Abstract— Modified inductance-capacitance voltagecontrolled oscillator (LC-VCO) topology is presented in this paper. NMOS only varactor has been used instead of variable capacitance. The frequency of oscillation of the VCO is 7.68GHz with a power dissipation of 5.68mW. The designed oscillator is characterized by a tuning range of 26.73%.The design is verified using SPICE simulations. Index Terms— Inductance Capacitance-Voltage Controlled Oscillator, Ultra-wideband frequency, VCO. I. INTRODUCTION Wave generators and voltage controlled oscillators (VCOs) play a vital role in the field of electronics and communication engineering. These have been used to generate signals from a few hertz to several gigahertz. High frequency VCOs are an integral part of various communication applications viz. biomedical implant communication, mobile, satellite and broadband communication etc [i]. Several researchers have proposed high frequency VCOs. A 1.4GHz CMOS LC low phase noise VCO using tapped bond wire inductances is proposed by Ahrens and Lee [ii]. An optimized analytical design of a 2.5GHz CMOS VCO is reported by Dehghani and Atarodi [iii]. Wu and Jian [iv] have presented a CMOS LC- VCO with novel negative impedance for wide-band operation. Graphically optimized design of a 4GHz CMOS LC-VCO is given by Issa et al [v]. Design of low power VCOs has also been reported in literature. Linten [vi] has designed low-power VCOs in standard CMOS technology. High quality thin film post processed inductors have been used in this design. Low power low phase noise differentially tuned quadrature VCO is designed in standard CMOS by Tiebout [vii]. Long et al [viii] have proposed a 2.4GHz low power low phase noise CMOS LC-VCO. A one volt ultra low power CMOS LC-VCO for UHF quadrature signal generation is reported by Wang et al [ix]. Even though significant amount of research work related to VCO design has been carried out, VCO is still a challenging component amongst radio frequency (RF) complementary metal oxide semiconductor (CMOS) very large scale integration (VLSI) designers [i,x]. The major issue in recent VCO research is to achieve monolithic integration of VCO with low-power consumption at higher frequencies [vii]. There are more stringent requirements imposed on VCOs as the need for wireless communications like global system for mobile telecommunication (GSM) and code division multiple access (CDMA) is increasing and new applications are coming into wireless market at higher frequencies [iii, vi]. The ultra wideband (UWB) communication devices need to operate at low power in GHz range. There is a need to reduce power for operations in gigahertz frequency range, for developing portable electronics and high frequency communication gadgets. This has been a source of research and hence the present work is a step in this direction. An oscillator is a circuit consists of an amplifier and a resonant element, as well as a feedback circuit. An amplifier is an electrical circuit with a defined input and output impedance which increases the level of the input signal to a predetermined value at the output. The energy required for this is taken from the DC power supply connected to the amplifier. If in a negative-feedback system, the open-loop gain has a total phase shift of 180 0 at some frequency w 0 , the system will oscillate provided that frequency provided the open-loop gain is unity. If the gain is less than unity at the frequency where A 6.1-7.68 GHZ CMOS LC-Voltage Controlled Oscillators (VCO) D. Solanki, R. Chandel, T. Alam, A. Nishad and P. Sharma Electronics & Communication Department, National Institute of Technology Hamirpur (H.P.)-India
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[email protected] the phase shift is 180 0 , the system will be stable, whereas if the gain is greater than unity, the system will be unstable. This statement is not correct for some complicated systems, but it is correct for those transfer functions normally encountered in oscillator design. The conditions for stability are also known as the Barkhausen criterion, which states that if the closed- loop transfer function is as 0 1 = ÷ i V V (1) where µ is the forward voltage gain and β is the feedback voltage gain, the system will oscillate provided that µβ=1. Design of LC-voltage controlled oscillators has posed many challenges to radio frequency (RF) designers. The design constraints are imposed on tuning range, start-up condition, power consumption and phase noise. So it is important to understand the specifications of the VCO II. SPECIFICATIONS OF OSCILLATORS AND VCOS The properties of an oscillator can be described in a set of parameters. The following are the important and relevant parameters as they needed to design an oscillatory circuit. These parameter will be used for designing of the oscillator circuit. In order to improve functionality of the oscillator these specification will be decided accordingly. In order to design an optimized design these parameter has to be satisfied. Start-up Condition- The start-up condition constraint is to guarantee enough of negative resistance generated by the cross coupled pair transistors NMOS and PMOS to compensate the tank resistance given by following relation as follows as tan > mn l k g g (2) Center Frequency - The output frequency of a VCO can vary over a wide range. The center frequency is determined by the architecture of the oscillator. A standard VCO has a center frequency range typically the range of few MHz to GHz. 1 f = (3) Power Consumption - The DC power, usually specified in mille-watts and sometimes qualified by operating voltage, required by the oscillator to function properly. = bias dd P I V (4) Tuning Characteristic - This specification shows the relationship between the VCO operating frequency and the tuning voltage applied. Ideally correspondence between operating frequency and tuning voltage is linear. tan tan ,min 2 max 1 k k L C w s (5) tan tan ,max 2 max 1 k k L C w > (6) Output Power as a Function of Temperature - All active circuits vary in performance as temperature range should vary less than a specified value as a function of temperature. Phase Noise - Unfortunately, oscillators do not generate perfect signals. The various noise sources in and outside of the transistor modulate the VCO, resulting in energy or spectral distribution on both sides of the carrier. This occurs via modulation and frequency conversion. The noise is expressed as the ratio of output power divided by the noise power relative to 1Hz bandwidth measured at an offset of the carrier. 2 0 2 ( ) 10 log 2 sig kT f L f P Q f A = A ( | | ( | \ . ( ¸ ¸ (7) III. RESULT AND DISCUSSIONS The complementary -g m oscillator consists of PEMOS and NEMOS cross-coupled transistors in parallel to generate the negative resistance. Fig. 1 shows the complementary LC-VCO circuit using varactor [v]. In this technique instead of using the NEMOS and PEMOS directly a varactor is used which is capable of reducing parasitic significantly. Here the negative compensating resistance is implemented by the cross coupled arrangement of NEMOS M1, M2 and PEMOS M3 and M4 [viii]. The transistors M5, M6, M7 and M8 are arranged in order to function as varactor. It can be seen in Fig. 1 that instead of using NEMOS as current source directly (as in case of VCO circuit in Fig. 1 NEMOS M3), an independent current source (Ibias) is used in order to have exact values of negative resistance [v]. The transistors M1, M2, M3 and M4 are connected in cross coupled fashion in order to further reduce parasitic present in the circuit due to use of inductors L1 and L2. The modified VCO considered in this work has been simulated and its performance is analyzed. SPICE simulations are carried out for 180nm technology node for supply voltage 1.8V [xv-xvi]. The phase noise of the LC-VCO has been analyzed. Phase noise is the frequency domain representation of rapid, short- term, random fluctuations in the phase of a waveform, caused by time domain instabilities. For this linear time varying (LTV) noise model has been used to calculated noise [ii]. 2 0 2 ( ) 10log 2 sig kT f L f P Q f A = A ( | | ( | \ . ( ¸ ¸ Fig. 2 shows LTI phase noise using eq.(7). The offset frequency f o been varied from 1Hz to 1kHz. The phase noise can be approximated at an offset of 1kHz as -175 dBc/Hz. Figure of merit (FOM) takes all important VCO parameters like power, phase noise and oscillation frequency into account. FOM is given as [v]: 0 20 log( / ) ( ) 10 log FOM f f L f P = A ÷ A ÷ (8) Using eq.(8) in order to decide the roll-off between various improvement constraints the FOM factor has been evaluated for frequency of oscillation of 7.68GHz with a power dissipation of 5.68mW and a phase noise of -175 dBc/Hz at 1kHz offset. FOM is found out to be 185. Fig. 1. Modified LC-VCO Schematic Vbias1 Vbias1 Vbias2 M12 NMOS M11 NMOS VDD L1 L2 Out M4 NMOS M3 NMOS M1 NMOS M2 NMOS Vtune Out1 M5 NMOS M7 NMOS M9 NMOS M10 NMOS M8 NMOS M6 NMOS VDD Fig. 3. Tuning Range of LC-VCO 6 6.4 6.8 7.2 7.6 8 0.3 0.6 0.9 1.2 1.5 1.8 F r e q u e n c y ( G H z ) Control Voltage (V) VCO Tuning Range Frequency Fig. 2. Phase Noise of the Proposed LC-VCO The tuning range of nearly 26.73% in the frequency of oscillation with control voltage varying from 0.5V to 1.8V [xvii] of LC-VCO is obtained as seen in Fig. 3. IV. CONCLUSION The modified VCO presented in this work is implemented in CMOS TSMC 0.18µm process and is designed using a negative-resistance accompanied by NMOS-only varactor to increase the tuning range. In addition to the reduced supply voltage and achieving low power consumption, the proposed technique also improves the phase noise. The phase noise is - 175 dBc/Hz at 1kHz offset, the tuning range is 6.06GHz~7.68GHz (26.73%) and the power consumption is 5.68 mW for 1.8V supply voltage. REFERENCES [i] T. H. Lee, “The Design of CMOS Radio-Frequency Integrated Circuits. Cambridge,” U.K.: Cambridge Univ. Press, 1998. [ii] T.I. Ahrens, and T.H. Lee, “A 1.4GHz, 3mW CMOS LC low phase noise VCO using tapped bond wire inductances,” in Proc. of Int. Sym. on Low Power Electronics and Design- ISLPED, pp. 16-19, Aug. 1998. [iii] R. Dehghani and S.M. Atarodi, “Optimised analytic designed 2.5GHz CMOS VCO,” Electronic Letters, vol.39, no.16, pp. 1160-1162, Aug. 2003. [iv] C.H. Wu, and G.X Jian, “A CMOS LC VCO with Novel Negative Impedance Design for Wide-Band Operation,” IEEE Radio Frequency Integrated Circuits Symposium (RFIC), pp. 537 – 540, 2010. [v] D.B. Issa, S. Akacha, A. Kachouri and M. Samet, “Graphical Optimization of 4GHz CMOS LC-VCO,” Design & Technology of Integrated Systems in Nanoscale Era 2009, pp. 33-37, May 2009. [vi] D. Linten, et al., “Low-power voltage-controlled oscillators in 90-nm CMOS using high-quality thin-film post processed inductors,” IEEE Journal of Solid State Circuits (JSSC), vol 40, no.9, pp.1922–1931, Sep. 2005. [vii] M. Tiebout, “Low-power low-phase-noise differentially tuned quadrature VCO design in standard CMOS,” IEEE J.Solid-State Circuits, vol.36, no.7, pp.1018-1024, July 2001. [viii] J. 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