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YXC Crystal Oscillator Knowledge Classroom Q&A
The role of two external capacitors
Q&A ：How to choose crystal capacitor
What is a capacitor
The load capacitance of a crystal element refers to the total external effective capacitance across the two ends of the crystal in the circuit, that is, the capacitance required for the crystal to oscillate normally. Generally, the external capacitor is used to make the equivalent capacitance at both ends of the crystal equal to or close to the load capacitance. In cases with high requirements, the capacitance to ground at the input end of the IC should also be considered. When applied, the precise frequency can be obtained by adjusting near the given load capacitance value. The size of this capacitor mainly affects the load resonant frequency and the equivalent load resonant resistance.
How to choose a capacitor
The capacitor and the internal circuit together form an oscillation of a certain frequency. This capacitor is hard-connected and has a strong fixed frequency capability, so it is difficult for other frequencies to interfere.
The crystal oscillator circuit is actually a capacitor three-point oscillation circuit. The output is a sine wave crystal equivalent to an inductor. Two slot voltage divider capacitors are added. The smaller the capacitance at the input end, the greater the positive feedback. Load capacitance is a parameter that each crystal oscillator will have, such as how many PPM the stability is, some people will call it frequency difference, the unit is PPM, how many PF the load capacitance is, etc. When the crystal oscillator is connected to the oscillation circuit, and the capacitance introduced in the oscillation circuit does not meet the capacity requirements of the crystal oscillator's load capacitance, the frequency of the oscillation circuit will be different from the frequency marked by the crystal oscillator.
For example:
A 4.0000MHz ±20PPM crystal oscillator with a load capacitance of 16PF, when the load capacitance is 10PF, the frequency of the oscillation circuit may be 4.0003MHz, and when the load capacitance is 20PF, the frequency of the oscillation circuit may be 3.9997MHz.
In some circuits with high frequency accuracy requirements, such as the PLL reference, multiple adjustable capacitors are used to fine-tune the frequency. If the frequency accuracy requirements are not high, fixed capacitors can be used. There are generally two ways to connect the crystal oscillator load capacitance: 1 in parallel with the crystal oscillator and 2 in series with the crystal oscillator. The second method is more commonly used to connect a capacitor to the AC ground on both legs.
The load capacitance of the crystal oscillator = [(Cd*Cg)/(Cd+Cg)]+Cic+△C, where Cd and Cg are the capacitances connected to the two legs of the crystal oscillator and to the ground, respectively, and Cic (internal capacitance of the integrated circuit) +△C (capacitance on the PCB). That is to say, if the load capacitance is 15pf, it is almost enough to connect 27pf on both sides, and a is generally 6.5~13.5pF.
The role of two external capacitors
01 Increase frequency stability
Under different working conditions, such as power supply voltage fluctuations or temperature changes, two capacitors can provide a better resonant frequency.
02 Manufacturing tolerance
The nominal load capacitance of a passive crystal oscillator is usually a range, not a fixed value. Two capacitors can flexibly adjust the actual load capacitance and compensate for the tolerance in the manufacturing process.
03 Flexible circuit design
Two capacitors are more flexible in design, and engineers can adjust the characteristics of the resonant circuit according to actual application requirements.
