Crystal load capacitors are capacitors connected to the two pins of the crystal and to ground, which are usually in the tens of PF. It will affect the resonant frequency and output amplitude of the crystal. When customers ask us for a crystal, we will ask them what the load capacitance of the crystal is. A resistor connected to the crystal is feedback, which makes the oscillator easy to start.

        The value of the crystal load capacitance is directly related to the accuracy of the frequency modulation. If the value of the load capacitance in the actual circuit is not accurate enough, the accuracy of the crystal purchased will be biased. The calculation method of the load capacitance is the sum of the capacitance seen from the two ends of the crystal.

       Calculation formula: load capacitance of the crystal = [(Cd * Cg) / (Cd + Cg)] + Cic + △ C; where Cd and Cg are the capacitors connected to the two pins of the crystal and to ground respectively; Internal circuit capacitance); △ C (capacitance on PCB) is generally 3 to 5pf.

       1. Matching capacitance: The load capacitance refers to the capacitance required for the crystal to oscillate normally. Generally, the external capacitance is to make the equivalent capacitance at both ends of the crystal equal to or close to the load capacitance. Where high requirements are required, the capacitance to ground at the ic input must also be considered. Generally, the capacitance connected to the two ends of the crystal is twice the required load capacitance. This closes the load capacitance in parallel.

       2. Load capacitance refers to the total effective external capacitance across the two ends of the crystal in the circuit. It is a test condition and a use condition. When applying, it is generally adjusted near the given load capacitance value to get accurate frequency. The size of this capacitor mainly affects the load resonance frequency and equivalent load resonance resistance.

       3. In general, increasing the load capacitance will decrease the oscillation frequency, while reducing the load capacitance will increase the oscillation frequency.

       4. The load capacitor refers to the sum of the two leads of the crystal oscillator connected to all the effective capacitances inside and outside the IC block. It can be regarded as the crystal chip connecting the capacitor in the circuit in series. Different load frequencies determine different oscillation frequencies of the oscillator. For crystals with the same nominal frequency, the load capacitance is not necessarily the same. Because a quartz crystal oscillator has two resonance frequencies, one is a low-load capacitor crystal in series with a oscillating crystal: the other is a high-load capacitor in parallel with a oscillating crystal. Therefore, when the crystals with the same nominal frequency are interchanged, the load capacitance must also be reached, and they cannot be interchanged, otherwise the electrical appliance will not work properly.