COVER STORY


High-performance mixer for 5G MIMO receiver applications


The global demand for ever-increasing data rates has pushed the limit of the current 4G wireless networks capacity. The next generation 5G networks will need to increase the capacity by more than 10 fold in order to keep up with future demand. Even though the 5G standard has not been finalised, most, if not all, market participants have concluded that the bandwidth will need to increase to at least 100MHz (from the current 20MHz spectrum), and some venturing to as high as 200MHz. Doing so will push the frequency spectrum upward to 3.6GHz or higher. Bill Beckwith, staff scientist, Xudong Wang, senior RFIC design engineer, and Tom Schiltz, RFIC design manager, Analog Devices Inc. (formally Linear Technology) talk about how they are dealing with this problem


T


o address this need, Analog Devices’ (formally Linear Technology) LTC5593 dual passive down-converting mixer


provides excellent linearity and dynamic range performance at 3.6GHz, while supporting more than 200MHz flat signal bandwidth that makes for a superbly robust MIMO (Multiple-Input Multiple- Output) receiver. MIMO technology has


demonstrated its usefulness by markedly boosting net data rate throughput and reception in systems such as Wi-Fi and 4G networks in times of limited spectrum bandwidth. As 5G systems migrate to higher frequencies, the LTC5593 provides continuous 50Ω matched from 2.3GHz to 4.5GHz, supporting multiband receivers at 2.6GHz and 3.6GHz bands. For lower bands, other pin- compatible mixers include the LTC5590, LTC5591, and LTC5592, which cover all the other LTE receivers. The frequency coverage and typical 3.3V 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 power conversion gain is 8dB with an input 3rd order intercept point (IIP3) of 26dBm, 10dB of noise figure and 1.3W power consumption. The LTC5593 family of dual high- performance mixers is ideal for wireless infrastructure MIMO receivers, such as in


8 December 2017/January 2018


a RRH (Remote Radio Head). Such systems are extremely compact and are in self-contained, weather-sealed casing, posing challenges in small size and thermal management from the large content of electronics. The dual channel


Table 1: LTC559x frequency coverage and 3.3V performance summary


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 7dB of conversion loss. However, in this case the loss is more than compensated by the gain of the subsequent on-chip IF amplifiers, resulting


The LO path uses a shared balun to convert the single-ended input to a differential LO, which then drives independent buffer amplifiers for each channel. This separate LO drive topology preserves the phase coherency of the LO signal to both mixers while providing excellent channel isolation. Additionally, to prevent unwanted load-pulling or disturbance to the VCO, a constant 50Ω LO input impedance matching is maintained in all operating modes, even when one or both mixer stages are turned on and off. A 50 impedance match from 2.1GHz to 3.4GHz is realised by adding a 1.5pF external series capacitor, C2. This capacitor is also needed for DC blocking. For the higher 3.6GHz band, adding a shunt inductor of 10nH on the source side of the capacitor provides good return loss at the LO. Figure 2 shows the LO input return loss of the LTC5593, under various operating conditions. This feature eliminates the need for an external LO buffer stage. Traditional basestations maintain a temperature- controlled environment and requires that components work up to +85°C. Smaller cells and remote radio heads, however, present a harsher environment for components, requiring operation up to +105°C. The LTC5593 mixers have been designed for, and tested at +105°C to meet this demand.


solution reduces parts count, simplifies routing of LO signals and reduces board area. Additionally, each LTC5593 incorporates integrated RF and LO baluns, double-balanced mixers, LO buffer amplifiers and differential IF amplifiers,


Components in Electronics


in overall power gain of about 8dB. The differential IF output has been optimised for a standard 200Ω interface, which can drive directly differential IF filters and variable gain amplifiers, minimising external components.


To minimise solution size, the LTC5593 family of mixers is assembled in a small 5mm x 5mm 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


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