Introduction

Crystal units and crystal oscillators are essential electronic components used in a wide range of devices, including smartphones, automotive electronics, and IoT equipment. Although both play a critical role in generating accurate clock signals, they differ significantly in their functions and structures.

This article explains the differences between crystal units and crystal oscillators, as well as their structures, operating principles, and the relationship between oscillation frequency and the thickness of the quartz blank. The content is intended to provide a clear and accessible introduction, making it easy for readers with varying levels of familiarity to understand the topic.

Differences Between Crystal Units and Crystal Oscillators

A crystal oscillator is another device that is often confused with a crystal unit because of its similar name. However, the two serve significantly different functions.

A crystal unit is a passive component that utilizes the piezoelectric properties of quartz to vibrate at a precise frequency. It cannot operate independently and must be used in combination with an external oscillation circuit.

In contrast, a crystal oscillator is an active component that integrates a crystal unit together with an oscillation circuit and an output circuit within a single package. By simply supplying power, it can generate a stable clock signal.

In general, the following guidelines apply:

Structure of a Crystal Unit

The structure of a crystal unit is illustrated in the figure below.

The crystal blank, which has been precisely processed to the required thickness and dimensions, is mounted on a ceramic base. Electrodes are then formed and the crystal blank is securely fixed in place.

The package is hermetically sealed with a metal cover after being filled with inert gas, protecting the crystal from external environmental influences and ensuring long-term frequency stability.

Structure of a Crystal Oscillator

The structure of a crystal oscillator is illustrated in the figure below.

A crystal unit does not contain an internal oscillation circuit and therefore cannot generate a clock signal on its own. To produce a stable frequency output, it must be be used together with an external oscillation circuit.

A crystal oscillator integrates a crystal unit, an oscillation circuit, and an output circuit into a single package. By simply supplying power, it delivers a highly stable clock signal, making it ideal for a wide range of electronic applications.

Crystal oscillators are also available with advanced features such as voltage control (VCXO) and temperature compensation (TCXO), providing even higher frequency stability for precision timing applications.

Relationship Between Crystal Device Frequency and Quartz Blank Thickness

The operating frequency of a crystal device, including both crystal units and crystal oscillators, is determined by the thickness of the quartz blank.

A quartz blank is cut from quartz at a specific angle, such as the AT-cut¹, and is precisely machined to the required thickness and dimensions to achieve the target frequency.

For an AT-cut crystal, the relationship between frequency and quartz blank thickness is expressed by the following equation:

As shown by this equation, the frequency is inversely proportional to the thickness of the quartz blank.

For example, when the quartz blank is 0.1 mm thick:

1670 ÷ 0.1 = 16,700 kHz (16.7 MHz)

If the thickness is reduced to 0.05 mm:

1670 ÷ 0.05 = 33,400 kHz (33.4 MHz)

This means that when the thickness is reduced by half, the frequency approximately doubles.

In other words:

Therefore, achieving higher frequencies requires the quartz blank to be processed to a thinner thickness.

In high-frequency quartz devices, the quartz blank can be thinned to approximately 10 μm (0.01 mm). This is about one-tenth the diameter of a human hair, which is approximately 100 μm thick.

In recent years, quartz devices themselves have also become increasingly miniaturized. Products with package sizes smaller than 1 mm, such as 0.8 mm × 0.6 mm, are now commercially available. Manufacturing such compact, high-frequency devices requires highly precise quartz blank processing technology.

1: An AT-cut is a quartz crystal cut at an angle of approximately 35°15′ relative to the Z-axis of a synthetic quartz crystal. It is the most widely used crystal cut for crystal units and crystal oscillators because of its excellent frequency stability.

Conclusion

Both crystal units and crystal oscillators are essential components used to generate the precise frequency signals required in electronic devices. Although they serve a similar purpose, they differ significantly in both structure and function.

A crystal unit is a passive component that utilizes the piezoelectric properties of quartz to vibrate at a specific frequency. It must be used together with an external oscillation circuit to generate a stable oscillation signal. In contrast, a crystal oscillator is an active component that integrates a crystal unit with an oscillator circuit and control circuitry. It can generate a stable clock signal simply by supplying power.

The performance of crystal devices is also greatly influenced by the machining accuracy and thickness of the quartz blank. As electronic products continue to become smaller and more powerful, crystal devices are increasingly required to provide higher precision, smaller package sizes, and greater frequency stability.

Selecting the appropriate device, a crystal unit or a crystal oscillator, according to the application and performance requirements is key to achieving stable operation and high performance in modern electronic equipment.