## 3 Current Status of the Circuit Component in the MMW Regime
### Voltage Control Oscillator (VCO)  
LC-tank oscillator [154], distributed oscillator [155, 156, 157] and harmonically pushed oscillator [158, 159, 160, 161] are popular topologies in MMW regime. These topologies will be introduced as follows.  
LC-tank oscillator is widely adopted due to its simplicity. The resonate frequency is dominated by the inductor and capacitor. Moreover, the loss of the passive components is compensated by the negative resistance, which is attributed to the cross-couple pair of the oscillator. A resonant circuit and an amplified stage constitute the LC tank VCO. The resonate circuit is usually consisted of the inductors and a variable capacitor for the purpose of tuning mechanism. This can alleviate the effects of the process variation and give some margins for the operation frequency. For instance, in most of case, there is a frequency shift between the designed frequency and the measured result. Thus, the variable capacitor makes us to adjust the resonate frequency of the VCO to fit the target frequency, in turn, minimize the difference between simulation and measured result. Moreover, the amplified stage cannot only be the cross-couple pair but also the other amplified configurations like a single transistors or cascode configuration. Similarly, there are three major factors, which affect the performance of the phase noise in this topology. The thermal noise from the inductors, the noise from the tail current of the VCO and the noise from the transistors are the major concerns while minimizing the phase noise in the topology [154].  
Although silicon-based technologies provide higher \(f_{\mathrm{T}}\) and \(f_{\mathrm{max}}\), passive components limit the frequency generation with low phase noise in MMW regime [162, 163]. The distributed oscillator is treated as a possible solution for silicon-based VCO for high frequency operation. There are two  
Figure 13: Comparison of the MMW high-order quadrature amplitude modulation QAM modulation. Data taken from [146, 150, 151, 152, 153].  
popular topologies for distributed VCO: Standing wave oscillator and traveling wave oscillators. The main concept of the standing wave oscillator is to design the position-dependent tailoring of conductive and resistive losses along the trans-mission line [155]. On the other hand, traveling wave oscillator means to utilize a distributed circuit to propagate the oscillation around a transmission line [156, 157]. Furthermore, the combination of these two topologies is rotary-wave oscillator [164].  
Harmonically pushed oscillator [158, 159, 160, 161] is another potential mechanism to generate signal above MMW frequencies. It generates N times fundamental frequency with the disadvantage of lower power level compared with the oscillator, which generates fundamental frequency. The LC-tank is designed to resonate at the harmonic of the target output frequency. The oscillation condition is easy to be satisfied for the low gain characteristic of the transistor at MMW or even higher frequencies. Additionally, the on-chip inductors and MOS-varcators have high quality factors at low frequencies; therefore, the better phase noise can be achieved. In contrast, the imperfect differential output of the VCO is the issue due to the process variation. As a result, an extra amplifier is necessary for compensating the loss, which is generated from the mismatch. More dc power consumption is required.