The crystal oscillator has seen tremendous advancements since its creation. It has gained the ability to operate with minimal power consumption and developed solutions for the aging problem. It is now widely used in various industries, including satellites, radar, wireless communications, and navigation.

What is a crystal oscillator?

Crystal oscillators are fundamental components in electronics that provide accurate and reliable reference clocks for use in a wide range of applications. These crystal oscillators are the primary choice when a high level of stability is required, as in digital circuitry and other wireless communication applications, such as radio frequency transceivers.

The quality factor, often known as the Q factor, is the most important parameter that determines the performance of a crystal oscillator. The crystals employed in this oscillator have an extremely high-quality factor, often in the range of 10,000 to 1,00,000. In addition, some crystals have an even higher quality factor.

Composition

A few naturally occurring crystals with piezoelectric qualities are Rochelle salt, quartz, and tourmaline. Among these, quartz crystals are more frequently used to manufacture crystal oscillators due to their stability, acceptable piezoelectric properties, easy accessibility, and low cost.

Despite having the highest piezoelectric property, Rochelle salt is not widely used due to its poor mechanical strength. Although tourmalines have a stronger structure, they are rarely used because of their poor piezoelectric properties.

The primary component used in the mechanical resonator of the crystal oscillator is crystalline quartz. The benefits of quartz-crystal oscillators are their great selectivity due to their extremely high Q factors and extremely stable output signal throughout time and temperature.

This crystalline quartz exhibits optical anisotropic properties, and through different cutting techniques in specific orientations, it can produce a variety of shapes and vibrations. The AT-cut and SC-cut chips are more frequently utilized cuts. Compared to the SC-cut chips, the AT-cut chip is easier to produce, smaller, and has better frequency stability over a wide temperature range.

In a crystal oscillator, a thin slice of quartz crystal is positioned between the two metalized electrodes for electrical contact, as depicted symbolically in Fig. 1

Fig. 1 Symbol of a Quartz Crystal. Source: Rakesh Kumar, Ph.D.

Working Principle

The working principle of the crystal oscillator is the inverse piezoelectric effect. When an AC signal of a particular frequency is applied, the piezoelectric material vibrates at that same frequency.

This phenomenon, in which the crystal switches from electrical energy to mechanical vibration and back again, is called the inverse piezoelectric effect. These vibrations are amplified and sent back by the oscillation circuit, which produces self-sustaining oscillations at the resonance frequency of the crystal.

Equivalent Model of a Crystal Oscillator

A parallel RLC network, as seen in Fig. 2, is the typical representation of the equivalent model of the crystal oscillator.

Fig. 2 Electrical equivalent model of quartz crystal. Source: MDPI

Motional Inductance (Lq):

This indicates the mechanical mass of the vibrating quartz crystal, which depends on the thickness of the quartz material.

Motional capacitance (Cq):

This indicates the elasticity of the quartz material, which is mostly dependent on the electrode's area and the thickness of the material.

Motional resistance (Rq):

This is the actual resistive loss that occurs in the crystal and depends on the crystal frequency.

Shunt capacitance (C0):

This indicates the capacitance between electrode plates when it is not vibrating.

Applications

Oscillators are employed in various domains of applications, such as electronic communication, testing, and measurement devices. Specifically, high-Q factor crystal oscillators are meant to be used as frequency and low-phase noise standards.

They find extensive applications in telecommunications systems, radio transmitters, and cellular devices. In addition, they have extensive practical usage in medical devices, various measuring instruments, and domestic appliances. They are employed in real-time clock modules in computers to provide clock signals for microprocessors or microcontrollers.

There is essentially no electronic system that does not include at least one oscillator circuit. For instance, electronic watches, thermal or electrochemical harvesting devices, and sensor technology all make extensive use of high-performance quartz crystal oscillators, which allow for the extremely accurate measurement of minute variations in capacitance and inductance, as well as other quantities.

Challenges

Crystal oscillators can only operate at a single frequency, making it difficult to implement them in a wider range of frequency tuning applications. The crystal oscillators are most frequently utilized in the frequency range between 100 kHz and many tens of megahertz. Frequency dividers or multipliers must typically be connected externally to control the frequency of the crystal oscillators.

The electromagnetic, temperature, and mechanical factors have an impact on the stability of the crystal oscillator's output. When subjected to a range of dynamic loads, the stability of crystal oscillators deteriorates, and the whole system's performance will decrease.

Oscillator instability caused by dynamic load can be classified as clock bias and drift. These deviations are determined by the magnitude and angular orientation of an intrinsic property known as a g-sensitivity vector. Each crystal has its own g-sensitivity vector, even if they are all identically cut, vibrate in the same way, and have the same overtone resonant frequency.

Furthermore, for a given crystal, different measuring methods produce different responses. Two methods are typically used to lessen the impact of dynamic loads on the stability of crystal oscillators: the passive control method and the active control method.

Summarizing the Key Points

● Crystal oscillators are essential components in various industries, providing accurate and stable reference clocks for electronic devices.

● Quartz crystals, with their high-quality factor and stability, are preferred for manufacturing crystal oscillators in diverse applications.

● Understanding the electrical equivalent model of quartz crystals helps optimize oscillator performance based on key parameters.

● The inverse piezoelectric effect forms the basis of crystal oscillator operation, converting electrical energy into mechanical vibrations.

● Challenges like frequency limitations and stability issues under dynamic loads highlight the need for innovative solutions in crystal oscillator design.