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MWEE MARAPR 2014

Emerging Wireless MIMO receivers demand high performance dual passive mixers By Bill Beckwith, Xudong Wang, Tom Schiltz, Linear Technology Corporation Multiple-Input Multiple-Output (MIMO) technology is increasingly being used in high data rate systems such as Wi-Fi and 3G/4G cellular technologies. The higher data rates of MIMO systems provide increased system capacity and improved levels of efficiency. In order to reduce system complexity and size, MIMO receivers require integrated circuits (ICs) that are capable of handling multiple channels. To address this need, the LTC559x family of dual passive downconverting mixers provides frequency coverage from 600 MHz to 4.5 GHz. The mixer family includes the LTC5590, LTC5591, LTC5592 and LTC5593. The frequency coverage and typical 3.3 V performance of each mixer is shown in Table 1. These mixers deliver high conversion gain, low noise figure (NF), and high linearity with low DC power consumption. Typical conversion gain is 8 dB with an input 3rd order intercept point (IIP3) of 26 dBm, 10 dB of noise figure and 1.3 W power consumption. The LTC559x family of dual highperformance mixers is ideal for wireless infrastructure MIMO receivers. The dual channel solution reduces parts count, simplifies routing of LO signals and reduces board area. Additionally, each LTC559x incorporates integrated RF and LO baluns, double-balanced mixers, LO buffer amplifiers and differential IF amplifiers, further reducing overall solution size, complexity, and cost. Mixer description The simplified block diagram in Figure 1 shows the dual-mixer topology, which uses passive double-balanced mixer cores driving IF output amplifiers. The mixer cores are switched-MOSFET quads, which typically have about 7 dB of conversion loss. However, in this case the loss is more than compensated by the gain of the subsequent IF amplifiers, resulting in overall gain of about 8 dB. The differential IF output has been optimized for 200 Ω loads. The LO path uses a shared balun to convert the single-ended input to a differential LO and then drives independent buffer amplifiers for each channel. To prevent unwanted loadpulling of the VCO, good LO impedance matching is maintained in all operating modes. Figure 2 shows the LO input return loss of the LTC5591, as an example, under various operating conditions. This feature eliminates the need for an external LO buffer stage. Traditional basestations maintain a temperature-controlled environment and require that components work up to +85°C. Smaller cells and remote radio heads, however, present a more harsh environment for components, requiring operation up to +105°C. The LTC559x mixers have been designed for, and tested at +105°C to meet this demand. To minimize solution size, the LTC559x Table 1: LTC559x frequency coverage and 3.3 V performance summary. mixers are assembled in a small 5 mm x 5 mm 24-lead QFN package. The small package size is only part of the total solution size reduction, however. The high integration level reduces the number of required external components to about 19, minimizing Figure 1: Block diagram of dual-channel mixer. board area, complexity, and cost. Receiver application The functional diagram of an LTC559x mixer in a two-channel receiver is shown in Figure 3. Single-ended RF signals are amplified and filtered before being applied to the mixer inputs. In this example, differential IF signal paths are shown, eliminating the need for an IF balun. The SAW filter, IF amplifier, and lumped-element bandpass filter are all differential. High-selectivity SAW filters are used in many MIMO receivers to block unwanted spurs and noise at the mixer output. The mixers’ 8 dB of conversion gain compensates for the high insertion loss of these filters and reduces their impact on the system noise floor. The overall mixer performance allows the filter loss to be accommodated while enabling the receiver to meet sensitivity and spurious requirements. Another important specification for multichannel receivers is the channelto channel isolation. The channel-tochannel isolation is the IF level at the undriven channel’s output relative to the IF level at the driven channel’s output. 16 Microwave Engineering Europe March-April 2014 www.microwave-eetimes.com


MWEE MARAPR 2014
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