Why do crystal oscillators need capacitors? How large should the capacitors be?
What is a crystal oscillator?
A crystal oscillator, also known as a crystal resonator, is an electromechanical device made by precisely cutting, grinding, and plating electrodes onto a quartz crystal with very low electrical loss, and then soldering leads onto it. This crystal has a crucial characteristic: when energized, it produces mechanical oscillations; conversely, when subjected to mechanical force, it generates electricity. This characteristic is called the electromechanical effect. A very important feature is that its oscillation frequency is closely related to its shape, material, and cutting direction. Because quartz crystals have very stable chemical properties and a very small coefficient of thermal expansion, their oscillation frequency is also very stable. Furthermore, because the geometric dimensions can be controlled with great precision, its resonant frequency is also very accurate.
A crystal oscillator is short for quartz oscillator. It is the most important component in clock circuits. Its function is to provide a reference frequency to various parts of components such as graphics cards, network cards, and motherboards. It acts like a ruler; an unstable operating frequency will cause instability in the operating frequency of related equipment, naturally leading to problems. Due to continuous improvements in manufacturing processes, crystal oscillators now exhibit excellent performance in key technical indicators such as frequency deviation, temperature stability, aging rate, and sealing, making them less prone to failure. However, it's still advisable to pay attention to the quality of the crystal oscillator when selecting one.
Crystal Oscillator Functions and Roles
Crystal oscillators play a crucial role in applications. Microcontroller clock sources can be categorized into two types: clock sources based on mechanical resonant devices, such as crystal oscillators and ceramic resonator circuits; and RC (resistor-capacitor) oscillators. One type is the Pierce oscillator configuration, suitable for crystal oscillators and ceramic resonator circuits. The other is a simple discrete RC oscillator. Oscillators based on crystal oscillators and ceramic resonator circuits typically offer very high initial accuracy and low temperature coefficients. RC oscillators offer fast start-up and relatively low cost, but their accuracy is generally poor across the entire temperature and operating power supply voltage range, varying from 5% to 50% of the nominal output frequency. Their performance is also affected by environmental conditions and the selection of circuit components. Careful consideration must be given to the selection of components and the layout of the oscillator circuit. When using oscillators, the ceramic resonator circuit and the corresponding load capacitors must be optimized according to the specific logic series. High-Q crystal oscillators are not sensitive to amplifier selection, but they are prone to frequency drift (and may even be damaged) when overdriven.
Environmental factors affecting oscillator operation include electromagnetic interference (EMI), mechanical vibration and shock, humidity, and temperature. These factors increase output frequency variations, increase instability, and in some cases, cause the oscillator to stop oscillating. Most of these problems can be avoided by using oscillator modules. These modules have built-in oscillators, provide low-impedance square wave outputs, and can guarantee operation under certain conditions. The two most common types are crystal oscillator modules and integrated RC oscillators (silicon oscillators). Crystal oscillator modules provide the same accuracy as discrete crystal oscillators. Silicon oscillators are more accurate than discrete RC oscillators, and in most cases, provide accuracy comparable to ceramic resonator circuits.
Power consumption must also be considered when selecting an oscillator. The power consumption of discrete oscillators is mainly determined by the power supply current of the feedback amplifier and the value of the internal capacitance of the circuit. The power consumption of CMOS amplifiers is proportional to the operating frequency and can be expressed as the power dissipation capacitance value. For example, the power dissipation capacitance value of the HC04 inverter gate circuit is 90pF. When operating at 4MHz and 5V, the equivalent current is 1.8mA. Adding the 20pF crystal load capacitor, the total current is 2.2mA. Ceramic resonant circuits generally have larger load capacitances, requiring correspondingly more current. In contrast, crystal oscillator modules typically require 10mA to 60mA of power supply current. The power supply current of a silicon oscillator depends on its type and function, ranging from a few microamps for low-frequency (fixed) devices to a few milliamps for programmable devices. A low-power silicon oscillator, such as the MAX7375, requires less than 2mA of current when operating at 4MHz. Optimizing the clock source for specific applications requires considering factors such as accuracy, cost, power consumption, and environmental requirements.
Crystal Oscillator Applications
1. General-purpose crystal oscillators, used in various circuits to generate oscillation frequencies.
2. Quartz crystal resonators for clock pulses, used in conjunction with other components to generate standard pulse signals, widely used in digital circuits.
3. Quartz crystal resonators for microprocessors.
4. CTVVTR uses a quartz crystal resonator.
5. Clocks use quartz crystal oscillators.
Why are capacitors added to crystal oscillators?
1. To meet the resonance condition. Specifically, the load capacitance of a crystal element refers to the total effective external capacitance across the crystal in the circuit. This is the capacitance required for the crystal oscillator to oscillate normally. Generally, external capacitors are added to make the equivalent capacitance across the crystal equal to or close to the load capacitance. Not all crystal oscillation circuits require matching capacitors. Whether or not a matching capacitor is needed depends on the form of the oscillation circuit; the equivalent model of the crystal must be used in the analysis.
2. Grounding: The two capacitors next to the crystal are grounded. This is actually the voltage divider capacitor in a three-point capacitor circuit; the grounding point is the voltage divider point. Using the ground point (voltage divider point) as a reference, the input and output of the oscillation pin are out of phase. However, from the perspective of the parallel resonant circuit (i.e., the two ends of the quartz crystal), a positive feedback is formed to ensure continuous circuit oscillation.
Alternatively, you can understand it this way:
The nominal value of a crystal oscillator has a "load capacitance" condition during testing. This condition must be met during operation for the oscillation frequency to match the nominal value. In other words, only by connecting a suitable capacitor can the crystal oscillator's start-up requirements be met, allowing it to function normally.
How much capacitor is needed for a crystal oscillator?
This depends on the crystal oscillator's specifications and factors in the circuit. Even for 16MHz crystal resonators, the load capacitance value may differ, such as 10pF or 20pF. These values are determined during manufacturing and cannot be changed. You should receive an accurate datasheet when purchasing a crystal oscillator.
When a crystal oscillator is used in a circuit, the following condition must be met: CL = C + CS.
CL is the load capacitance value specified in the crystal's datasheet; C is the external capacitance value in the circuit (usually obtained by connecting two capacitors in series and parallel); and CS is the distributed capacitance of the circuit. This value is uncertain and depends on factors such as circuit design and component distribution, but is generally between 3 and 5 pF.
Therefore, the appropriate capacitance value can be roughly calculated using the above formula, and this capacitance value will allow the crystal oscillator to operate near its nominal frequency.
A crystal oscillator, also known as a crystal resonator, is an electromechanical device made by precisely cutting, grinding, and plating electrodes onto a quartz crystal with very low electrical loss, and then soldering leads onto it. This crystal has a crucial characteristic: when energized, it produces mechanical oscillations; conversely, when subjected to mechanical force, it generates electricity. This characteristic is called the electromechanical effect. A very important feature is that its oscillation frequency is closely related to its shape, material, and cutting direction. Because quartz crystals have very stable chemical properties and a very small coefficient of thermal expansion, their oscillation frequency is also very stable. Furthermore, because the geometric dimensions can be controlled with great precision, its resonant frequency is also very accurate.
A crystal oscillator is short for quartz oscillator. It is the most important component in clock circuits. Its function is to provide a reference frequency to various parts of components such as graphics cards, network cards, and motherboards. It acts like a ruler; an unstable operating frequency will cause instability in the operating frequency of related equipment, naturally leading to problems. Due to continuous improvements in manufacturing processes, crystal oscillators now exhibit excellent performance in key technical indicators such as frequency deviation, temperature stability, aging rate, and sealing, making them less prone to failure. However, it's still advisable to pay attention to the quality of the crystal oscillator when selecting one.
Crystal Oscillator Functions and Roles
Crystal oscillators play a crucial role in applications. Microcontroller clock sources can be categorized into two types: clock sources based on mechanical resonant devices, such as crystal oscillators and ceramic resonator circuits; and RC (resistor-capacitor) oscillators. One type is the Pierce oscillator configuration, suitable for crystal oscillators and ceramic resonator circuits. The other is a simple discrete RC oscillator. Oscillators based on crystal oscillators and ceramic resonator circuits typically offer very high initial accuracy and low temperature coefficients. RC oscillators offer fast start-up and relatively low cost, but their accuracy is generally poor across the entire temperature and operating power supply voltage range, varying from 5% to 50% of the nominal output frequency. Their performance is also affected by environmental conditions and the selection of circuit components. Careful consideration must be given to the selection of components and the layout of the oscillator circuit. When using oscillators, the ceramic resonator circuit and the corresponding load capacitors must be optimized according to the specific logic series. High-Q crystal oscillators are not sensitive to amplifier selection, but they are prone to frequency drift (and may even be damaged) when overdriven.
Environmental factors affecting oscillator operation include electromagnetic interference (EMI), mechanical vibration and shock, humidity, and temperature. These factors increase output frequency variations, increase instability, and in some cases, cause the oscillator to stop oscillating. Most of these problems can be avoided by using oscillator modules. These modules have built-in oscillators, provide low-impedance square wave outputs, and can guarantee operation under certain conditions. The two most common types are crystal oscillator modules and integrated RC oscillators (silicon oscillators). Crystal oscillator modules provide the same accuracy as discrete crystal oscillators. Silicon oscillators are more accurate than discrete RC oscillators, and in most cases, provide accuracy comparable to ceramic resonator circuits.
Power consumption must also be considered when selecting an oscillator. The power consumption of discrete oscillators is mainly determined by the power supply current of the feedback amplifier and the value of the internal capacitance of the circuit. The power consumption of CMOS amplifiers is proportional to the operating frequency and can be expressed as the power dissipation capacitance value. For example, the power dissipation capacitance value of the HC04 inverter gate circuit is 90pF. When operating at 4MHz and 5V, the equivalent current is 1.8mA. Adding the 20pF crystal load capacitor, the total current is 2.2mA. Ceramic resonant circuits generally have larger load capacitances, requiring correspondingly more current. In contrast, crystal oscillator modules typically require 10mA to 60mA of power supply current. The power supply current of a silicon oscillator depends on its type and function, ranging from a few microamps for low-frequency (fixed) devices to a few milliamps for programmable devices. A low-power silicon oscillator, such as the MAX7375, requires less than 2mA of current when operating at 4MHz. Optimizing the clock source for specific applications requires considering factors such as accuracy, cost, power consumption, and environmental requirements.
Crystal Oscillator Applications
1. General-purpose crystal oscillators, used in various circuits to generate oscillation frequencies.
2. Quartz crystal resonators for clock pulses, used in conjunction with other components to generate standard pulse signals, widely used in digital circuits.
3. Quartz crystal resonators for microprocessors.
4. CTVVTR uses a quartz crystal resonator.
5. Clocks use quartz crystal oscillators.
Why are capacitors added to crystal oscillators?
1. To meet the resonance condition. Specifically, the load capacitance of a crystal element refers to the total effective external capacitance across the crystal in the circuit. This is the capacitance required for the crystal oscillator to oscillate normally. Generally, external capacitors are added to make the equivalent capacitance across the crystal equal to or close to the load capacitance. Not all crystal oscillation circuits require matching capacitors. Whether or not a matching capacitor is needed depends on the form of the oscillation circuit; the equivalent model of the crystal must be used in the analysis.
2. Grounding: The two capacitors next to the crystal are grounded. This is actually the voltage divider capacitor in a three-point capacitor circuit; the grounding point is the voltage divider point. Using the ground point (voltage divider point) as a reference, the input and output of the oscillation pin are out of phase. However, from the perspective of the parallel resonant circuit (i.e., the two ends of the quartz crystal), a positive feedback is formed to ensure continuous circuit oscillation.
Alternatively, you can understand it this way:
The nominal value of a crystal oscillator has a "load capacitance" condition during testing. This condition must be met during operation for the oscillation frequency to match the nominal value. In other words, only by connecting a suitable capacitor can the crystal oscillator's start-up requirements be met, allowing it to function normally.
How much capacitor is needed for a crystal oscillator?
This depends on the crystal oscillator's specifications and factors in the circuit. Even for 16MHz crystal resonators, the load capacitance value may differ, such as 10pF or 20pF. These values are determined during manufacturing and cannot be changed. You should receive an accurate datasheet when purchasing a crystal oscillator.
When a crystal oscillator is used in a circuit, the following condition must be met: CL = C + CS.
CL is the load capacitance value specified in the crystal's datasheet; C is the external capacitance value in the circuit (usually obtained by connecting two capacitors in series and parallel); and CS is the distributed capacitance of the circuit. This value is uncertain and depends on factors such as circuit design and component distribution, but is generally between 3 and 5 pF.
Therefore, the appropriate capacitance value can be roughly calculated using the above formula, and this capacitance value will allow the crystal oscillator to operate near its nominal frequency.
Aug 03,2026