Each new generation of wireless access technology has presented greater challenges than the previous generation for designers of RF power transistors and power amplifiers.
For example, the emergence of digital modulation schemes and multi-carrier amplifiers placed more stringent requirements on linearity. Now, WiMAX is raising the bar even higher, as it employs orthogonal frequency division multiplexing (OFDM), which requires exceptionally high levels of linearity and efficiency, along with high output RF power capabilities. To meet the demands of the WiMAX environment, Freescale Semiconductor has introduced two families of LDMOS FETs that satisfy the demanding operational requirements of WiMAX in both the 2.7 and 3.5 GHz WiMAX bands.
The reason for OFDM’s increasing popularity is that it can deliver high data rates with efficient spectrum use by using multiple overlapping carrier signals instead of a single carrier as in current WCDMA and cdma2000 wireless systems. It can accomplish this even in non-line-of-sight (NLOS) links that suffer multipath conditions, which can significantly degrade network performance, without resorting to complex equalization filters that are expensive and difficult to design. It also eliminates crosstalk, and its low symbol rate helps keep time-domain spreading encountered in multipath propagation to a minimum, thanks to a guard interval between symbols.
As a result, in addition to its use in WiMAX, OFDM is rapidly becoming the modulation scheme of choice in coming generations of wireless systems, regardless of the more stringent requirements it places on RF designs and the potential increase in overall system cost. It is also used in wired broadband applications such as DSL and cable. OFDM signals have a high peak-to-average power ratio (PAR) because the phase of multiple sub-carriers can combine constructively. Therefore, from a design perspective, it necessitates the use of a high resolution digital-to-analog converter in the transmitter, a high resolution analog-to-digital converter in the receiver and highly linear devices in the transmit section. Levels of nonlinearity that are acceptable in the latest 3G wireless systems result in intermodulation distortion in WiMAX systems that can cause intersymbol interference along with out-of-band spurious signals.
Design Challenges
For designers of RF power devices and amplifiers, the challenge is to deliver a level of linearity that satisfies the difficult requirements of OFDM, while still producing acceptable RF output power with the fewest number of power devices in order to reduce power consumption, system complexity and cost. To achieve this performance, RF power devices are operated in backoff conditions to varying degrees from their rated P1dB gain compression power capability, depending on the chosen amplifier design. The Freescale devices have been designed with various P1dB power levels as high as 160 W CW at 2.7 GHz—believed to be the highest power currently available in an RF power device suitable for WiMAX operation.
Prior to the release of Freescale’s LDMOS FETs, the feeling among some was that while LDMOS could deliver the required performance at 2.7 GHz, higher frequencies were beyond its capabilities. However, this is hardly the first time the limits of LDMOS have been proclaimed and then exceeded. The technology has shown repeatedly over the years that its capabilities can be extended to higher frequencies (in this case 3.5 GHz), and that its practical limitations are not likely to be reached anytime soon.
At 3.6 GHz, the MRF7S38075H delivers P1dB output power of 75 W and a WiMAX compliant –49 dBc ACPR at a 12 W average, gain of 14 dB and efficiency of 14 percent. The MRF7S38010H delivers P1dB output power of 10 W and a WiMAX compliant –49 dBc ACPR at a 2 W average, gain of 15 dB and efficiency of 17 percent. Both achieve this performance while meeting linearity and power consumption requirements in spectral form with mask requirements as well as in quadrature form, in terms of error vector magnitude (EVM).
High Efficiency at 2.7 GHz
At 2.7 GHz, the frequency band used in North America, Freescale has introduced the MRF6P27160H, which delivers 160 W CW at a 35 W average power output level, has a gain of 14.6 dB and efficiency of 22.6 percent with an ACPR of –47.8 dBc. It can deliver its rated power output into a 10:1 VSWR without damage. The device is housed in a low thermal-resistance package, is internally matched, operates at 28 VDC (and up to 32 VDC) and is available on tape and reel.
Freescale has introduced devices with lower power outputs as well, including the MRF6S27085H and MRF6S27015N, which deliver 85 W P1dB CW (20 W average) and 15 W P1dB CW (3 W average) respectively at 28 VDC. The MRF6S27085H delivers a gain of 15.5 dB and efficiency of 23.5 percent, while the MRF6S27015N provides 14 dB of gain with efficiency of 22 percent.
The MRF6S27085H is housed in a low thermal-resistance air-cavity package, and the MRF6S27015N in Freescale’s cost-effective over-molded plastic packaging. Both are available in tape and reel format.
The performance of the devices lends them well to being cascaded and combined to produce efficient, high gain power amplification solutions under various interstage backoff conditions. Two of these scenarios at 2.7 GHz are shown in Table 1. In the first, a single MRF6S27015N drives two MRFG35010N gain blocks, followed by two MRF6S27085H devices for final amplification. The MRFG35010N is an inexpensive, over-molded plastic-packaged GaAs FET device delivering 10 dB of gain, 1.8 W of RF output and 25 percent efficiency.
Total gain for the line-up chain is 38 dB and RF output power is 44 W. In the second scenario, an MRF6S27015N drives two MRF6S27015Ns followed by two MRF6S27085H devices. Total line-up gain is 42 dB and RF output power is 44 W. While these are not the only possible device combinations, they are representative of the performance that can be achieved using the new Freescale devices. Detailed specifications for all of the devices are shown in Table 2.
Freescale Semiconductor,
RF Division,
Tempe, AZ (800) 521-6274,
RS No. 301