A Brief Overview of Novel Approaches in Designing High Performance VCOs <s> IV. CONCLUSION <s> A resonant tunnelling diode terahertz voltage-controlled oscillator integrated with a varactor diode (RTD VCO) for a wide tuning range was proposed and fabricated. In single RTD VCO, a tuning range of 120 GHz of the centre frequency of 640 GHz was achieved by a good combination of the RTD and varactor capacitances. A multi-element array of RTD VCOs was fabricated to increase the tuning range. A very wide frequency tuning of 320 GHz (580–900 GHz) was achieved with a four-element RTD VCO array. <s> BIB001 </s> A Brief Overview of Novel Approaches in Designing High Performance VCOs <s> IV. CONCLUSION <s> This letter presents a low-power low-phase-noise VCO with self-adjusted active resistor (SAAR). A pair of PMOS transistors is introduced between cross-coupled pairs and LC-tank, serving as the SAAR. When cross-coupled transistor resides in the saturation region, SAAR exhibits small resistance, enabling fast switching and suppressing flicker noise up-conversion to $1/f^{3}$ phase noise. Moreover, as cross-coupled transistor enters triode region, SAAR will adjust to larger resistance and prevent the small conducting resistance of the cross-coupled transistor degrading the quality factor of LC tank. Fabricated in a 65 nm CMOS technology, the proposed VCO demonstrates a tuning range of 5.07–6.35 GHz (22.4%) with only 0.42 mW power consumption at 0.6 V supply. The phase noise in the worst case is $-$ 40.8 dBc/Hz at 1 kHz and $-$ 111 dBc/Hz at 1 MHz, respectively. <s> BIB002
In this paper, I have introduced the VCO and its applications. Besides, I have presented the theoretical modeling for phase noise and FTR as well as the novel modifications for them. In addition, the impact of technology scaling on the phase noise of a VCO has been discussed briefly. Furthermore, as shown in section II of this paper, the RTD VCO combined with varactor diode which is proposed in BIB001 can enhance the frequency tuning range in THz applications. Also, in section III I have investigated three of the state-of-the-art VCO designs which were proposed recently. Eventually, as shown in Table I ., the VCO design using the proposed method in BIB002 , can bring higher FTR than the other methods at the expense of phase noise performance.