A Low Power Voltage Controlled Oscillator Design

The performance of voltage controlled oscillator (VCO) is of great importance for any telecommunication or data transmission network. Here, voltage controlled oscillators (VCOs) using three-transistor NAND gates have been designed. New delay cell with three-transistorNANDgate has been used for designing the ring basedVCOcircuits.Three-, five-, and seven-stageVCOs have been proposed. Output frequency has been controlled with supply voltage variation from 1.8V to 2.4V. Three stage VCO shows output frequency variation in the range of 3.2909GHz to 4.2280GHz whereas power consumption varies in the range of 335.4071 μW to 486.1816 μW. Five-stage VCO depicts frequency in the range of 1.9406GHz to 2.5769GHz with power consumption variation from 559.0118 μW to 810.3027 μW.Moreover a seven-stage VCO shows frequency variation from 1.3984GHz to 1.8077GHz. Power consumption of seven-stage VCO varies from 782.6165 μW to 1134.400 μW. Phase noise results for these VCOs have also been obtained. Power consumption, output frequency, and phase noise results of proposed circuits have been compared with earlier reported circuits, and the proposed circuits show significant improvements.


Introduction
Oscillators are the most fundamental blocks in various communication systems.With each generation of communication and microprocessor technology data rates are increasing at a very fast pace.In modern high performance systems phase-locked loops (PLLs) are the commonly used circuit component with wide application in frequency synthesis, clock, and data recovery [1][2][3].PLL block contains a phase detector, a charge pump, a loop filter, and voltage controlled oscillator circuit.VCO is the major part of PLL circuit and it affects the system performance in terms of power consumption and noise performance.In modern VCO designs power consumption and high output frequency range have become important performance metrics.Two widely used VCO types are LC tank and CMOS ring based circuits.Combination of capacitor and inductor on integrated circuits consumes large layout area in LC tank based VCO designs [4][5][6].CMOS ring based oscillators show advantages due to ease of controlling the output frequency and nonrequirement for on chip inductors [7,8].With the beginning of very large scale integration (VLSI) technology CMOS based VCOs are more accepted in PLL systems.These are also easier to integrate and provide wide tuning range.Further, with the rising demand of portable devices like cellular phones, notebooks, and personal communication devices, the need for power saving has also increased many times.Power consumption in very large scale integration (VLSI) systems includes dynamic, static power and leakage power consumption.Total power consumption in any CMOS circuit is given as )  is the switching activity,   is the capacitance of the load,  is the clock frequency, and   is the supply voltage. sc is the short circuit current, which flows directly from the power supply to ground terminal when NMOS subnetwork and PMOS subnetwork conduct simultaneously. sub is the leakage current which results from substrate injection and subthreshold effects.The reverse biased p-n junction current is the static dissipation due to reverse biased diode leakage between the diffusion regions, wells, and substrate. gateleakage is the gate leakage current which arises from gate oxide which is mostly dependant on gate oxide thickness.The first two components in (1) represent the dynamic power consumption and the remaining two components show static power consumption.In ring based oscillator design output of last stage is fed back to input of first stage.A VCO block diagram with single ended N-delay inverter stages is shown in Figure 1.The ring structure must provide a phase shift of 2 and unity voltage gain for oscillation occurrence.Each delay cell should provide a phase shift of /, where  is the total number of delay stages.The remaining  phase shift is provided by dc inversion of the inverter delay cells.In single ended oscillator designs the odd numbers of delay stages are required for dc inversion.Frequency of oscillation of VCO designed with N-single ended delay stages is given by   = 1/2  , where  is the total number of delay stages and   is delay of each stage [9,10].Different types of delay cells have been reported in the literature for oscillator design including multiple-feedback loops, dual-delay paths, and single ended delays [11][12][13][14][15][16][17][18][19][20].Delay cells have been implemented by different approaches like inverter stages, latches, cross-coupled cells and so forth.Delay stages are the fundamental building blocks in any VCO design and improved design of these delay cells affects the overall performance of VCO design.In the present work a new delay cell has been designed considering the importance of power consumption and frequency range.
The paper is organized as follows: in Section 2, a threetransistor NAND gate has been discussed.Further, three-, five-, and seven-stage VCOs have been designed with NAND delay cell.In Section 3 results of proposed circuits have been described and compared with the earlier circuits.Finally conclusions have been presented in Section 4.

Circuit Description
The output frequency of ring VCO depends on the delay provided by each inverter delay cell.In the proposed designs delay cells based on three-transistor NAND gates are used.Inverter operation has been obtained by three-transistor NAND gate as shown in Figure 2. The circuits have been designed in 0.18 m CMOS technology with supply voltage of 1.8 V. Supply voltage/control voltage has been varied from 1.8 V to 2.4 V to obtain the different output frequency components.Direct path between   and ground has been eliminated in the delay cells, due to which leakage power is reduced and the designs are power efficient.NAND delay stage is made up of two PMOS transistors and one NMOS transistor.Out of two input terminals of NAND gate, one is connected to logic 1 (i.e., 1.0 V) and feedback signal is applied to the other terminal.This circuits works as inverter without having direct path between   and ground with saving in power consumption.The gate lengths of all three transistors have been taken as 0.18 m.Width (  ) of NMOS transistor (N1) has been taken as 0.25 m.Width (  ) of transistors P1 and P2 has been taken as 1.25 m.Output frequency has been controlled by varying the supply voltage (  ) of NAND delay stages.Three-and five-stage VCOs have been shown in Figures 3(a) and 3(b).Seven-stage VCOs have also been designed with the same concept.
The proposed NAND gate has only three transistors so the design is more power efficient and requires less area as compared to conventional four-transistor NAND or NOR gates.The proposed NAND gate design is based on pass transistor logic which has reduced internal capacitance and is suitable for power efficient circuits [15].In NAND gate design direct connection to   is eliminated and there is only one NMOS transistor connected to ground so the design is power efficient as compared to that implemented with conventional gates.

Results and Discussions
Simulations have been carried out using SPICE based on TSMC 0.18 m technology with supply voltage variations from 1.8 V to 2.4 V. Table 1 shows the results of power consumption and output frequency for three-, five-, and seven-stage VCOs.
Output frequency of three-stage VCO shows variation from 3.2909 GHz to 4.2280 GHz with power consumption variation from 335.4071 W to 486.1816 W.In five-stage ring VCO frequency varies from 1.9406 GHz to 2.5769 GHz with power consumption variation from 559.0118 W to 810.3027 W.Finally, in seven-stage ring VCO frequency varies from 1.3984 GHz to 1.8077 GHz with power consumption variation from 782.6165 W to 1134.400 W.Figures 4(a) and 4(b) show frequency and power consumption variation for three-, five-, and seven-stage VCOs. Figure 5 shows output waveforms for three-, five-, and seven-stage NAND VCOs at supply voltage of 1.8 V. Table 2 shows results of phase noise performance for three-, five-, and seven-stage ring VCOs designed with  In the reported circuits, power consumption is showing upward trend with increase in number of delay stages whereas output frequency is showing downward trend.The number of stages may be decreased or increased depending upon the application and requirement for output frequency range.A comparison with earlier reported circuits in terms of power consumption and output frequency range is given in Table 3.
The proposed circuits show superior performance in terms of power consumption and output frequency range than the compared circuits.

Conclusions
In the reported work CMOS ring VCO designs have been improved with three-transistor NAND gates.Three-, fiveand seven-stage VCOs have been reported with reduced

Table 1 :
Results for NAND delay based VCOs.

Table 3 :
Comparison of VCO performances.