Design and Research of Two High Frequency CMOS Voltage Controlled Oscillators

Phase-locked loops play an important role in communication technology and play an irreplaceable role in modulation, demodulation, clock recovery, and frequency synthesis. Controllable oscillators are the core of phase-locked loops. Recently, in view of the pursuit of low power consumption and high integration of integrated circuits, more and more researchers have invested in the design of voltage-controlled oscillators based on CMOS technology. Ring voltage-controlled oscillators have wide tuning range and small chip area, and can also have good phase noise performance under careful circuit design, so they are widely used in digital communication systems. As the feature size of CMOS technology continues to decrease, according to the theory of CMOS technology scaling down, the power supply voltage should also be reduced in the same proportion. Compared with the 0.18 μm CMOS process with a power supply voltage of 1.8 V, the swing of the output signal of the traditional fully differential delay unit structure is limited to a very small area, which not only reduces the signal-to-noise ratio (SNR) of the output signal, but also must be amplified and processed before it can be sent to the next level circuit. This paper analyzes the important parameters that affect the performance of voltage-controlled oscillators, and designs and implements two multi-harmonic voltage-controlled oscillators, and gives the corresponding experimental results.

1 VCO working principle and performance indicators

VCO is a voltage/frequency conversion circuit. As a controlled oscillator in the loop, its output frequency should change linearly with the control voltage. The relationship between the output frequency and input frequency of an ideal VCO is

ωout=ω0+KVCOVcont (1)

Where ω0 is the fixed frequency of the oscillator when the control voltage Vcont is zero, and KVCO is the gain or sensitivity of the VCO (in rad/s·V-1).

From equation (1), the transfer function of the VCO can be derived:

From equation (2), we can conclude that when the VCO is placed in a phase-locked loop, its output is connected to the input of the phase detector after passing through a frequency divider. What affects the output of the phase detector is not its frequency but its phase. Therefore, in a phase-locked loop, the VCO is usually regarded as a system whose input is a control voltage and whose output is a phase.

Therefore, the VCO is like an ideal integrator in the phase-locked loop system, and its transfer function can be expressed as

In practical applications, the linear range of VCO is limited. Beyond this range, the parameters of the loop will change greatly, which is not conducive to loop design. Generally, the quality of VCO is mainly evaluated by the following characteristics:

(1) Low jitter or low phase noise: Due to the influence of factors such as circuit structure, power supply noise, and ground noise, the output signal of the VCO is not an ideal square wave or sine wave. Its output signal has a certain jitter. After conversion to the frequency domain, it can be seen that there is also a large energy distribution near the center frequency of the signal, that is, phase noise. The jitter of the VCO output signal directly affects the design of other circuits. It is usually hoped that the VCO jitter is as small as possible;

(2) Wide locking range: The adjustment range of the VCO directly affects the adjustment range of the phase-locked loop. Usually, the locking range of the VCO will change with changes in process deviation, temperature, and power supply voltage. Therefore, the VCO is required to have a wide enough adjustment range to ensure that the VCO output frequency can meet the design requirements.

(3) Stable gain: The voltage-frequency nonlinearity of the VCO is one of the main causes of noise. At the same time, this nonlinearity will also bring uncertainty to the circuit design. The changing VCO gain will affect the loop parameters and thus the stability of the phase-locked loop. Therefore, it is hoped that the VCO gain change is as small as possible.

2 VCO Design

Ring oscillator is a common type of oscillator, which is composed of several gain stage circuits connected in series. Generally, its oscillation frequency is very high, and its structure is simple and easy to implement. The basic component unit can be an inverter or a differential pair.

2.1 Inverter Ring VCO Design

A single-stage inverter can only provide a phase difference of 180°. In order to meet the phase condition, the simplest ring oscillator should be composed of at least three inverters in series. As the amplitude continues to increase, each stage of the circuit will experience nonlinearity and reach saturation, at which time the amplitude and frequency are both in a stable state. Use a large signal to analyze its oscillation period. Assuming that the delay time of each stage of the inverter is T, it can be concluded through analysis that each inverter returns to the initial state after 6T time, so the oscillation period is 6T. Similarly, the period of the IV-stage inverter is 2NT. It is deduced that the frequency of the oscillation circuit composed of the IV-stage inverter is 1/2NT.

The number of loop inversions must be an odd number, otherwise the phase condition of the Bahausen law is not met. In the design, 3 or 5 levels of inversion can achieve a relatively good effect. Of course, if the design requires, more inverters can be cascaded.

The delay time of each unit is related to the current flowing through the inverter, the aspect ratio of the inverter, the voltage, and the process. VCO1 uses a single inverter delay unit in series to form the simplest multivibrator with a maximum frequency of 3.3 GHz. The structure is shown in Figure 1. This oscillator uses a special mechanism, which is divided into two parts: control and delay feedback. The short channel effect of the control MOS tube is used to determine the oscillation frequency of the entire circuit through the current mirror. Since there are no external components and the structure is simple, the extremely small parasitic parameters increase the operating frequency.

2.2 Differential Ring VCO Design

The differential pair type VCO is mainly composed of a differential pair delay, and the differential delay unit is composed of a voltage-controlled current source, a resistive load, and an NMOS tube. The oscillation frequency of the loop can be controlled by controlling the current of the voltage-controlled current source. The differential delay unit circuit of the saturated dual-delay structure adopted by VCO2 is shown in Figure 2, and the ring voltage-controlled oscillator circuit structure composed of 4 levels of the delay unit is shown in Figure 3. In Figure 2, the circuit forms a CMOS latch (Latch) through two PMOS loads M3 and M4, and the cross-connected NMOS transistors M7 and M8 control the gate voltage of the PMOS load and limit the latch strength of the latch. Through the positive feedback of the latch, the delay unit works in a full switch state, reducing the proportion of the open time in the oscillation cycle. The external voltage controls the gate voltage of the PMOS load field effect tubes M3 and M4 through the cross-connected NMOS field effect tubes M7 and M8, thereby adjusting the delay of the unit; the gates of the field effect tubes M5 and M6 are connected to additional inter-stage positive feedback, which can reduce the rise and fall time of the signal, increase the oscillation frequency of the oscillator and reduce phase noise. The ring voltage-controlled oscillator structure composed of 4-stage delay units is shown in Figure 3, OUT+ and OUT- are the differential outputs of the oscillator, and V_cd is the voltage control terminal.

3 Simulation Results and Performance Analysis

Two multi-vibrator VCOs are presented in this paper: one is a three-stage inverter ring oscillator (VCO1); the other is a four-stage differential ring oscillator (VCO2). The output waveforms of these two multi-vibrators at their center frequencies are shown in Figure 4(a) and Figure 4(b). The voltage-frequency characteristics of VCO1 and VCO2 are shown in Figure 5(a) and Figure 5(b). This design uses a standard 0.18μm n-well three-layer metal CMOS process, extracts the netlist and simulation parameters of the layout, and performs post-simulation. Figure 6(a) and Figure 6(b) are the layouts of VCO1 and VCO2, respectively. Table 1 lists the main characteristics of these two VCOs.

Through the above analysis of the performance of the two VCOs, we can draw the following conclusions: The advantage of the inverter ring VCO is that the circuit design is simple and the oscillation frequency can be designed to be very high, but it is more sensitive to power supply or ground noise and has a larger phase jitter. The advantage of the differential pair VCO is that the differential signal can suppress ground noise or power supply noise and has a smaller phase jitter. The disadvantage is that the bandwidth is limited and it is not suitable for high-frequency applications.

4 Conclusion

Two high-speed CMOS multi-resonant voltage-controlled oscillators are presented in this paper, which use standard 0.18μm CMOS manufacturing process to achieve high operating frequency and low power consumption. Since the circuit does not require any external components, it is easy to achieve high integration density.