What Is a Voltage-Controlled Oscillator?

Voltage controlled oscillator refers to an oscillator circuit (VCO) whose output frequency corresponds to the input control voltage. The oscillator VCO whose frequency is a function of the input signal voltage. The operating state of the oscillator or the component parameters of the oscillation circuit are controlled by the input control voltage. To form a voltage controlled oscillator.

Voltage controlled oscillator

Voltage controlled oscillator

In any kind of LC oscillator, an LC voltage controlled oscillator can be formed by inserting a voltage controlled variable reactance element into the oscillation circuit. The early voltage-controlled variable reactance elements were reactors, and most of them later used varactor diodes. Figure 2 is

Using

Center frequency

It refers to the middle value of the frequency adjustment range, that is, the middle value of the maximum and minimum values of the oscillator frequency. The size of the center frequency depends on the structure and component parameters of the oscillator, and also changes accordingly with the process and temperature. With the continuous development of science and technology and the improvement of product performance, the center frequency of CMOS voltage-controlled oscillators can now reach 10GHz.

2. Tuning range

It refers to the range of adjustment of the output frequency, that is, the difference between the maximum tuning frequency and the minimum tuning frequency of the oscillator

The voltage-controlled oscillator must have a sufficiently large tuning range to meet the required value of the output frequency.

3. Tuning gain

That is, the sensitivity of the voltage-controlled oscillator refers to the change in the unit's input voltage and output frequency. It is generally expressed in Kv and the unit is Hz / V. In practical applications, the higher the sensitivity of the voltage controller, the more the noise response will be on the control circuit. The stronger it is, the larger the interference output frequency will be, which will reduce the noise performance of the voltage controlled oscillator. So we need to find the balance between the gain and noise performance of the VCO.

4. Output amplitude

That is, the peak value of the VCO output spectrum. By optimizing the phase noise, it is necessary to increase the amplitude of the output voltage as much as possible, so that the voltage control gain

reduce. Continuously decreasing, it is especially important to increase the output amplitude. With the continuous development of CMOS technology, the input voltage is continuously decreasing, and it is especially important to increase the output amplitude.

5. Tuning linearity

Voltage controlled gain

The ideal voltage-controlled oscillator is a constant, and the performance of the voltage-controlled oscillator is non-linear in actual work.

. As a constant, try to minimize the variation in the tuning range.

6. Phase noise

When the oscillator enters a stable state, the noise in the circuit interferes with the circuit operation, which is the phase noise. The unit is dBc / Hz.

7. Power consumption

In the work, noise in the circuit and reduction of power consumption are the main research directions of CMOS voltage controlled oscillators. The power consumption of the oscillator is closely related to the operating frequency, input voltage and output frequency. Current oscillator power consumption can reach one to several tens of mW.

8. Other performance indicators

The output frequency's spectral density, due to other effects such as noise, the output waveform is not an ideal waveform. In order to make it reach the ideal waveform, the existence of harmonics should be suppressed when designing the circuit. The impact is also large. In order to achieve better common mode rejection, a differential line or other line should be adopted in the design. ^[1]

Voltage controlled oscillators are often used in:

1. Signal generator.

2. Used in electronic music to create transposition.

3. Phase locked loop.

4. Frequency synthesizer in communication equipment.

Using a Voltage Controlled Oscillator to Control Frequency

The voltage-controlled frequency part of the high-frequency voltage-controlled oscillator is usually an LC resonant circuit connected by a varactor C and an inductor L. Increasing the reverse bias voltage of the varactor diode will increase the empty area in the diode. As the distance between the two conductor surfaces becomes longer, the capacitance will decrease, and the resonant frequency of this LC circuit will be increased. Conversely, the reverse bias voltage is reduced As the capacitance in the diode increases, the frequency decreases.

The low-frequency voltage-controlled oscillator selects different methods according to different frequencies. For example, a voltage-controlled current source is obtained by changing the charging rate of a capacitor. See Waveform Generator.

Voltage controlled crystal oscillator

A "voltage-controlled crystal oscillator (VCXO)" is usually used in the following situations: when the frequency needs to be adjusted in a small range, when the correct frequency or phase is very important for the oscillator The oscillator that uses different voltages as the control source and is used to disperse the interference in a certain frequency range so that the frequency band is not greatly affected. The typical frequency variation of a voltage controlled quartz oscillator is between tens of ppm. This is because a high quality factor (or Q Factor) quartz oscillator will only produce a small amount of frequency range shift.

When a radio frequency circuit transmits (transmitter) radio waves, heat is generated and frequency drift occurs, which makes "temperature-compensated VCXO (TCVCXO)" widely used because TCVCXO is not affected by temperature. Change its piezoelectric characteristics. ^[2]