Crystal Oscillators

1. Definition

1.1 Crystal Oscillator

A crystal oscillator is an electronic device that uses the mechanical resonance of a vibrating crystal (typically quartz) to generate a precise and stable frequency signal.
It is commonly used for clock generation in microcontrollers, communication systems, and timing applications.

crystal oscillators

Crystal oscillator symbol

crystal oscillator symbol

Crystal oscillator Circuit Schematic symbol and equivalent circuit for a quartz crystal in an oscillator

crystal oscillator symbol

1.2 Resonant Frequency

The resonant frequency is the frequency at which the crystal naturally vibrates when an electrical signal is applied.
It is determined by the crystal’s cut, shape, and size and is measured in hertz (Hz), typically in kHz or MHz for practical use.

2. Features

  • Frequency Range: most common are 32.768 kHz (for real-time clocks) and several MHz (for microcontrollers).
  • Frequency Stability: how much the output drifts with temperature, load, or aging.
  • Load Capacitance: external capacitance needed to achieve the specified frequency.
  • Drive Level: the amount of power applied to the crystal, usually measured in microwatts.
  • Aging Rate: slow change in frequency over time.
  • Package Type: through-hole (metal can) or SMD (surface mount).

4. How to Use

Safety Note

Crystals are physically delicate; excessive mechanical shock, bending, or overheating during soldering can damage them.
Also, driving a crystal beyond its rated power can crack it internally.

4.1. Identify the crystal oscillator

  • Frequency is usually printed on the metal can or package (e.g., 8.000 MHz, 32.768 kHz).
  • Package style can help identify the mounting method (e.g., HC-49, SMD).

crystal markings

4.2. Choose the right crystal oscillator

  • Frequency (must match the circuit’s design requirements).
  • Load capacitance (specified in the datasheet; match with circuit capacitors).
  • Stability (ppm rating, e.g., ±20 ppm).
  • Package type (through-hole or surface-mount).
  • Temperature range (especially for outdoor or industrial use).

4.3. Connect it correctly

  • Crystals are non-polarized, so orientation doesn’t matter electrically.
  • Usually connected across the input and output pins of an oscillator circuit, with capacitors to ground.
  • For microcontrollers: connect to the XTAL pins and follow datasheet capacitor recommendations.

4.4. Test and verify

  • Use an oscilloscope or frequency counter to confirm output frequency.
  • Ensure proper oscillation startup, incorrect load capacitors can prevent operation.
  • Check for drift with temperature if stability is critical.

4.5. Install in circuit

  • On breadboard: not recommended for high-frequency crystals due to stray capacitance.
  • On PCB: keep leads short and capacitors close to reduce noise and instability.

5. When to Use

  • Microcontroller Clock Source: to provide precise timing for CPUs.
  • Real-Time Clocks (RTC): 32.768 kHz crystals for low-power timekeeping.
  • Communication Systems: for frequency generation in transmitters and receivers.
  • Signal Processing: as part of frequency synthesizers and filters.
  • Test Equipment: in frequency standards.

6. Video Explanation

video coming soon