A crystal oscillator[^1] is a quartz crystal resonator, which is a thin slice (referred to as a wafer) cut from a quartz crystal at a certain azimuth angle. It is also referred to as a quartz crystal or crystal or crystal oscillator. A crystal element that adds an IC to form an oscillation circuit inside the package is called a crystal oscillator. Its products are generally packaged in metal shells, but they can also be packaged in glass shells, ceramics or plastics.
Crystal oscillator is one of the most commonly used electronic components in electronic circuits. It is known as the "heart of electronic equipment". It is the most commonly used and indispensable component in electronic products. From mobile phones and computers to missiles and satellites, crystal oscillators are needed. As a (prospective) PCB design engineer, we must master the knowledge about crystal oscillators.
What types of crystal oscillators are there?
Crystal oscillators in electronic circuits are divided into two types: passive crystal oscillator[^2]s and active crystal oscillators. Passive crystal oscillators and active crystal oscillators have different English names. Passive crystal oscillators are called crystals, while active crystal oscillators are called oscillators.
- Passive crystal oscillator: Passive crystal oscillator is a non-polar component with 2 pins. It needs the help of clock circuit to generate oscillation signal. It cannot oscillate by itself, so the term "passive crystal oscillator" is not accurate.
The signal quality of passive crystal oscillator is poor, and it usually requires precise matching of peripheral circuits (capacitors, inductors, resistors, etc. for signal matching). When replacing crystals of different frequencies, the peripheral configuration circuits also need to be adjusted accordingly. It is generally recommended to use quartz crystals with higher precision, and try not to use ceramic crystals with low precision.
- Active crystal oscillator: Active crystal oscillator has 4 pins and is a complete oscillator. In addition to quartz crystal, it also has transistors and resistors and capacitors, so it is larger in size. The package of active crystal oscillator has 4 pins, namely VCC (voltage), GND (ground), OUT (clock signal output), and NC (empty pin).
Active crystal oscillators do not require the CPU's internal oscillator, have stable signals, good quality, and a relatively simple connection method (mainly power supply filtering, usually using a capacitor and inductor to form a filter network, and a small resistance resistor at the output end to filter the signal), and do not require complex configuration circuits.
What is the role of the crystal oscillator?
The crystal oscillator is the most important component in the clock circuit. Its main function is to provide reference frequency to various parts of accessories such as graphics cards, network cards, and motherboards. It is like a ruler. Unstable working frequency will cause unstable working frequency of related equipment, which is naturally prone to problems. Under normal working conditions, the absolute accuracy of ordinary crystal oscillator frequency can reach 50 parts per million, and the advanced ones have higher accuracy.
Another function of the crystal oscillator is to provide a basic clock signal for the system. Usually a system shares a crystal oscillator to facilitate synchronization of various parts. Some communication systems use different crystal oscillators for baseband and radio frequency, and synchronize them by electronically adjusting the frequency.
What should be paid attention to in the PCB design of the crystal oscillator circuit?
Choose the right position: There is a quartz crystal inside the crystal oscillator. If it is accidentally dropped or hit by an unknown impact, the quartz crystal is easy to break and damage, so the crystal oscillator is placed away from the edge of the board and close to the MCU.
Two close: The coupling capacitor should be as close to the power pin of the crystal oscillator as possible. If there are multiple coupling capacitors, they should be placed in order from large to small according to the direction of power inflow; the crystal oscillator should be as close to the MCU as possible.
Short wiring: All wires connecting the input/output ends of the crystal oscillator are as short as possible to reduce noise interference and the influence of distributed capacitance on the crystal oscillator.
High independence: Ensure that there are no other components around the crystal oscillator as much as possible. Prevent mutual interference between devices and affect the quality of clocks and other signals. 1mm around the crystal oscillator is prohibited from laying devices, 0.5mm is prohibited from laying vias, and no vias are drilled under all crystal oscillators (including ground vias).
The shell must be grounded: The shell of the crystal oscillator must be grounded. In addition to preventing the crystal oscillator from radiating outward, it can also shield external interference.
[^1]: Understanding crystal oscillators is crucial for anyone in electronics, as they are fundamental components in many devices.
[^2]: Exploring passive crystal oscillators will enhance your knowledge of their limitations and applications in electronic circuits.
