In VCO the output frequency is a linear function of its input : 

The correct answer is Voltage.

Key Points

• A Voltage-Controlled Oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input.
  • The output frequency of a VCO is directly proportional to the input voltage, meaning that as the input voltage changes, the output frequency changes linearly.
  • VCOs are widely used in communication systems, function generators, and phase-locked loops (PLLs).
  • The input voltage is typically applied to a varactor diode or a voltage-controlled capacitor, which adjusts the frequency of the oscillator circuit.
  • VCOs can generate a wide range of frequencies, making them versatile in various applications.

Additional Information

• VCOs can be designed using different technologies, including analog and digital circuits.
• In analog VCOs, the frequency is usually adjusted by changing the capacitance or inductance in the circuit.
• Digital VCOs use digital-to-analog converters (DACs) to convert a digital input into a corresponding voltage that controls the frequency.
• VCOs are critical components in frequency modulation (FM) and phase modulation (PM) systems.
• Modern VCOs offer high-frequency stability and low phase noise, which are essential for high-performance communication systems.