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This parameter is usually specified to be 10 dB better than the antenna-toantenna isolation to avoid degrading system performance. Based on its precise IC design, the LTC559x mixers achieve greater than 45 dB of channel-to-channel isolation, which satisfies most multichannel application requirements. Power consumption and size With the maturing of multiband/multimode base station topologies and a more refined system definition of 4G networks, wireless infrastructure systems are moving toward platform configurations that allow implementation of various band or mode requirements with minimal hardware and software changes. The LTC559x mixers all share a common pinout, making it easy to use the same board layout for all bands. The continued growth of wireless communications has also spurred the use of smaller cells such as picocells and femtocells. The need for more, and smaller, cells plus the increased use of remote radio heads has placed additional constraints on infrastructure systems, demanding higher integration and smaller solution size. As the number of cells grows, power consumption has also become increasingly important as energy costs go up proportionally. In remote radio heads, on the other hand, thermal stress is a major concern due to reliance on passive cooling. Simply reducing the solution size is not sufficient, as reduced system size would result in higher power density, higher junction temperatures and potentially reduced component reliability. Thus, it is necessary to simultaneously reduce system power consumption and size. This goal is challenging, because the RF performance must not be compromised. In the past, combining two individual mixers on one chip would result in total power consumption of 2 W. To reduce power consumption, the LTC559x mixers have been designed for 3.3 V operation instead of 5 V. Low voltage circuit design techniques reduce power dissipation without impacting conversion gain, IIP3 or noise figure performance. The only parameter affected by the lower supply voltage is the P1dB performance, which is approximately 11 dBm. The P1dB performance is output limited by the voltage swing at the open collectors of the IF amplifiers when driving the 200 Ω load impedance. For applications where higher P1dB is necessary, the mixers have been specifically designed to allow the use of a 5 V supply on the IF amplifier. Emerging Wireless The higher voltage improves the P1dB to greater than 14 dBm. As shown in Table 1, the dual-mixers achieve excellent performance while using just over 1.3 W of power, with both channels enabled. For additional power savings, each channel can be independently shut down as desired by using the independent enable controls. In instances where reduced linearity requirements are acceptable, the ISEL pin allows the user to switch to low current mode and further decrease DC power consumption. Conclusion The LTC559x family of dual passive downconverting mixers delivers the high performance needed to meet the demanding requirements of today’s multichannel infrastructure receivers. The mixers’ combination of high conversion gain, low NF, and high linearity improves overall system performance, and low power consumption and small solution size meet the more stringent needs of today’s smaller base stations and remote radio heads. The authors, Bill Beckwith is a Senior RFIC Design Engineer, Xudong Wang a Senior RFIC Design Engineer, and Tom Schiltz a Senior RFIC Design Engineer at Linear Technology Corporation www.linear.com Figure 2: LTC5591 LO return loss for different operating states. Figure 3: LTC559x dual passive mixer in a receiver application. www.microwave-eetimes.com Microwave Engineering Europe March-April 2014 17


MWEE MARAPR 2014
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