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The load capacitance formula for parallel vibration is obtained
The internal capacitance of the oscillator is approximately equal to
NaN PF
The formula for calculating the load capacitance is calculated
The load capacitance of the crystal oscillator is respectively connected to the two feet of the crystal oscillator and the capacitance to ground, generally tens of PF. It will affect the resonant frequency of the crystal oscillator and the output amplitude, the average customer asked us about the crystal oscillator, we will ask them what is the required load capacitance of the crystal. A resistance connected to the crystal is a feedback effect, making the oscillator easy to start oscillating.
Crystal load capacitance value is directly related to the accuracy of FM. If the load capacitance is not accurate enough, then buy the accuracy of the crystal will be poor, on the calculation method of load capacitance that is seen from both ends of the crystal capacitance into the sum.
Calculated formula: Crystal load capacitance = [(Cd * Cg) / (Cd + Cg)] + Cic + △ C; where Cd, Cg are respectively connected to the crystal two feet and the ground capacitance; Cic Circuit internal capacitance; △ C (PCB capacitor) is generally 3 to 5pf.
Matching capacitance: The load capacitance refers to the capacitance that the crystal oscillator needs to be in normal oscillation. General external capacitor is to make the equivalent capacitance of the crystal at both ends equal to or close to the load capacitance. High occasions also need to consider ic input capacitance to ground. The capacitance of both ends of the common crystal is double the required load capacitance. This is close to the load capacitance in parallel.
2. Load capacitance refers to the total external effective capacitance across the crystal in the circuit. He is a test condition, but also a condition of use. Application is generally given in the vicinity of the load capacitance value can be adjusted to get the exact frequency. The size of this capacitor mainly affects the load resonant frequency and the equivalent load resonant resistance.
3. In general, increasing the load capacitance causes the oscillation frequency to drop, while decreasing the load capacitance causes the oscillation frequency to increase.
4. load capacitance refers to the two leads of the crystal oscillator connected to the IC block and the outside of all the effective capacitance and can be seen as crystal in the circuit capacitor in series. Different load frequency determines the oscillation frequency of the oscillator is different. The same nominal crystal frequency, the load capacitance may not be the same. Because the quartz crystal oscillator has two resonant frequencies, one is a low-load capacitance crystal in series with a vibrating crystal and the other is a high-load capacitive crystal in parallel with a vibrating crystal. Therefore, the same nominal frequency of the crystal oscillator must also be required when the load capacitance to one, can not be used interchangeably, otherwise it will cause the electrical work is not normal.