It is a crystal oscillator circuit that can be used as a reference oscillator. The circuit is based on the CMOS integrated circuit 4001. The basic version of the oscillator shown in Figure 1 uses an 1MHz crystal. However, the circuit can work equally well at different frequencies with crystals up to 3.5MHz.
Figure 1. The CMOS reference oscillator circuit
The power supply voltage can be from 5 to 15V. The oscillator is actually implemented with the U1A, NOR logic gate. U1B and U1C logic gates are used as an isolation stage (buffer), to isolate the crystal oscillator from the next stages. With the C2 variable capacitor, the oscillation frequency can be slightly drifted from the nominal frequency of the crystal.
