Stable clock signals are essential for MCUs, communication modules, industrial control systems, networking equipment, and many other electronic devices.
However, choosing the right timing component involves more than simply selecting a frequency and package size. The first step is to understand the difference between a quartz crystal (crystal resonator) and a crystal oscillator, and then evaluate the parameters that matter for each type.
The main difference is simple:
A quartz crystal is a passive resonator that requires an oscillator circuit, while a crystal oscillator integrates the oscillator circuitry and provides a clock output directly.
A quartz crystal works with the oscillator circuit inside an MCU, SoC, or other IC and cannot provide a complete clock signal by itself.
Crystals are widely used in MCUs, consumer electronics, IoT devices, and industrial control systems because of their compact size, flexibility, and cost-effectiveness.
A crystal oscillator integrates the resonator and oscillator circuitry and can provide a clock signal directly to the receiving IC.
It is commonly used when a system has specific requirements for clock output, frequency stability, or signal quality, such as communication equipment, networking devices, industrial systems, and high-speed interfaces.
Depending on the application, designers may choose a single-ended or differential oscillator.
When selecting a quartz crystal, consider these key parameters:
1. Frequency
Confirm the frequency required by the IC, such as 12 MHz, 24 MHz, 25 MHz, or 40 MHz.
2. Load Capacitance (CL)
CL is a key parameter for passive crystals. It represents the effective load capacitance seen by the crystal, including external capacitors, IC pin capacitance, and PCB parasitic capacitance.
3. Frequency Tolerance and Stability
Select the appropriate ppm specification according to the system requirements and operating temperature range.
4. Operating Temperature
Industrial applications may require -40°C to +85°C or a wider temperature range.
5. Package Size
Common SMD packages include 1612, 2012, 2520, 3225, 5032, and 7050. Consider PCB space, manufacturing, reliability, and cost when selecting the package.
For more detailed designs, ESR and drive level should also be checked against the IC manufacturer's oscillator requirements.
For an active crystal oscillator, the selection criteria are slightly different:
1. Output Frequency
Choose the frequency required by the receiving IC.
2. Supply Voltage
Common options include 1.8 V, 2.5 V, and 3.3 V.
3. Output Type
Select an output compatible with the clock input, such as LVCMOS, LVDS, LVPECL, or HCSL. Differential outputs are commonly used in high-speed communication and networking applications.
4. Frequency Stability
Choose the appropriate ppm specification according to the application's timing requirements and operating temperature.
5. Jitter and Phase Noise
For high-speed communication, SerDes, RF, and other performance-sensitive applications, jitter and phase noise may also be important selection criteria.
A simple way to start is:
| System Requirement | Typical Choice |
|---|---|
| IC has an oscillator circuit and requires an external resonator | Quartz Crystal |
| A ready-to-use clock output is required | Crystal Oscillator |
| Compact and cost-effective design | Quartz Crystal |
| Defined clock output characteristics | Crystal Oscillator |
| High-speed differential clock | Differential Oscillator |
This is only a general guide. The receiving IC datasheet should always be the primary reference for final selection.
A Simple Crystal and Oscillator Selection ProcessA practical selection process can be summarized in six steps:
For high-performance applications, also check specifications such as ESR, drive level, jitter, phase noise, and aging when required.
The key to crystal and oscillator selection is to start with the clock requirements of the receiving IC rather than frequency and package size alone.
YXC provides quartz crystals, crystal oscillators, TCXOs, OCXOs, differential oscillators, and programmable timing solutions for a wide range of electronic applications.
For product selection, provide your frequency, package, load capacitance or supply voltage, frequency stability, operating temperature, output type, and application requirements to help identify a suitable timing solution.
