Principle of single chip microcomputer crystal oscillator circuit

　　Crystal oscillators generally use a three-terminal (Colpitts) AC equivalent oscillation circuit; in the actual crystal oscillator AC equivalent circuit, Cv is used to adjust the oscillation frequency, which is generally achieved by adding different reverse bias voltages to a varactor diode, which is also the mechanism of voltage control; after replacing the crystal with the equivalent circuit of the crystal. Among them, Co, C1, L1, and RR are the equivalent circuits of the crystal.

　　Analysis of the entire oscillation tank shows that using Cv to change the frequency is limited: the entire tank circuit that determines the oscillation frequency C=Cbe, Cce, and Cv are connected in series, then connected in parallel with Co and then in series with C1. It can be seen that: the smaller C1 is, the larger Co is, the smaller the effect of Cv on the entire tank circuit when it changes. Therefore, the frequency range that can be "voltage controlled" is also smaller. In fact, since C1 is very small (1E-15 order of magnitude), Co cannot be ignored (1E-12 order of magnitude, a few PF). Therefore, when Cv becomes larger, the effect of reducing the tank circuit frequency becomes smaller and smaller, and when Cv becomes smaller, the effect of increasing the tank circuit frequency becomes larger and larger. On the one hand, this causes the nonlinearity of the voltage control characteristics. The larger the voltage control range, the greater the nonlinearity; on the other hand, the feedback voltage (the voltage on Cbe) allocated to the oscillation becomes smaller and smaller, and finally causes the oscillation to stop. Through the schematic diagram of the crystal oscillator, you should have a general understanding of the role and working process of the crystal oscillator. The higher the number of overtones of the crystal oscillator, the smaller its equivalent C1; therefore, the smaller the frequency variation range.