A 67-86 GHz Spectrum-Efficient CMOS Transmitter Supporting 1024-QAM with a Process-Variation-Tolerant Design

This paper describes a direct-conversion E-band transmitter (TX) in 65-nm CMOS. To demonstrate the feasibility of E-band 1024-quadrature amplitude modulation (QAM), a 67 to 86 GHz direct-conversion CMOS transmitter with broadband image rejection is presented. The transmitter contains an I (in-phase) and Q (quadrature-phase) modulator and a five-stage power amplifier. To achieve a 40 dBc image-rejection ratio (IRR) of the I/Q modulator for E-band, a broadband half-quadrature generator (HQG), which contains a composite right/left-handed (CRLH) based divider, quadrature couplers, and baluns is proposed. For the purpose of minimizing the phase imbalance, phase balance lines are utilized in HQG to achieve a 1-degree phase accuracy under different tuning lines. Subsequently, the reflection can be mitigated by optimizing the impedance matching between the HQG and the mixer core. Meanwhile, because millimeter-wave (MMW) circuits are susceptible to process variations, a process-variation-tolerant design is introduced to mitigate IRR variation, especially under 40 dBc. The transmitter demonstrates a measured flat conversion gain (CG) of 23±2 dB from 56 to 86 GHz. The proposed TX achieves an IRR better than 40 dBc from 67 to 86 GHz (bandwidth of 19 GHz) with a peak IRR of 56.2 dBc at 70 GHz. Furthermore, the proposed TX has exhibited a 6 bit/s/Hz spectral efficiency with 1024-QAM under the orthogonal frequency-division multiplexing (OFDM) modulation format. The 1.129 mm²E-band TX achieves a measured output power of 6.5 dBm with a total dc power consumption of 164 mW from a 1.2 V supply voltage.