Quartz crystal is used as time or frequency reference source in the circuit, which can be called the heart of equipment. Quartz crystal resonator is also called passive crystal oscillator or resonator
In 1880, Curie brothers Jacques and Pierre Curie discovered that when quartz crystal was under pressure, the surface would generate charge, and the amount of charge was proportional to the external mechanical force. This phenomenon is piezoelectricity. Objects with piezoelectric effect are called piezoelectric bodies.
If pressure is applied to the quartz wafer, an electric charge will be generated on the wafer; On the contrary, when voltage is applied to the electrodes at both ends of the wafer, the crystal will be mechanically deformed.
The frequency of the wafer depends on the cutting and size of the wafer. The higher the Q value of the wafer, the more stable the frequency. Since there is no IC inside the passive crystal oscillator, it needs to be used with external circuits. Therefore, the stability of the passive crystal oscillator largely depends on the matching of users’ external circuits.
The piezoelectric resonance of quartz crystal can be simulated by equivalent circuit. The equivalent circuit includes a static capacitor C0, a dynamic capacitor C1, a resonant resistor R1, and a dynamic inductor L1.
Passive crystal itself can not produce oscillation signal, so it needs to add external oscillation circuit, which can be divided into series oscillation circuit and parallel oscillation circuit.
Series crystal oscillator is a kind of feedback oscillator, which is composed of amplifier and feedback network. Quartz resonator and load capacitor are connected in series. The resonator is purely resistive.
In the parallel crystal oscillator, the quartz resonator is connected in the feedback branch and operates between the series and parallel resonant frequencies. The resonator is inductive. It is used as an inductive element in a circuit. The circuit matching capacitors C1 and C2 plus the stray capacitance cstray of the circuit, the closer to the load capacitance of the crystal, the more accurate the frequency of the crystal output.