Terahertz Up-conversion Mixers Using Varactors in CMOS and Their Applications




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Wireless communication at frequencies above 100 GHz is drawing attention due to its high data rate capability resulting from the wide available bandwidth. The recent advances of the high frequency performance of complementary metal oxide semiconductor (CMOS) technology have made it an affordable way for implementing the wireless systems. In order to support high-order modulations to increase the data rate, and an increased range, the transmitter must have a high output 1-dB compression point (OP1dB) and a wide bandwidth. Since the transistor fmax in CMOS has peaked at ~350 GHz, it is challenging to implement 300-GHz transmitters in CMOS. Consequently, the performance of the last up-conversion mixer in a transmitter is a key factor determining its performance. A 300-GHz sub-harmonic up-conversion mixer using symmetric varactors (SVAR’s) is demonstrated. This mixer takes an IF signal centered at 150 GHz and up-converts to RF at 290 GHz with an LO of 70 GHz. Implemented in 65-nm CMOS, the mixer achieves the maximum conversion gain (CG) of -16 dB and OP1dB of -11.4 dB. The OP1dB when reported was more than 10 dB higher compared to that of the other CMOS sub-harmonic up-conversion mixers in the literature. Fundamental mixing has superior conversion efficiency and output power. To increase CG and OP1dB, a fundamental up-conversion mixer with a similar structure using asymmetric varactors (ASVAR’s) is demonstrated. Using a similar transformer-based hybrid structure, this mixer achieves measured CG of -12.5 dB. The OP1dB is greater than -2 dBm with LO power of 15 dBm at 140 GHz. Due to the imbalance, a -21-dBm leakage at 2fLO is presented at the output. To reduce the generation of unwanted harmonic terms, a double-balanced up-conversion mixer using ASVAR is demonstrated in 65-nm CMOS. It utilizes a power-splitting-transformer hybrid for differential signal isolation. The up-converter achieves measured OP1dB of -6.2 dBm and maximum CG of -11.2 dB including input and output baluns, and a 3-dB bandwidth of ~25 GHz. The CG and OP1dB are the highest among all up-converters in CMOS with RF at ~300 GHz. These results are particularly critical for mixer-last transmitters operating near 300 GHz for high datarate communication. A 280-GHz transmitter using the proposed double-balanced mixer is experimentally demonstrated in 65-nm CMOS. The transmitter has a maximum output power of -8 dBm. The spectrum measurement shows the capability of transmitting 30-Gbps QPSK signals. This transmitter is the first ever demonstration of transmitters using varactor-based mixer above 100 GHz and supporting such a data rate.



Engineering, Electronics and Electrical