popular because superficially at least they are a more or less “plug ‘n play” solution. While these amplifier modules provide the amplifier and matching circuits, they still require other components such as the transmit/receive switch and receive and harmonic filtering to be designed and assembled. To achieve their “zero engineering” benefits, they sacrifice performance such as efficiency, which generally ranges between 25 and 40%. In contrast, the efficiency of the Airfast devices is typically 65% or greater, which is not only higher than modules can provide but higher than competing discrete LDMOS devices as well.

The Airfast devices make the process of employing discrete RF power transistors easier and less expensive. Only a few external inductors and capacitors are required to complete the amplifier and Freescale provides detailed reference designs as guides. The result is an amplifier that delivers higher performance in the same footprint as a module at less than half the cost, including the external components and the time required to assemble them.

The new devices will be supported by reference designs and other documentation, are currently sampling, and full production of the AFT05MS031N and AFT09MS031N 31-W devices is expected in the second quarter of 2012 with the AFT05MP075 75 watt device following in the fourth quarter of 2012. More information is available at www.freescale.com/rfpower.

FREESCALE SEMICONDUCTOR
www.freescale.com
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