Piezo Crystal Oscillator: Understanding the Basics

When it comes to designing a crystal oscillator, there are several key factors to consider in order to ensure optimal performance. One of the most critical components in this process is the piezo crystal oscillator, which is widely used due to its high quality factor (QF), excellent frequency stability, tight tolerance, and relatively low cost.

In addition to these benefits, quartz crystals have piezoelectric properties that allow them to be used as electrical circuit elements. This property allows electric potential across the crystal to be proportional to mechanical deformation, making them an ideal choice for resonant elements in oscillators.

When designing a simple crystal oscillator using AT-cut crystals, there are several key considerations to keep in mind. One important aspect is understanding the overtone models that include additional series RLC branches in parallel with the elements shown in Figure 1. These models also apply to overtone operation of crystal resonators, but the model shown in Figure 1 applies specifically to fundamental-mode operation.

The electrical model of a fundamental-mode quartz crystal consists of a shunt capacitance that represents the physical capacitance formed by both the parallel plate capacitance of the electrode metallization and the stray package capacitance. The series RLC branch, often referred to as the motional arm, models the piezoelectric coupling to the mechanical quartz resonator.

When designing a crystal oscillator, there are several topics that must be taken into consideration, including load capacitance, negative resistance, startup time, frequency stability versus temperature, drive-level dependency, crystal aging, frequency error, and spurious modes. By understanding these factors and how they can affect performance in various applications, designers can create highly reliable and accurate crystal oscillators.

In conclusion, the piezo crystal oscillator is a critical component in many electronic designs, offering high quality factor, excellent frequency stability, tight tolerance, and relatively low cost. By understanding the electrical model of a fundamental-mode quartz crystal and considering key factors such as load capacitance and frequency stability versus temperature, designers can create highly effective and reliable crystal oscillators using AT-cut crystals.

Piezo crystal oscillator