Application of Voltage Controlled Oscillator in RF Communication Circuit

With the development of communication technology, the frequency used in communication equipment is increasing day by day. Radio frequency (RF) and microwave (MW) circuits have been widely used in communication systems. High-frequency circuit design has also received special attention from the industry. The continuous development of new semiconductor device manufacturing technology has made the application fields of high-speed digital systems and high-frequency analog systems continue to expand. Usually, the operating frequency of these circuits is above 1 GHz, and this trend will continue with the development of communication technology. The main application field of RF circuits is wireless communication. A typical wireless communication transceiver system includes a transmitter circuit, a receiver circuit and a communication antenna, which can be used in personal communication and wireless local area networks. The analog circuit is divided into two parts: the transmitting part and the receiving part. In the transmitting part, the low-frequency analog signal output by the digital-to-analog conversion and the high-frequency carrier provided by the local oscillator are up-converted into a radio frequency modulated signal through a mixer, and then radiated into space through the antenna; in the receiving part, the spatial radiation signal coupled from the antenna is first amplified by a low-noise amplifier, and then down-converted with the local oscillator signal through a mixer to a signal containing an intermediate frequency signal component, and then input into the analog-to-digital converter after filtering to convert it into a digital signal, and then enter the digital processing part for processing. It can be seen that the voltage-controlled oscillator that provides local oscillation is an indispensable part of the RF circuit. At the same time, the RF voltage-controlled oscillator VCO (Voltage Controlled Oscillator) is a key module of phase-locked loop, frequency synthesis and clock recovery circuits. It is widely used in electronic systems such as mobile phones, satellite communication terminals, base stations, radars, missile guidance systems, military communication systems, digital wireless communications, optical multiplexers, optical transmitters, etc. It has a decisive impact on the performance, size, weight and cost of electronic systems, and is a difficulty in RF circuit design and integration. Although VCO can be composed of discrete components, due to too many parameters considered in the design, complex circuits, large circuit size, and long design cycle, it is difficult to meet the requirements of low power consumption, low cost, miniaturization, lightweight, and high performance of today's portable wireless communication equipment. Therefore, the design of a fully integrated voltage-controlled oscillator that meets the requirements in the RF field is particularly important for promoting the development of portable mobile communication equipment and has broad market prospects. In recent years, people's research on voltage-controlled oscillators has become increasingly in-depth.

Voltage controlled oscillators can be divided into ring oscillators and LC oscillators. Ring oscillators are easy to integrate, but their phase noise performance is worse than that of LC oscillators. In order to make the phase noise meet the requirements of communication standards, the negative resistance LC voltage controlled oscillator is analyzed here, and a voltage controlled oscillator with excellent performance is designed using Agilent's ADS software, and then simulated and verified.

1 Circuit Principle and Design

1.1 Buffer design

The emitter follower (also known as the emitter output, or simply the emitter follower or follower) is a circuit with a common collector connection, as shown in Figure 1. It inputs the signal from the base and outputs the signal from the emitter. Its input impedance is high, and it has little effect on the previous stage circuit, so it can be used as the first stage of a multi-stage amplifier; its output impedance is low, and it has strong load capacity, so it can be used as the output stage of a multi-stage amplifier. Due to the above two characteristics, it can be used as a buffer stage in a multi-stage amplifier. An amplifier whose signal is output from the emitter. It is characterized by high input impedance, low output impedance, a voltage amplification factor slightly lower than 1, and strong load capacity. It can also be considered as a current amplifier, which is often used for impedance transformation and inter-stage isolation. The triode is connected in a common collector manner, that is, the base and the emitter are grounded, the base is input, and the emitter is output, which is also called a common collector amplifier. The dynamic voltage amplification factor is less than 1 and close to 1, and the output voltage is in phase with the input voltage, but the output resistance is low, and it has a current amplification effect, so it has a power amplification effect.

Figure 2 is a simulation of the buffer isolation effect. Through simulation, it is found that when the voltage-controlled oscillator is connected to an external circuit, the change of the external circuit impedance will not affect the impedance of the voltage-controlled oscillator.

1.2 Circuit Principle and Design Simulation

Voltage controlled oscillators can be divided into two categories according to their construction principles: feedback oscillators and negative resistance oscillators. A negative resistance oscillator is used here, which is mainly composed of a negative resistance device and a resonant circuit. The negative resistance effect of the negative resistance device is offset by the loss positive resistance in the resonant circuit to maintain stable oscillation of the resonant circuit. Figure 3 shows the voltage controlled oscillator circuit.

The negative transconductance of VQ5 and VQ6 in Figure 3 can compensate for the circuit loss in the oscillation and provide energy for the oscillation. The control voltage Vr controls the change of the capacitance of the varactor diode to achieve the purpose of controlling the oscillation frequency. VQ5 and VQ6 have the same size and are cross-coupled. Ignoring the second-order effects such as the channel modulation effect and the body effect, their equivalent circuit can be obtained, as shown in Figure 4.

Since Vce5=Vbe6, Vce6=Vbe5, when the oscillation is balanced, the voltage amplitudes at points A and B are symmetrical and equal, so Vce5=Vce6. Then the AC equivalent conductance from the collector to the emitter (i.e., both ends of AM) of VQ5 is:

In the formula, the reason for adding a negative sign in front of gmVbe5 is that when this current source increases, Vce5 decreases. Simplifying formula (1), we can get:

This is a negative conductance. Positive resistance absorbs energy, negative resistance provides energy, and here the negative conductance from the collector to the emitter of VQ5 indicates that the transistor provides energy conversion, converting the energy of the DC power supply into AC energy. Similarly, the equivalent conductance from the collector to the emitter of VQ6 (i.e., across BM) is -gm. Then the input resistance of the single-port network AB is the series connection of the emitter output resistances at both ends of VQ5 and VQ6, that is

When the negative resistance Rm provided by the single-port network is equal to the resistance of the parallel resonant circuit, the energy provided by the negative resistance compensates for the loss of the parallel resonant circuit, and the oscillation is maintained. The resonant frequency of the oscillator is equal to the parallel resonant frequency, and its output frequency is:

The circuit is simulated using Agilent's ADS software. From Figure 5, it can be seen that when the control voltage of the voltage-controlled oscillator changes from 1.98 to 3.98 V, the oscillator tuning range is 1.14 to 1.18 GHz. Figure 6 is the circuit layout of the voltage-controlled oscillator.

2 Conclusion

This paper introduces the design of a wide frequency modulation high frequency LC VCO. Its core circuit is realized by differential cross-coupling circuit structure. When connecting with external circuit, considering the influence of external circuit changes on voltage controlled oscillator, a buffer composed of emitter follower as the main structure is adopted to eliminate the influence of external circuit. In addition, on-chip integrated inductor is adopted in layout design to realize on-chip integration of the entire VC0 circuit and meet the design requirements.