Definition of voltage controlled oscillator (VCO) Working principle and application of voltage controlled oscillator

What is a voltage controlled oscillator?

Refers to an oscillator circuit in which the output frequency has a corresponding relationship with the input control voltage, often represented by the symbol (VCO) (Voltage Controlled Oscillator).

Its characteristics are represented by the relationship curve between the output angular frequency ω0 and the input control voltage uc (Figure 1). In Figure 1, the angular frequency ω0,0 when uc is zero is called the free oscillation angular frequency; the slope K0 of the curve at ω0,0 is called the control sensitivity. A voltage-controlled oscillator can be constructed by making the working state of the oscillator or the component parameters of the oscillation circuit controlled by the input control voltage. In communication or measuring instruments, the input control voltage is the signal to be transmitted or measured (modulation signal). People usually call voltage-controlled oscillators frequency modulators to generate frequency-modulated signals. In automatic frequency control loops and phase-locked loops, the input control voltage is the error signal voltage, and the voltage-controlled oscillator is a controlled component in the loop.

Figure 1 Control characteristics of a voltage-controlled oscillator

In electronic equipment, voltage-controlled oscillators are widely used, such as the local oscillator circuit in the high-frequency head of a color TV receiver, the oscillator circuit in various automatic frequency control (AFC) systems, and the oscillator circuit used in a phase-locked loop (PLL). The waveform output by the oscillator is either sinusoidal or square. The
basic working principle of the varactor diode voltage-controlled oscillator
After a voltage-controlled reactance element is connected in parallel or in series to the oscillation circuit of the oscillator, the oscillation frequency can be controlled. The controlled reactance element is often replaced by a varactor diode.

Figure 2

The capacitance Cj of the varactor diode depends on the size of the external control voltage. The change of the control voltage will change the Cj of the varactor tube, and the change of Cj will cause the change of the oscillation frequency.
For the figure, if the values ​​of C1 and C2 are large, and C4 is a DC blocking capacitor with a large capacity, the total capacitance in parallel with L in the oscillation circuit is:

The varactor is a crystal diode made by using the characteristic that the junction capacitance of the semiconductor PN junction is controlled by the external reverse voltage. It is a voltage-controlled variable reactance device. The typical curve of its voltage-controlled characteristic is shown in the figure. In the figure, when the reverse bias voltage increases from 3V to 30V, the junction capacitance Cj decreases from 18pF to 3pF, and the capacitance change ratio is about 6 times.
For different Cj, the corresponding oscillation frequency is
(VR is the minimum)

(VR is the maximum)
The ratio of fmax and fmin is usually called the frequency coverage factor, represented by the symbol Kf. The frequency coverage factor of the above oscillation circuit is

Practical application circuit of VCO
The local oscillator circuit of the 6th to 12th frequency band in the VHF tuner of a color TV receiver is shown in the circuit. The control voltage VC is 0.5-30V. Changing this voltage changes the junction capacitance of the varactor tube, thereby obtaining a change in frequency. As can be seen from Figure (3), this is a typical Siler oscillation circuit. The oscillator tube is in a common collector configuration, and the oscillation frequency is about 170-220MHz. This method of achieving frequency adjustment by changing the DC voltage is usually called electrical tuning, which has great advantages over mechanical tuning.

Figure 3 Figure 4

TV receiver VHF local oscillator circuit diagram