VCO vs VCXO: Key Differences Explained

This article compares Voltage Controlled Oscillators (VCOs) and Voltage Controlled Crystal Oscillators (VCXOs), highlighting the differences between them.

VCO (Voltage Controlled Oscillator)

Definition

A Voltage Controlled Oscillator (VCO) is an oscillator whose frequency is controlled by an external input DC voltage.

Applications

VCOs are utilized in various applications, including:

• Voltage-to-Frequency Conversion: Ideal when a voltage level needs to be translated to a corresponding frequency.
• FM Modulators: Used to generate frequency-modulated signals.
• RF Synthesizer Design: A critical component in generating desired radio frequencies.

Examples

Examples of VCOs include:

• SN74LS624 from TI (Texas Instruments)
• LM566C from National Semiconductor

VCXO (Voltage Controlled Crystal Oscillator)

Definition

A Voltage Controlled Crystal Oscillator (VCXO) is an oscillator that utilizes a crystal to determine its base oscillating frequency. This frequency can be subtly adjusted by varying an applied control voltage.

Functionality

The core oscillation frequency is determined by the crystal. A small adjustment to this frequency is achieved by applying a DC voltage. This allows for fine-tuning and compensation.

Implementation in Mobile Phones

In mobile phone design, the VCXO is often interfaced with a Digital-to-Analog Converter (DAC). The DAC, in turn, is connected to a microcontroller. The microcontroller provides a digital input, which the DAC converts into an analog voltage. This analog voltage then controls the VCXO’s output frequency. Any deviation in the frequency can be compensated for using the voltage control pin.

Applications

VCXO-based clock generators are found in:

• ADSL (Asymmetric Digital Subscriber Line)
• STB (Set-Top Box)
• Digital Audio/TV
• And other applications requiring precise clock signals.

Example

An example of a VCXO is the MAX9485 from Maxim Integrated, an IC specifically designed as a VCXO clock generator.