The crystal oscillator generates its own clock signal and serves as a timing reference for various microprocessor chips. It acts like the heart of these chips—without it, they cannot function.
The primary function of a crystal oscillator is to provide a basic clock signal for the system. Usually, a single oscillator is shared across a system to maintain synchronization. In some communication systems, different oscillators are used for baseband and radio frequency, and synchronization is achieved by electronically adjusting frequencies.
Crystal oscillators are mainly used in microcontrollers, DSPs, ARM, PowerPC, CPLD/FPGA CPUs, as well as communication interface circuits like PCI and CAN.
A crystal oscillator, often referred to as an active oscillator or simply an oscillator, is technically known as a Crystal Oscillator (XO). It requires power and contains an internal oscillation circuit.
A crystal, often called a passive oscillator or resonator, is technically referred to as a Crystal Resonator (XTAL). It doesn’t require power and needs an external oscillation circuit to function.
1. Working Principle of Crystal Oscillator
A quartz crystal oscillator is a resonating component utilizing the piezoelectric effect of quartz. Its basic construction involves slicing a quartz crystal at a specific angle, coating the opposite faces with silver to form electrodes, soldering wires to these electrodes, and enclosing it in a package, forming a crystal resonator—commonly referred to as a crystal or crystal oscillator. These are generally packaged in metal but can also be found in glass, ceramic, or plastic packages.
Quartz crystals exhibit a key property: when an alternating voltage is applied, they undergo mechanical vibration; conversely, mechanical vibration induces an alternating voltage—this is known as the piezoelectric effect.
Typically, the mechanical vibration amplitude and alternating field amplitude are minimal, but their frequency is highly stable. When the frequency of the applied voltage matches a specific value, the vibration amplitude sharply increases—this is called piezoelectric resonance, similar to LC circuit resonance.
A quartz oscillator can be electrically modeled as a network consisting of a capacitor and resistor in parallel, with another capacitor in series. This network has two resonant frequencies, fs and fp, with fp slightly higher than fs.
The lower frequency fs is the series resonant frequency; the higher fp is the parallel resonant frequency. These two are very close due to the crystal’s properties. Within this narrow frequency range, the oscillator can also be modeled as an inductor.
By adding suitable capacitors in parallel with the oscillator, a parallel resonance circuit is formed. Combined with a negative feedback circuit, it forms a sinusoidal oscillator.
Thanks to quartz’s stable chemical properties and low thermal expansion coefficient, the effective inductance region is narrow, leading to high frequency stability. The resonant frequency depends on the crystal’s shape, material, and cut angle. With precise geometry control, frequency remains stable even if other component parameters vary, ensuring a stable oscillator frequency.
2. Main Categories of Crystal Oscillators
- Passive Crystal Resonator (XTAL):
A crystal resonator that doesn’t operate on its own and requires an external circuit (e.g., oscillator circuit inside a chip) to generate a clock signal. Its frequency stability depends on the crystal’s properties and the external circuit design, typically within the 10⁻⁵ to 10⁻⁶ range. Simple in structure—just the crystal and its casing—it relies on an external excitation signal to function. - Simple Packaged Crystal Oscillator (SPXO):
A crystal oscillator without temperature compensation. Its frequency stability across temperatures depends on the internal crystal’s quality, usually around 10⁻⁵. Used in standard environments as a local oscillator or for intermediate frequency signals, it’s a low-cost solution for applications with low stability demands. - Temperature Compensated Crystal Oscillator (TCXO):
Features internal compensation for temperature-related frequency variation, operating accurately between -40℃ and 105℃ with frequency stability ranging from 5×10⁻⁷ to 5×10⁻⁸. Analog TCXOs use thermistor compensation networks that control a VCO or varactor diode. These are widely used for their good startup performance, cost-efficiency, low power, small size, and environmental adaptability. - Voltage Controlled Crystal Oscillator (VCXO):
A crystal oscillator whose output frequency can be tuned via an external voltage. The tuning range and linearity depend on the varactor diode and crystal properties. Mainly used in phase-locked loops and frequency fine-tuning. - Oven Controlled Crystal Oscillator (OCXO):
Maintains crystal temperature at the point of zero thermal coefficient for maximum frequency stability. Medium-precision OCXOs achieve 10⁻⁸ to 10⁻⁹ stability, and high-precision ones reach 10⁻¹⁰ or better.
By enclosing the crystal in a temperature-controlled oven, frequency drift from external temperature changes is minimized. The OCXO consists of a temperature control circuit and an oscillator circuit, often using a thermistor bridge in a differential amplifier configuration. It’s typically used as a frequency source or reference signal.
OCXOs are considered the pinnacle of crystal oscillator technology. Their crystals require high Q and low aging characteristics, combining cutting-edge industry techniques. OCXOs are essentially small electronic systems, critical like GPSDOs and rubidium clocks, but more cost-effective and versatile. With the expansion of 5G, demand is rising rapidly—a single small 5G base station requires at least one OCXO, and macro base stations may need over ten.
Different characteristics define application scenarios:
- For plug-and-play requirements: SPXO, VCXO, and TCXO are suitable.
- For high clock stability: TCXO and OCXO are recommended.
3. Applications of Crystal Oscillators
Oscillators are widely used across industries, with application-specific parameters like output frequency, frequency stability, operating temperature, input voltage and power, output waveform, package size and shape. Common application areas include:
- Research & Metrology: Atomic clocks, measuring devices, telemetry, remote sensing, remote control
- Industrial: Communication, telecom, mobile/cellular/portable terminals, aviation, marine, navigation, instruments, computers, digital devices, displays, disk drives, modems, sensors
- Consumer Electronics: Wearables, smart home, audio systems, cable TV, TVs, PCs, cameras, wireless communication, toys, medical devices
- Automotive: ADAS, infotainment systems, dashboards, digital mirrors, in-car cameras, mmWave radar, LiDAR, vehicle networks, tire pressure monitoring, in-car navigation systems.
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