Abstract
In this study, a new structure of Voltage Control Oscillator (VCO) to reduce the phase noise using two plans of a variable voltage capacitor is proposed. The aim of the current paper is to analyse two structure of a completely integrated 3.7 GHz LC-VCO based on TSMC 0.18 um technology. In the first plan for different voltages with variable voltage capacitors in this circuit can be achieved to a suitable phase noise of about 125 dB per Hz and in the second plan, a noise of -123 dB per Hz is achieved in the deviation frequency of 1 MHz. Also, the Figure of Merit (FOM) values for the first plan of -4/186 and for the second plan of –4/184 are achieved and the power consumption is 10 mW.
Keywords: Voltage Control Oscillator, power consumption, phase noise, variable voltage capacitors, FOM.

Introduction
Voltage controlled oscillators are one of the most important components in telecommunication systems. Most telecommunication systems now use the inactive elements on the chip in their oscillator structure 1, 2. Two types of oscillators on the chip include ring oscillators and LC oscillators. In order to select the type of oscillator, after determining the scope of the oscillator, requirements such as power consumption, silicon consumption rate, and phase noise will be considered. For example, it is clear that the proper oscillator architecture is the use of LC oscillators for frequency synthesizers. These oscillators can have a good phase noise at microwave frequencies 2. Ring oscillators have a simpler structure than LC oscillator. Ring oscillators have advantages such as low power consumption, low silicon consumption rate, high adjustment range, but their phase noise performance is not suitable for applications in the range of gigahertz 3. As mentioned, one of the most important features of the VCO is phase noise 4. Due to the low-noise performance of the LC oscillators compared to ring oscillators. Therefore, these oscillators are widely used in telecommunication systems. Another method for designing broadband oscillators is the use of magnetic adjusting, which has a wider range of tuning than ordinary LC VCO. A method for reducing phase noise in a VCO is to use a cross-coupled series LC receiver 5. Another way to reduce the phase noise in the LC VCO is to use the Switch Current Source. Using this method, the current duty cycle decreases 6. The most important features of VCO design is low power consumption, low phase noise, wide-band frequency tuning, tuning linearity and low area occupation 4. One of the important applications of voltage controlled oscillators is their application in PLL circuits, telecommunication systems, Wireless Local Area Network (WLAN), GPS receivers, Wireless Personal Area Network (WPAN) and IEEE 802.11g applications 7-10. Another method for designing broadband oscillators is the use of magnetic adjusting, which has a wider range of tuning than ordinary LC VCO.
Therefore, one of the common ways to increase the tuning band of VCO is the use of Switched-Inductor and Switched-Capacitor 8. One of the common ways to increase the tuning range of VCO is to use the switched capacitors 11. During these years, several methods for implementing this technique are presented in accordance with the IEEE 802.11g standard 9, 10. There is a tradeoff between tuning linearity and the frequency tuning range 12. Recently, the use of millimeter wave applications (mm-wave) has increased dramatically in broadband circuits. The III-V or SiGe technology is used in the design of VCO operating in the mm-wave frequency range 13.
Another VCO design approach for broadband users is using quadrature voltage controlled oscillator (QVCO), which consists of serially coupled. In this VCO, SiGe heterojunction bipolar transistors are used in QVCO for oscillation 14, 15. In order to achieve low phase noise and large output swing in QVCO the p-core indirect back-gate coupled Technique is used 16, this method can access a strong coupling trans conductance for a quadrature phase 16. Another way to increase the range of regulators of oscillators is to use a higher order tank circuit that is implemented by transformers on the chip 17, this technique can access to the low phase noise triple-band. Therefore, tank-based circuit-based oscillators show higher potentials for use in broadband applications and multiple band applications 18. One of the ways to reduce the area occupation in the VCO is to use a helical inductor based filtering technique 19. This method reduces the phase noise without increasing power consumption 19, 20. In order to achieve wide-frequency frequency response and low-noise noise, a dual-mode VCO based on mm-wave topology is designed using parasitic capacitances of cross-coupled pair to sense dual-mode operation 21. In 22, in order to achieve a wide frequency response range in the Ku and Ka bands, a LC push pull VCO has been designed. The main purpose of this paper is to introduce and design a voltage controlled oscillator with low phase noise characteristic in the C frequency band. The voltage controlled oscillator (VCO) is an electronic oscillator designed to control the oscillation frequency by voltage. The oscillation frequency changes proportionally to the input DC voltage. A message signal can be given to the VCO (Voltage Controlled Oscillator), and Frequency Modulation (FM), Phase Modulation (PM), and Pulse Width Modulation (PWM) can be achieved. For this reason, we will first introduce the structure of a variable capacitor with a new voltage for applications of voltage controlled oscillators. The feature of this structure compared to similar structures is the wide tuning rate.
Design and Analysis of Proposed Voltage Control Oscillator
Figure 1 shows a circuit of Ordinary oscillator. In order to analyse this circuit, it is necessary to obtain the differential amplifier equivalent resistance seen from both X and Y. As shown in Figure 1, it can be proven that the equivalent resistance seen from both X and is equal to:
-V_gs2+V_(YX )+V_gs1=0?(??(V_gs1=-V_gs2 ) ) V_YX+?2V?_gs1=0?(??(V_gs1=I_YX/g_m ) ) V_YX/I_YX =(-2)/g_m (1)
In this case, you can obtain the oscillation condition. To have stable oscillations, it is required that in the parallel resonant RLC circuit resistance must be greater than the size of the differential amplifier equivalent resistance seen from both X andY. Therefore, the circuit oscillation condition is written as follows:
2R_P?|(-2)/g_m | ?(?) R_P?1/g_m (2)

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Fig. 1. Ordinary oscillator circuit

First Proposed Design
The first proposed circuit for a variable voltage capacitor is based on the use of capacitive property of the density layer in a CMOS transistor based on 0.18 ?m technology and the setting and tuning of capacitive property with a voltage change.
Given that our ultimate goal is to design VCO with a low phase noise, we used an oscillator with a differential structure.
For this reason, the symmetry in the proposed form for a variable capacitor seems to be necessary to achieve the goal of a low phase noise and differential structure. Figure 2 shows the first proposed structure for a variable voltage capacitor. The number of transistors in this topology is based on the achievement of proper tuning and symmetric structure. Table 1 shows the equivalent capacitance seen from both A and B in terms of voltage variable voltage capacitor for the proposed topology.
According to Table 1, the results indicate a change from 0.11 to 0.18 pF (range of capacitance variation is 0.07 pF) for an equivalent capacitor in design frequency (about 3.8 GHz).

Fig. 2. The first proposed plan for a variable voltage capacitor

Table 1. Capacitor Variations In Terms Of Voltage In The First Proposed Structure At The Frequency of 3.8 GHz.

Voltage (in terms of Volt)
Varactor Capacitor (in terms of Pico Farad)
0 0.18
1 0.15
2 0.13
3 0.12
4 0.11
0 0.18
1 0.15

LC cross-oscillators play an important role in the design of high frequency circuits due to a relatively good performance of the phase noise and ease of implementation. In this paper, differential cross-oscillators are used.
Figure 3 shows the first proposed low phase noise VCO circuit schematic. This oscillator consists of three parts with a variable voltage capacitor: 1- A constant current source for feeding LC; 2- A differential amplifier; and 3- A suitable differential amplifier circuit (the bottom of the circuit).

Fig. 3. A proposed oscillator controlled by voltage using the first variable capacitor topology.

The equivalent inductor value in accordance to L2, L5 and L4 inductor in Figure 3 is achieved as follows:
L_eq=(1/5+1/0.95+0.95)=2.2nH (3)
If we consider ?_0=3.7 GHz then the equivalent capacitance can be obtained as follow:

?_0=1/?(L_eq C_eq )?C_eq=1/(??_0?^2 L_eq )=0.82PF (4)
where ?_0 is the frequency of an oscillation.
The equivalent capacitor required in theoretical calculations is equal to0.82PF. It can be said that the results of analysis and simulation are relatively good in this case. Figure 4 shows the frequency of oscillation in term voltage control.

Fig. 4. Freuency of Oscillation Versus Voltage Control

Second Proposed Design
In the second proposed circuit, the goal was to increase the tuning rate. To do this, we used the topology presented in Figure 5. In this topology, the capacitance and its variations were increased in accordance to the simulation results, which is given in Table 2. The results indicate changes from 0.21 to 0.33 Pico Farad (0.12PF) for an equivalent capacitor at design frequency (about 3.8 GHz).

Fig. 5. The Second Proposed Topology for a Variable Voltage Capacitor

The second proposed VCO circuit topology is shown in Figure 6. The difference between this circuit and the first Topology of VCO circuit as shown in Figure 3 is in the variable capacitor topology. This oscillator has more frequency setting range than the first design. But at the same time, the cost increases due to the increased number of transistors.

Table 2. Capacitor Variations in Terms of Voltage in the Second Proposed Structure at the Frequency of 3.8 GHz.

Voltage (in terms of Volt)
Varactor Capacitor (in terms of Pico Farad)
0 0.33
1 0.26
2 0.24
3 0.23
4 0.21

Fig. 6. A proposed oscillator controlled by voltage using the second variable capacitor topology.

Results and Discussion
In Advance Design System (ADS) software, first DC point of first design was examined. The results indicate the proper operation of the circuit, and especially the differential amplifier in terms of DC; that is, as expected, the circuit differential part works quite symmetrically in terms of DC. The current in both differential branches is equal and about 6.439 mA. Thus, from a 0.8 volt circuit power supply, a current as 12.88 mA is taken equivalent to a relatively low and suitable power consumption as 10 mW.
In the first design, results show that for different values of the voltage control, an appropriate phase noise of about -125 dBc/Hz can be obtained at the frequency of 1 MHz in the first design. In addition, by changing the voltage control of the variable capacitor, the oscillation frequency can be set from 3.775 to 3.905 GHz. In other words, the tuning rang of the oscillation frequency is about 130 MHz.

In the second design, for different values of the variable voltage control, an appropriate phase noise as -123 dBc/Hz can be obtained at the frequency of 1 MHz in the second design. In addition, by changing the voltage control of the variable capacitor, the oscillation frequency can be set from 3.651 to 3.901 GHz. In other words, the tuning range of the second design is nearly two times the value of this parameter in the first design. In order to arrive at a general conclusion, the comparison between the proposed oscillator and similar cases has been made. In Table 3, the frequency parameters, the setting range of frequency, phase noise, and power consumption are compared. It is also a key factor in determining the performance of the VCO (FOM) calculated by the following formula:
FOM=L(?f)-20 log?(f_0/?f)+10 log?(P_dc/1mW) (5)
In this formula L (?f), the phase noise at the deviation frequency ?f, f0 is the oscillation frequency and P_dc of the DC power consumption is in terms of mw. In fact, in the FOM parameter, the effect of frequency factors, DC power consumption, and phase noise is hidden. Therefore, the FOM parameter is also used for comparison in Table 3.
The results show that the VCO in this paper are better than references 5-22 in terms of phase noise and have the best status.

Table 3. Comparison of Oscillator Performance with References 5-22.
Reference Frequency (GHz) Frequency Setting Range (MHz) DC Power
Consumption (mW) Phase Noise at Deviation Frequency 1MHz (dBc/Hz) FOM
(dBc/Hz)
5 3.87 – 5.6 -122 -201
6 5.36 – 3 -121.3 -198.8
7 5.2 780 (14.7%) – -118 -188.6
8 2.4 190 (8%) 0.66 -121 -190.4
9 1.57 240 (15%) 3.06 -120 -179
10 2.6 560 (22%) 2.7 -122.3 -186.3
11 5.8 1140 (19%) 10.8 -117 -181.9
12 2.4 390 (16%) 10 -115.7 in 600 KHz -177.7
13 57 2000 (3.5%) 15 -96 -179.3
14 5 700 (14.6%) 19.8 -114
in 2 KHz -169
15 2.36 300 (12.7%) 16.25 -104.33 in 600 KHz -164
16 5.96 0.64 4.4 -120.5 -190.4
17 3.49 5 4.2 -123.06 -180
18 5.2 870 (16.7%) 9.7 -113.7 -178
19 2.5 131 (5%) 1.5 -119.2 -185.4
20 5.32 260 (4.9%) 5.7 -116 -183
21 55.7 17.2% – -93.5 -163
22 15 – 8.1 -109.30 -172
First proposed oscillator 3.7 130 (3.5%) 10 -125 -186.4
Second proposed oscillator 3.7 250 (7%) 10 -123 -184.4

Conclusions
In this paper, a new structure of Voltage Control Oscillator (VCO) to reduce the phase noise using two plans of a variable voltage capacitor is proposed.
The proposed oscillators are considered as low power oscillators in terms of DC power consumption and in terms of the FOM key factor among the compared references, and they are also in a very good condition. The results show that the VCO in this paper are better than references 5-22 in terms of phase noise and have the best status.
The frequency setting range of 3.5 and 7% is moderate. Generally, the voltages controlled by the proposed voltage in this paper are suitable for low power and low phase noise applications with the moderate tuning range.
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