Terminology » RF Components » Crystal Oscillator vs. Frequency Synthesizer: Key Differences Explained

Crystal Oscillator vs. Frequency Synthesizer: Key Differences Explained

crystal oscillator frequency synthesizer rf oscillator frequency

This article breaks down the differences between a crystal oscillator and a frequency synthesizer. Both serve as frequency generation sources, but they cater to different needs and applications.

Crystal Oscillator

A crystal oscillator is primarily used to generate a very stable, fixed low-frequency signal. Typically, these frequencies are around 10 MHz, 100 MHz, or similar values. The output of a crystal oscillator is fixed, and it’s available in various output formats like square wave, sine wave, or triangular wave.

  • Key Features:

    • Fixed frequency output.
    • High stability.
    • Various output waveforms available.
    • Often used as a reference clock.

  • Applications:

    • Clock reference in embedded circuits.
    • Reference oscillator for PLL circuits.
    • Stable reference frequency in Vector Signal Generators (VSGs) and Vector Signal Analyzers (VSAs) for accurate measurements.

  • Types of Crystal Oscillators:

    • OCXO (Oven Controlled Crystal Oscillator)
    • EMXO (Evacuated Miniature Crystal Oscillator)
    • TCXO (Temperature Compensated Crystal Oscillator)
    • VCXO (Voltage Controlled Crystal Oscillator)
    • VCSO (Voltage Controlled SAW Oscillator)
    • XO (Crystal Oscillator)

Frequency Synthesizer

A frequency synthesizer is designed to generate different frequencies within a specific band. The selection of frequencies depends on the step size for which it’s designed. Think of it as a variable frequency generator.

  • Key Features:

    • Variable frequency output within a band.
    • Frequency selection based on step size.
    • Uses a crystal oscillator as a reference.

  • Applications:

    • RF channel selection in modems and RF transceivers.
    • Variable RF frequency generation.

  • Relationship with Crystal Oscillators: A crystal oscillator is often used as the reference input frequency source for a frequency synthesizer.

Crystal Oscillator vs. Frequency Synthesizer: The Core Difference

In essence, a crystal oscillator provides a stable, fixed frequency, while a frequency synthesizer provides a range of selectable frequencies. The synthesizer often relies on the stability of a crystal oscillator for its reference.