When we look at the specifications of the crystal, we will find that there is a parameter called load capacitance. For the layman, you may not understand the usefulness of these two load capacitors. Today, Hongjingbao Jingzhen will introduce it to everyone.

These capacitors called the crystal load capacitance, are connected to the two feet and the crystal to ground capacitance, typically tens of fat in the skin. It will affect the resonant frequency and output amplitude crystal, crystal general order supplier will ask you when the load capacitance is.

Crystal load capacitance = [(Cd * Cg) / (Cd + Cg)] + Cic + △ C where Cd, Cg is respectively connected to two feet of the crystal and a capacitor to ground, Cic (IC internal capacitance) + △ C (the capacitor PCB) empirical value 3 to 5pf.

Various logic chip pin crystal oscillator equivalent capacitance three. Pin internal oscillator typically an inverter, or an odd number of inverters in series. A resistor is connected between the crystal output pin XO and the crystal input pin XI. For CMOS chips, it is usually between several tens and several tens of ohms. Many internal pin of the chip has been included in this resistance, the external pin would not pick up. This resistor is to make the inverter in a linear and linear state at the beginning of the oscillation. The inverter is like an amplifier with a large gain to facilitate oscillation. equivalent to a parallel resonant circuit, the oscillation frequency should be a parallel resonant frequency of a quartz crystal.

The two capacitors next to the crystal are grounded, which is actually the voltage-dividing capacitor of the capacitor three-point circuit. The ground point is the voltage-dividing point. The ground point is the voltage-dividing point as the reference point. The input and output of the oscillation pin are inverse Phase, but from the two ends of the parallel resonant circuit, that is, the quartz crystal, a positive feedback is formed to ensure that the circuit continues to oscillate. When the chip is designed, these two capacitors have already been formed. Generally, the two capacitors have the same capacity. The size of the capacitors varies according to the process and layout, but it is relatively small, and it may not be suitable for a wide frequency range. When connected externally, it is about several PF to dozens of PF, depending on the frequency and the characteristics of the quartz crystal. It should be noted that the value of these two capacitors in series is connected in parallel on the resonance circuit, which will affect the oscillation frequency. When the two capacitances are equal, the feedback coefficient is 0.5, which can generally meet the oscillation conditions, but if it is not easy to start or the oscillation is unstable, you can reduce the capacitance of the input to ground and increase the value of the output to increase the feedback.