## 3 Current Status of the Circuit Component in the MMW Regime
### Modulators  
Quadrature phase (I/Q) modulator (or demodulator) [141, 142] is composed of two mixers, which are combined with a 90\({}^{\circ}\) hybrid at the input port; meanwhile, an in-phase power divider is utilized at the output (Figure 12). Figure 11: CG _versus_ fractional bandwidth of the recent published MMW SiGe mixers. Data taken from [80, 136, 137, 138, 139, 140]. Figure 12: Block diagram of the MMW I/Q modulator. The Marchand balun is designed to provide the differential LO signal for the double-balanced mixer architecture. The Wilkinson power divider furnish a good isolation between the LO port and the other two mixers. The modulator (demodulator) can perform the modulation (demodulation) of the received (or transmit) RF signal by following the design procedures in the textbook [143]. However, there are some design challenges for the modulator or demodulator. The Wilkinson power divider at the output port, which limits the overall bandwidth of the modulator (demodulator), should be designed carefully. The performances could be further improved in both the area and bandwidth. Moreover, the bandwidth extension techniques such as right/left hand transmission line and naturalization can be applied to the circuit for improving the bandwidth of the passive components and maintaining good isolation performances [144]. The other concern is that the mixer core is the most critical part of the modulator (demodulator). It dominates the critical performances like CG. Furthermore, the better mixer core is developed, the less LO power is needed to drive the modulator (demodulator). For example, in a passive mixer, adjusting the bias condition of the mixer core can intensify the CG. The transistors in mixer core can be biased around their threshold voltage to increase the CG due to their low channel resistance in this operation region. As for the low LO power driven mixer core, it is able to achieve by reducing the number of transistors in the mixer cores of the modulator (demodulator). Furthermore, in the upcoming communication standard for V-band, it is desire to have a 64-quadrature amplitude modulation (64-QAM) [145]. The nonlinearity and IQ mismatch damage the RF signal especially for high QAM modulators. However, the IQ mismatch can be reduced by proper selection of the circuit topology. Likewise, power back-off can be leveraged the linearity constrain [146]. The solution to the IQ mismatch can be provided by a type of calibration technique [116, 147, 148, 149, 150, 147, 148, 149, 151]. For the in-phase/quadrature (IQ) mismatch of the IF path, digital [147, 148] and mixed-mode [149] techniques are exploited below 2 GHz. However, more power consumption and area are necessary for the digital compensation technique as well as the additional control circuits. Also, the limitation of bandwidth are the major issue for most IF compensation techniques. For MMW wideband communication, IQ magnitude mismatches can be achieved with phase calibration mechanisms on the local oscillator (LO) signal path and variable gain amplifiers (VGAs) on the IF path [150]. The image rejection ratio (IRR) can be suppressed below 40 dBc with this calibration mechanism at W-band. A 256-QAM constellation with 3% error vector magnitude (EVM) is demonstrated at 90 GHz. In addition, the 60-GHz direct conversion transmitter with 33-dB conversion gain and the 11-dBm saturated power enlarges the dynamic range for the measurement consideration. By using the characteristics of the high image rejection and good LO signal suppression, the direct-conversion transmitter MMIC demonstrates a 1024-QAM modulated signal (modulation capability) with a data rate of 500 Mb/s and the EVM results are within 1.7% at 65 GHz. Figure 13 shows a comparison between our design and other works of MMW high-order QAM modulation [150, 151, 152, 153]. Those solutions for improving the performances in the major categories are introduced.