Single-Objective Optimization of a CMOS VCO Considering PVT and Monte Carlo Simulations

: The optimization of analog integrated circuits requires to take into account a number of considerations and trade-offs that are speciﬁc to each circuit, meaning that each case of design may be subject to different constraints to accomplish target speciﬁcations. This paper shows the single-objective optimization of a complementary metal-oxide-semiconductor (CMOS) four-stage voltage-controlled oscillator (VCO) to maximize the oscillation frequency. The stages are designed by using CMOS current-mode logic or differential pairs and are connected in a ring structure. The optimization is performed by applying differential evolution (DE) algorithm, in which the design variables are the control voltage and the transistors’ widths and lengths. The objective is maximizing the oscillation frequency under the constraints so that the CMOS VCO be robust to Monte Carlo simulations and to process-voltage-temperature (PVT) variations. The optimization results show that DE provides feasible solutions oscillating at 5 GHz with a wide control voltage range and robust to both Monte Carlo and PVT analyses.


Introduction
The voltage-controlled oscillator (VCO) is quite useful in applications such as: analog-to-digital converters [1][2][3], phase-locked loops [4], and so on. The VCO can be implemented by using complementary metal-oxide-semiconductor (CMOS) technology of integrated circuits, as already shown in [5], and also by using LC-tank structures. Several CMOS VCO designs can be classified by using single-ended stages [6,7], differential stages [8,9] and pseudo-differential stages [10]. Among the currently available VCO topologies, the one consisting of a ring structure [11], and using CMOS differential stages has the advantage of providing great immunity to supply disturbances [12]. Other desired features in designing a VCO are associated to accomplish low-power consumption, minimum layout area, high-frequency and wide control voltage range. These target specifications become difficult to achieve due to the continuous down scaling of silicon CMOS technologies. Besides, designing a VCO in a ring topology is frequently a more attractive alternative because it allows accomplishing a wide tuning (control voltage) range, small layout area, high gain, low cost, robustness to variations, simplicity and scalability in nanoscale CMOS processes [13,14]. The three principal causes of alteration on the performace for a circuit are the variations in the fabrication process, power supply and operation temperature, these constitute PVT variations and their impact is increased with the devices' downscaling [15]. Process variations include wafer defects or may be produced by certain chemical procedures causing some circuit's paremeters to change, voltage fluctuations in the circuit take place for a variety of reasons such as supply noise and can be compensated with a voltage regulator to prevent the transistor's operating point from being affected, last but not least temperature variations can be caused by external sources or by the circuit's own power dissipation. These PVT variations can be minimized by a proper design and layout placement and routing. Among the currently available designs, the authors in [5] introduced a wide-band VCO implemented by CMOS differential stages connected in a ring topology. Other design guidelines to improve the VCO's performance can be found in [16][17][18][19].
The oscillation frequency f osc of a VCO can be evaluated by (1), where N indicates the number of stages and τ is a time constant that depends on the associated resistance of the active load and the value of the capacitor load. f osc varies in a range determined by a control voltage V ctrl [14], and depends on the number of CMOS differential stages N, but decreasing N yields a reduction in gain, which may result in the oscillation mitigation. This trade-off can be improved by applying metaheuristics to maximize f osc under a wide range of V ctrl , and low silicon area or number of CMOS differential stages N. Different metaheuristics have been applied to the optimization of CMOS integrated circuits in previous works due to the complexity involved in the design processes [7,[20][21][22]. In this manner, the differential evolution (DE) algorithm is applied herein to vary the sizes of the transistors in the CMOS differential stages to maximize the oscillation frequency of a CMOS VCO f osc . The electrical characteristics of the VCO are evaluated by linking the simulation program with integrated circuit emphasis (SPICE).
The rest of the paper is organized as follows: Section 2 describes the considerations taken for the design of both the CMOS differential pair stage and the VCO in a ring topology. The DE algorithm is detailed in Section 4. The single-objective optimization is described in Section 5. Section 6 describes a brief disscussion about this work. Finally, Section 7 summarizes the conclusions.

Ring VCO-Based on CMOS Differential Stages
In this paper, the main objective in designing a CMOS differential stage as the one shown in Figure 1, which will be used to implement a ring VCO, is oriented to achieve the highest oscillation frequency f osc given in (1), which is inversely proportional to both the number of CMOS stages N and the propagation delay τ. Supposing N constant, then the delay generated by the differential pair must be minimized [14,23]. Some authors recommend that the delay can be reduced by augmenting the output transconductance g ds of the active MOS transistor and by reducing the equivalent capacitance, where the load capacitance C L could be the dominant one [13,23,24]. The trade-off here is that augmenting g ds leads to increase the sizes of the MOS transistors and this generates larger parasitic capacitance values. Therefore, this problem is quite suitable for applying metaheuristics, like the DE algorithm. If the MOS transistors M N1 and M N2 operate in their saturation region, then they must accomplish |V DS | > (|V GS | − |V TH |) and |V GS | > |V TH |, where the voltages are associated to the drain (D), gate (G) and source (S) terminals of the MOS transistors, and its associated threshold voltage V TH . The width (W) and length (L) sizes of the MOS transistors can be evaluated by (2), where I D is the drain current, and µ n C ox are parameters provided by the CMOS technology foundry. In this work the sizing is performed by using 180 nanometers (nm) from United Microelectronics Corporation (UMC).
As already shown in [5], the active loads are implemented by P-type MOS transistors (M P3 and M P4 ) operating in the triode region, and their sizing accomplish |V DS | < (|V GS | − |V TH |) and (3) [25]. The equivalent resistance is tuned by the control voltage V ctrl at the gates of the PMOS transistors [14,26], and the output conductance of the PMOS transistor can be approached as 1/g o = 1/g ds = 1/µC ox (|V ctrl − V s | − |V th |).
The propagation delay τ is directly related to the dominant pole, and it has been approximated as in (4), which depends on C L , the transconductance g m of the CMOS differential pair, and g ds of the active load [27], so that the reduction of the transistors' sizes leads to an increase of the dominant pole ω p .
3.29·10 54 (gds 2 +gds 4 )+1.46·10 56 (gds 4 gm 2 +gds 2 gds 4 +gds 4 gmb 2 ) The delay cell shown in Figure 1 can therefore be characterized by measuring the open-loop gain A OL and the dominant pole ω p . For instance, the gain-bandwidth product (GBW) of the delay cell, is the frequency at which A OL becomes 0 dB [28]. Its design including process, voltage and temperature (PVT) variations is given in [5], and in this paper the delay cell is optimized to provide the smallest propagation delay τ to increase f osc . The CMOS differential stage with active load is used to design the four-stages (N = 4) VCO shown in Figure 2.

VCO Optimization Methods
The VCO optimization has been carried out through different approaches, such as metaheuristics [7,22]. In [7], a ring VCO's operation improvement is performed through particle swam optimization (PSO) and non-dominated sorting genetic algorithm (NSGA-II), to minimize both the phase noise and the power consumption. This is carried out through the use of symbolic modeling techniques to obtain the total output noise density and VCO's phase noise expressions by doing this the run time is reduced and the noise expression is simplified. Achieving also an improvement in tuning range without being an objective and also performing both Monte Carlo and process corners analyses to the final design. Similarly, in [22] the optimal sizing of a differential ring VCO is carried out through multi-objective particle swam optimization (MOPSO) and infeasibility-driven evolutionary algorithm (IDEA) to improve its performances by minimizing both the phase noise and the power consumption while maintaining a given oscillation frequency. Noise modeling is also carried out, to obtain the simplified noise expressions and solve the equations' system the determinate decision diagram (DDD) symbolic technique is used. Furthermore, Monte Carlo and PVT variations analyses were performed to guarantee the design robustness.
In [29], an algorithm that performs RF circuits sizing by using evolutionary strategies and simulating annealing in the search and selection parts, respectively, is implemented in Matlab. The optimization is carried out taking into account the parasitics caused by the passive elements' layout through physical based equivalent parasitic models, by doing this the number of iterations between circuit sizing and layout generation is reduced (reducing the synthesis time) since the difference between synthesis and post-layout results is decreased. The use of simplified models through RF circuit synthesis to approximate layout-induced parasitics lead to unrealistic outcomes. An LC cross-coupled oscillator was optimized using this approach, where the restrictions are: oscillation frequency, phase noise, power consumption, and oscillation amplitude.
In [30], the circuit optimization tool AIDA-C is used to carry out a multi-objective optimization and perform the sizing of an LC-tank VCO with the aim to minimize two compromised objectives, which are phase noise and power consumption. This optimization process achieves a good balance between the two objectives, since there is a trade-off between them, the optimization execution takes several hours to run. In [31], two design tools AIDA and SIDe-O to design a robust LC-tank VCO are introduced. SIDe-O is employed to face the problems relative to the passive elements and through AIDA a robust design is assured due to its corner-aware approach and NSGA-II is employed for the phase noise, power consumption and area minimization, as in the previous case the algorithm takes several hours to run, in both algorithms none of the objectives are focused on achieving a higher oscillation frequency.

Problem Formulation for the Optimization of the VCO by Applying DE
The single-objective function g(x) is formulated by (5), where µ is a constant established to one and r(x) stands for the constraints. One can see that when all the constraints are fulfilled then the second term of the function is equal to 0 and the objective function is the oscillating-period of the ring VCO g(x) = f (x). Therefore, the sizing optimization problem can be defined by (6).
By applying the DE algorithm, which is described below, the sizing optimization process requires a population of I n individuals, a maximum number of generations maxGen, and the objective function g(x). Two of the main factors guaranteeing that global optimality is achievable by a metaheuristic like DE are the selection of the best solutions and randomization, where the former ensures that the solution converges to an optimum value while the later keeps the solution from getting halted at local optima [32]. To maximize f osc , this paper minimizes the oscillating period of the ring VCO, which is subject to the constraints of maintaining the load MOS transistors M P3 and M P4 operating in the triode region and the rest N-type MOS transistors operating in the saturation region. The SPICE simulator is linked within the optimization loop to evaluate the delay cell's gain A OL to be maintained within 1 and 5 dB.
The DE algorithm is a metaheuristic that performs an iterative optimization based on the evolution of a population of individuals under the concept of competition. The initial population is randomly generated where each individual represents a tentative solution that is associated to a fitness value through an objective function to point out the individual's suitability to a particular problem. The individuals with better fitness are more likely to be selected as parents, the chosen ones are reproduced using genetic operators (crossover, mutation) to produce new offsprings, which will also be evaluated to determine its survival. This represents a generation and this process is repeated until a stop criteria is met [33][34][35][36]. The DE algorithm is suitable for continuous optimization problems, like sizing analog CMOS integrated circuits as the VCO. In the DE algorithm, a vector population is altered through a vector of differences, which translates to a two operators: the first one being a recombination operator of two or more solutions and the second one coming as a self-referential mutation operator that conducts the algorithm unto finding acceptable solutions. Each individual is encoded as a vector of real numbers that are within the limits defined for each design variable (as the widths (W) and lengths (L) of the MOS transistors). The crossover operator defines the offspring-associated variable to be a a linear combination of three randomly selected individuals or an inheritance of its parents value while guaranteeing that at least one of the offspring's variable will be different from its parent. A scaling factor is employed to prevent stagnation of the search process [33,37].
In the DE algorithm, if a variable's magnitude is out of range, the recombination and mutation operators can be employed to reset the value. For instance the value can be established to the limit it exceeds, however this diminish the population's diversity. Other approaches reset it to a random value or initializing this value to a mid point between its previous value and the violated bound. In the latter the limits are approached asymptotically leading to diminish the amount of disruption [33]. In our current DE implementation, the individual is reset randomly within the search bounds. Other guidelines to design a DE algorithm may include to set the population number to ten times the amount of decision variables and initialize the weighting factor, P f to 0.8 and the crossover constant, P c to 0.9. If no convergence is achieved an increase in population may be necessary, however frequently the weighting factor is the one that has to be modified to be a little lower or higher than 0.8. The relation between convergence speed and robustness features is a trade-off, if the amount of population increments and the weighting factor decrements then convergence is more likely to occur but within a longer period of time. The performance of DE is more sensitive to the value of the weighting factor than the value of the crossover constant, and the range of both is generally in [0.5, 1]. A faster convergence may occur with higher values of the crossover constant [33].
The usefulness of the DE algorithm in sizing CMOS integrated circuits has been proved in [38][39][40]. Algorithm 1 describes its adaptation to maximize the oscillation frequency of the ring VCO shown in Figure 2. As mentioned above, herein the objective function is associated to minimize the propagation delay τ that is accomplished by measuring the oscillating period by using SPICE. Generate the SPICE netlist of the ring VCO 3: for i = 1 : I n do 4: Initialize the population randomly and replace the initial individuals (Ws, Ls, V ctrl ) into the netlist 5: Evaluate the VCO's delay cell and check the constraints 6: if constraints = 0 then 7: Simulate the VCO and evaluate the objective function 8: end if 9: end for 10: while j < maxGen do 11: for i = 1 : I n do 12: Create a trial solution from three randomly selected parents using (7) 13: Apply crossover using (8) 14: Replace the new individual into the netlist 15: Simulate the VCO's delay cell and count the constraints 16: if constraints = 0 then 17: Simulate the VCO and evaluate the objective function 18: end if 19: if the individual's objective function is lower than that of the parent then 20: The new individual replaces the parent using (9) 21: end if 22: end for 23: end while 24: end procedure In the optimization process the individuals I n of the population generated by the DE algorithm are replaced into the netlist file of the VCO's delay cell and each individual is simulated in SPICE. The electrical characteristics are obtained from the (.lis) output SPICE-file to verify that all the MOS transistors are working in the appropriate region of operation and that the gain is within the range of 5 > A OL > 1. A flag assigns 0 to a fulfilled constraint and 1 to a not fulfilled one. The period of the sinusoidal wave is associated to the function f (x). If the VCO is not oscillating then a high value is assigned to f (x). In the DE algorithm each individual is mutated to generate an adaptive solution v ij from three randomly selected parents, as given in (7). Afterwards, the crossover takes place creating a trial solution, through the recombination of a mutated solution v ij with an individual x ij , given by (8). Finally, the replacement is carried out employing an elitist selection, where the new individual will replace its parent if its objective function value is better than the parent, as given in (9) [33].

Optimizing the CMOS VCO by Applying DE Algorithm
The sizing optimization problem defined by (6), requires the sizes of the design variables (widths W and lengths L) of the MOS transistors, but one must determine the search space ranges. For instance, the limits of the sizes are set to: 2λ ≤ W ≤ 1000λ and 2λ ≤ L ≤ 10λ, respectively, where λ = 90 nm for the UMC CMOS technology of 180 nm. Another design variable is the control voltage, which bounds are set to V SS ≤ V ctrl ≤ V DD , and where V SS = −0.9 V is the lower supply voltage and V DD = 0.9 V the higher supply voltage.
The DE algorithm was calibrated by adjusting P c , P f and I n to 0.7, 0.6 and 50, respectively. The maximum number of generations is set to 50. In total, 30 runs of DE were performed. The best feasible solution provided an oscillation frequency of 5 GHz, as shown in Figure 3. In such a case the obtained parameter values are: I bias = I MN3 = 4 mA, W MN1 = W MN2 = 40 µm, W MN3 = 500 µm, W MP3 = W MP4 = 17 µm, L MN1 = L MN2 = L MN3 = L MP3 = L MP4 = 0.18 µm, V ctrl = −0.8 V and C L = 31.39 fF. The V BI AS is created from Figure 1, in which the CMOS differential stage with active load is biased with I bias = 2 mA, and the sizes of M bn are W = 200 µm and L = 180 µm.
The SPICE simulation result of the best solution of the DE algorithm is shown in Figure 3.  Monte Carlo is an integrated circuits' statistical analysis in which a circuit devices' parameters and mismatch are varied randomly. Monte Carlo simulation allows the designer to consider the possible effects of a random variation of certain circuit's parameter over its performance. Monte Carlo analysis is carried out through the variation of W and L for each one of the 30 feasible solutions over 1000 runs, and considering a Gaussian distribution with 10% deviation. The outcome of the Monte Carlo simulations is employed to compute the mean and the standard deviation of the objetive function value, those results are sketched in Figure 4.  The feasible sized solutions that accomplished the lower time delay τ of the CMOS differential stages are analyzed and their statistics related to the mean and standard deviation of the period of the sinusoidal wave are summarized in Table 1. From this table, the Monte Carlo simulation of the best solution of the DE algorithm is shown in Figure 5.  The parameters of each one of the five best feasible sized solutions and the simulated period, frequency and gain of the VCO and the CMOS delay cell, respectively, are summarized in Table 2.   A PVT simulation of the ten best feasible sized solutions was also performed to assure that the CMOS VCO is robust to variations. The PVT variations are simulated by setting V ctrl = −0.8 V. Considering five process corners (typical-typical (TT), slow-slow (SS), slow N-type MOS transistor and fast P-type MOS transistor (SNFP), fast N-type MOS transistor and slow P-type MOS transistor (FNSP), and fast-fast (FF)), three voltage variations (±10% of ±V supply = 0.9 V), and three temperature variations (T− = −20 • C, T = 60 • C and T+ = 120 • C) [41], Figure 6 shows the higher and lower gain and oscillation frequency values provided by the DE algorithm. Table 3 summarizes PVT simulation results, where the five corners (TT, SS, SNFP, FNSP and FF) correspond to the MOS transistor models provided by the UMC foundry.   As one can see from Table 3 solution number 4 is the most robust to PVT. This solution has the greater frequency with all the gains been positive. Figure 6 depicts the higher and lower gains and dominant pole frequencies for solution number 1 since this is the one that provides the higher oscillation frequency, as one can see the greater gains occur at the FNSP process-corner (in Figure 6a) while the lower gains for the most part occur at SNFP process-corner (see Figure 6b. Furthermore, the greater ω p takes place mostly at FF process-corner, while the lower ω p mostly takes place at SS process-corner. Figure 7 depicts the higher and lower gains and dominant pole frequencies for solution number 4 since is the most robust one. As one can see in Figure 7a the greater gains occur mostly at the FNSP process-corner, while the lower gains, in Figure 7b, for the most part occur at SNFP process-corner. The greater ω p takes place mostly at the FF process-corner (in Figure 7b), while the lower ω p mostly takes place at the SS process-corner (in Figure 7a). Table 4 shows the oscillation frequency and power dissipation corresponding to each control voltage V ctrl value for the best 5 feasible sized solutions.

Discussion
The proposed methodology to circuit design here is: (1) apply DE at least 30 times. This give us 30 solutions to our design problem, considering only the best solutions according to the objective function. (2) From the best 10 solutions, apply the Monte Carlo (MT) analysis. (3) From the best 10 solutions of the MC analysis apply the PVT analysis. Finally, (4) select the best solution according to the showed variations in the PVT analysis.
We apply the MC analysis to vary the dimension for all the circuit's transistors up to 10% of their value. As shown in Figure 5, these variations are not too high to move the operating point of the MOS transistors, and still the order of the obtained solution according to the objective function is kept after the MC analysis.
Then we apply the PVT analysis: The five process corners employed for this simulation are the ones provided by the foundry which are typical-typical (TT), slow NMOS transistor and fast PMOS transistor (SNFP), fast NMOS transistor and slow PMOS transistor (FNSP), slow-slow (SS) and fast-fast (FF), these account for the variation of fabrication parameters. A circuit can also be subject to temperature (considering three temperatures −20 • , 60 • and 120 • ) and voltage variations (considering a variation of ±10%) in its operation environment therefore each corner is simulated with each temperature and voltage variation.
The chosen solution is the one with lower time period (or higher operation frequency) while all the gains are positive, within the gain constraint of 1 < A OL < 5.
In Table 3 are shown only the first five solutions, although 10 analyses were performed. We use a DE version programmed in C language. One single run (50 individuals, and 50 generations) took around 32 min.
The MC and PVT analyses could be incorporated within the optimization loop, as another set of constraints. This idea also will increase the simulation time to several hours. We are going to analyse this idea as a future work.

Conclusions
The application of the DE algorithm has proven to be effective in the minimization of the time period of a CMOS VCO designed with CMOS differential delay cells in a ring topology. We use the Monte Carlo analysis over the sized transistor dimensions to rank the obtained DE solutions. Then we apply the PVT analyses to the 10 best solutions according to the Monte Carlo analysis. The most robust solution to PVT, provides an oscillation frequency up to 4.25 GHz (corresponding to a time period of 0.235 ns), and it has a wider tunning range, of 2.72-4.44 GHz, corresponding to V ctrl of −0.36 to −0.9V.