25mHZ quartz crystal oscillator circuit diagram

This article mainly focuses on the circuit diagram of the 25mHZ quartz crystal oscillator, and analyzes and explains it:

Fundamental frequency oscillation circuit

Third overtone oscillator circuit

Circuit description:

R172 is used to match the negative feedback and phase displacement of the IC's internal circuit. C186 and C187 are the crystal load capacitance CL. The formula for determining the capacitance value is:

In the fundamental frequency circuit diagram, R1, R2, and R3 provide bias for the circuit, L1 and C1 form the oscillation tank, C2 is the bypass capacitor, C3 is the coupling capacitor, and CL is the crystal load capacitance.

In the FEIN circuit diagram, R1, R2, R3, and R4 provide bias for the circuit, C1, C2, and L1 form the frequency selection network, C3 is the bypass capacitor, C4 is the coupling capacitor, and CL is the crystal load capacitance.

When selecting transistors in the circuit, their characteristic frequency should be greater than 50-100 times the oscillation frequency of the circuit, that is, not less than 1.2G. The coupling capacitor can be smaller without affecting the output amplitude. In short, the higher the frequency, the smaller the capacitance, and the lower the frequency, the larger the capacitance value. If the capacitance is too large, the output noise will increase. You can also consider adding a filter network here.

It should also be noted that the frequency stability of the parallel crystal oscillator is not as high as that of the series crystal oscillator.