It is a crystal oscillator circuit that can be used as a reference oscillator. The circuit is based on the TTL integrated circuit 7404. 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 10MHz.
The circuit must be powered from a 5V voltage source. The oscillator is implemented with the NOT logic gates U1A and U1B. U1C and U1D are used as an isolation stage (buffer), to isolate the crystal oscillator from the next stages. With the C1 variable capacitor the oscillation frequency can be slightly drifted from the nominal frequency of the crystal.