Crystal oscillator products involve more than a dozen key parameters, such as operating frequency, frequency stability, temperature frequency deviation, load capacitance, motional resistance, package type, aging rate, etc. Since all electronic circuits rely on repetitive and stable frequency signals as references, selecting an appropriate crystal oscillator is a fundamental prerequisite for ensuring long-term, efficient, and reliable system operation.
Package Size
The package size of a crystal oscillator determines the PCB space it occupies. For most modern electronic devices, common package types include surface-mount devices (SMD), such as 2.5 × 2.0 mm, 3.2 × 2.5 mm, and 5.0 × 3.2 mm, as well as through-hole DIP packages, which can meet the requirements of different application scenarios.
Operating Frequency
This is the most basic parameter of a crystal oscillator and the primary consideration for engineers during circuit design and component selection. Common frequencies include 32.768 kHz, 8 MHz, 12 MHz, 16 MHz, 24 MHz, 32 MHz, 125 MHz, and others.
Frequency Stability
Frequency stability refers to the maximum deviation of the operating frequency from the nominal frequency at the reference temperature, expressed in parts per million (ppm). A smaller ppm value indicates higher accuracy.
For example, a 12 MHz crystal oscillator with a tolerance of ±20 ppm has a frequency deviation of ±240 Hz (12 MHz × 20 ppm), meaning the actual frequency range is 11,999,760 Hz to 12,000,240 Hz.
Temperature Frequency Deviation
This parameter describes the allowable frequency deviation relative to the reference temperature over a specified temperature range, also expressed in ppm.
Load Capacitance
Load capacitance refers to the total effective capacitance connected across the crystal terminals in the circuit (not simply the external matching capacitors). It mainly affects the load resonant frequency and equivalent load resistance. Together with the crystal, it determines the operating frequency of the oscillator circuit. By adjusting the load capacitance, the oscillation frequency can be fine-tuned to the nominal value.
Operating Voltage
The supply voltage required for crystal oscillator operation, with common values including 1.8 V, 3.3 V, and 5 V.
Operating Temperature Range
This defines the temperature conditions under which the crystal oscillator can operate reliably. Typical ranges include standard temperature (0 to +70 °C), industrial temperature (−40 to +85 °C), and extended temperature (−40 to +125 °C).
Static Capacitance (C₀)
The capacitance of the static arm in the equivalent circuit. Its value depends on electrode area, crystal thickness, and manufacturing process.
Motional Capacitance (C₁)
The capacitance of the motional arm in the equivalent circuit. It is related to electrode area, crystal plate parallelism, and frequency trimming range.
Start-up Time
The time required for the crystal oscillator to reach stable operation after power is applied. Generally, shorter start-up times are preferred.
Drive Level / Drive Capability
Drive capability indicates the maximum current or power the crystal can safely withstand, usually specified in microamperes (µA).
Output Waveform
Common output waveforms include sine wave, square wave, and differential signals. Different applications have different waveform requirements.
Phase Noise
Phase noise refers to the random phase fluctuations of the output signal caused by various noise sources. It is a key indicator for evaluating the frequency stability and spectral purity of precision frequency sources and is typically expressed in dBc/Hz.
Q Factor (Quality Factor)
The Q factor represents the quality of the crystal oscillator. A higher Q value indicates better frequency stability and higher precision.
Motional Resistance (Rr)
The equivalent resistance of the crystal element at its resonant frequency. This parameter directly affects the quality factor of the crystal.
Load Resonance Resistance (RL)
The resistance of the crystal element at the load resonant frequency (FL) when connected in series with the specified external load capacitance. For a given crystal, RL depends on the load capacitance used. The load resonant resistance is always higher than the crystal’s intrinsic motional resistance.
Aging Rate
Aging refers to the gradual change in frequency over time, typically specified per week, month, or year. It is independent of temperature, voltage, and other operating conditions.
