The characteristics of a voltage-controlled oscillator are represented by a curve (Figure 1) showing the relationship between the output angular frequency ω0 and the input control voltage uc. In the figure, the angular frequency ω0,0 when uc is zero is called the free-running angular frequency; the slope K0 of the curve at ω0,0 is called the control sensitivity. In communications or measuring instruments, the input control voltage is the signal to be transmitted or measured (the modulating signal).
A voltage-controlled oscillator is commonly called a frequency modulator, used to generate a frequency-modulated signal. 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.
VCO types include LC, RC, and crystal. The main technical requirements for voltage-controlled oscillators include good frequency stability, high control sensitivity, a wide frequency modulation range, a linear relationship between frequency deviation and control voltage, and ease of integration. Crystal voltage-controlled oscillators have high frequency stability but a narrow frequency modulation range; RC voltage-controlled oscillators have low frequency stability but a wide frequency modulation range; and LC voltage-controlled oscillators fall somewhere in between.