A Brief Overview of Novel Approaches in Designing High Performance VCOs <s> B. Tuning Range <s> To provide wide frequency tuning range (FTR) with compact implementation area, a new inductive tuning method is introduced in this paper for CMOS 60 GHz voltage controlled oscillator (VCO). The inductive tuning is based on a switching inductor-loaded transformer by configuring different current return-paths in the secondary coil of the transformer. Different from previous inductive tuning methods, the proposed VCO topology can achieve wide FTR for multiple sub-bands at 60 GHz within compact area by only one transformer. Two 60 GHz VCOs are demonstrated in 65 nm CMOS with design targets for the maximum FTR and the balanced phase noise in each sub-band, respectively. As measured by experiments, the first VCO (asymmetric) achieves a wide FTR of 25.8% from 51.9 to 67.3 GHz with phase noise variation of ±8.2 dB ( -90.2 to -106.7 dBc/Hz at 10 MHz offset) in all sub-bands; and the second VCO (symmetric) realizes a low phase noise variation of ±2.5 dB ( -105.9 to -110.8 dBc/Hz at 10 MHz offset) in all sub-bands with a FTR of 14.2% from 57.0 GHz to 65.5 GHz. <s> BIB001 </s> A Brief Overview of Novel Approaches in Designing High Performance VCOs <s> B. Tuning Range <s> This paper reviews CMOS LC Voltage Controlled Oscillators (VCO) for wireless multi-standard transceivers and wireless communications. The main parameters, such as phase noise, carrier frequency, supply voltage, tuning range, power dissipation, figure of merit (FOMt) were reviewed in this paper. <s> BIB002 </s> A Brief Overview of Novel Approaches in Designing High Performance VCOs <s> B. Tuning Range <s> The impact of CMOS technology scaling, on the tuning range and phase noise performance of mm-wave LC voltage controlled oscillators (LC-VCOs) is presented. As a preliminary step, the fundamental LC-VCO elements (i) tank inductor, (ii) fixed and variable capacitor elements, and (iii) cross-coupled transistor pair are analytically modeled across the frequency range 10-50GHz. These models are then exploited to analyze the tuning range and phase noise revealing the ultimate performance bounds for simultaneously achieving low phase noise and wide tuning range in mm-wave CMOS LC-VCOs across the CMOS technology scaling (from 130nm down to 45nm) are explored. The analysis demonstrates the improvement of the maximum achievable tuning range, phase noise, and figures-of-merit (FoM and FoMT) with the technology down scaling. Finally, the performance trend of the mm-wave CMOS LC-VCOs implemented using both thin and thick gate cross-coupled pair is compared. The analysis indicates that thick gate cross-coupled pair VCOs achieve better phase noise at the expense of power consumption and maximum tuning range. HighlightsMotivations and challenges of mm-wave CMOS LC-VCOs.Study of the limitations imposed by the LC-VCO elements across the CMOS processes.Tuning range quantitative analysis and minimum bounds for phase noise, FoM, FoMT.Comparing thin and thick gate cross-coupled pairs LC-VCOs across the CMOS nodes. <s> BIB003 </s> A Brief Overview of Novel Approaches in Designing High Performance VCOs <s> B. Tuning Range <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> BIB004
The majority of today's telecommunication applications require a tunable oscillator. In an ideal case, the output frequency of a VCO is a linear function of a control voltage or current inputs. However, having a good Frequency Tuning Range (FTR) is not achievable without degrading the phase noise or other parameters of the VCO. As shown in  , the simple definition for a VCO tuning range can be determined by Eq. (4). Additionally, the output frequency can be expressed by Eq. (5), in which f0 is the center oscillation frequency, V CO K is the VCO gain and con V is the input voltage or control voltage. 2 1 Also it is worth to mention here that having a satisfying FTR in VCO is very important, due to covering the band width of the desired application or for compensating the misalignment of center frequency. Usually tuning a VCO is implemented through exploiting the switched capacitor banks as shown in BIB002 . In BIB001 a novel method for tuning the frequency is proposed which employs the inductive tuning shown in Fig.4 . which is an alternative method for conventional capacitive tuning. In this method, the FTR can be determined directly from Eq. BIB003 in which k is coupling factor for loaded transformer.  It is clear from Eq. (8) that increasing coupling factor k will result in FTR enhancement. Also, it should be considered that Eq. (9) is the optimum design condition for the proposed circuit presented in BIB001 .  2 In BIB004 , a varactor diode combined with multi-element array of resonant tunneling diode (RTD) VCO has been empirically proposed to increase the tuning range in Terahertz applications. In this method, the mechanism for frequency variation is based on the depletion layer of the varactor in which the maximum frequency is achieved when the varactor diode is entirely depleted and vice versa. Indeed, the output frequency is function of the bias voltage of varactor diode which can be controlled by changing the slot antenna width. In fact, slot antenna acts as a resonator and radiator as shown in Fig.5 . 