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RF Crystal Oscillators

Key Takeaways

  • Crystal oscillators utilize the inverse piezoelectric effect of quartz crystals to produce electrical signals at precise frequencies

  • A variety of crystal oscillators, including XO, VCXO, TCXO, OCXO, and others, cater to diverse applications ranging from general electronics to high-precision needs 

  • The key specifications of crystal oscillators, such as frequency stability and phase noise, are crucial for determining their suitability in high-precision applications

RF crystal oscillator with equivalent LCR circuit model

RF crystal oscillator with equivalent LCR circuit model

RF Oscillator Key Specs


The operational frequency of the oscillator, measured in megahertz or gigahertz for RF applications.

Output Waveform

Oscillators produce specific output waveforms, including Sinewave, Clipped Sinewave, and more.

Supply Voltage (V)

The required voltage to power the crystal oscillator is measured in volts (V).

Frequency Stability (ppm)

This parameter indicates the variance in the oscillator's output frequency and is crucial for accuracy. It is measured in parts per million (ppm) and is vital for applications requiring high precision.

Phase Noise

A measure of the noise produced along with the desired signal. Lower phase noise is desirable as it signifies a clearer and more accurate signal. This is one of the most important specifications of a crystal oscillator.

RF Crystal Oscillator Circuits

The crystal oscillator circuit maintains oscillation by capturing a voltage signal from the quartz resonator, amplifying this signal, and then reintroducing it to the resonator. The resonant frequency depends on the crystal's size and the manner in which it is cut. Oscillation is achieved when the energy of the output frequencies produced equals the losses within the circuit.

In an RF crystal oscillator structure, a slice or tuning fork made of a quartz crystal is positioned between two electrically conductive plates. At the initial phase, the control circuit disrupts the crystal's equilibrium. Due to the system's positive feedback, even minor noise is magnified, escalating the oscillation process. In this context, the crystal resonator acts as an extremely selective frequency filter, permitting only a narrow range of frequencies close to the resonant frequency while damping others. Over time, the resonant frequency becomes predominant. The amplifier in the oscillator strengthens the signals emitted by the crystal, and as a result, the signals within the crystal’s frequency range increasingly dominate the oscillator's output.

Crystal Oscillators for Radio Frequencies Applications

Crystal oscillators are used in a variety of RF applications, including:

  • Precise Frequency Generation: Essential for producing stable RF signals in radio communications.
  • Carrier Frequency Production: Used in radio systems to ensure accurate transmission and reception.
  • Low Phase Noise: Vital for maintaining high-quality RF signals, reducing frequency fluctuations.
  • Frequency Stability: Crucial in telecommunications and GPS systems for reliable performance. 

RF Crystal Oscillator Operating Principles

Quartz crystals are the most commonly used piezoelectric resonators in oscillator circuits, giving rise to the term "crystal oscillators." Although quartz is predominant, other piezoelectric materials, such as polycrystalline ceramics, are also employed in similar circuits. These oscillators work based on the inverse piezoelectric effect, where a quartz crystal changes shape slightly under an electric field. Applying voltage to the crystal's electrodes causes it to deform, and removing the voltage induces a small voltage as the crystal elastically reverts to its original shape. This oscillation process in quartz mirrors that of an RLC circuit (comprising an inductor, capacitor, and resistor) with a significantly higher Q factor, meaning less energy is lost with each oscillation cycle. Once calibrated to a specific frequency, influenced by various factors, including electrode mass, crystal orientation, and temperature, the quartz crystal reliably maintains this frequency with high precision.

All objects, including crystals, have innate resonant frequencies of vibration. High-frequency crystals are often shaped as simple rectangles or circular disks, while low-frequency varieties, such as those in digital watches, are typically crafted in a tuning fork shape. 

Other Crystal Oscillator Types and Applications


Full Name



Crystal Oscillator

General use in electronics without temperature compensation, fixed frequency applications


Voltage Controlled Crystal Oscillator

Fine adjustments in frequency, mobile phones, RF interference reduction


Temperature Compensated Crystal Oscillator

Devices requiring stable frequency in varying temperatures, portable electronics


Oven Controlled Crystal Oscillator

Satellites, telecom, and devices needing highly stable frequency references


Voltage Controlled Temperature Compensated Crystal Oscillator

Applications requiring both temperature compensation and fine frequency adjustments


Digitally Controlled Crystal Oscillator

Digital fine-tuning of frequency in communication and precision equipment


Voltage Controlled SAW Oscillator

High-frequency applications, telecommunication, and precision equipment

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