※The image is for illustration purposes.

Introduction

In recent years, new audio formats, such as high-resolution audio, spatial audio, and DSP support, have become increasingly widespread across various streaming services and smartphones. To preserve the original sound quality and expressive characteristics of these audio sources, it is essential to reduce clock jitter in playback devices such as DA converters and DD converters. Moreover, jitter reduction is not only important for audio playback but also plays a critical role in recording and mastering processes, as well as in multimedia systems that require synchronization among multiple devices.
This column introduces the concept of jitter and explains how the quality of crystal oscillators contributes to achieving low jitter in electronic systems.

Jitter: The Main Obstacle to Digital Audio

In a digital audio system, the clock can be described as "a signal that determines the timing of signal conversion." The conversion between analog audio signals and digital audio data is carried out based on the sampling frequency, which is supplied by the audio master clock.

Ideally, the clock should provide perfectly stable timing, and conversions should occur precisely according to it. However, in reality, slight deviations in timing occur. These deviations are referred to as jitter.

When jitter increases, the misalignment in sampling timing causes distortion in the converted waveform. As a result, audio performance deteriorates in terms of total harmonic distortion and signal-to-noise ratio (S/N ratio). Perceptually, this manifests as a reduced sense of spatial expression and a limited dynamic range.

The Key to Reducing Jitter: Not Just Circuit Design

Jitter can be classified by its origin into two main categories: one caused by circuit and power supply noise, and the other originating from the clock source itself, which serves as the starting point of the signal.

A commonly used low-jitter clock source is the crystal oscillator. Crystals exhibit a property known as the piezoelectric effect, which allows them to vibrate at a stable, fixed frequency when a voltage is applied. Leveraging this vibration enables the generation of highly accurate clock signals, making crystal oscillators widely used not only in audio equipment, but also in communication and measurement devices.

A crystal oscillator is a module that combines a crystal unit with an oscillation circuit. It offers advantages such as reduced design costs and smaller PCB footprint by eliminating the need to design a separate oscillation circuit.

The crystal unit embedded within the oscillator is a pure vibrating element that operates in conjunction with the oscillation circuit. From a functional standpoint, it is a core component responsible for generating stable frequency. To achieve low jitter, not only must the performance of the oscillation circuit be optimized, but selecting a high-quality crystal unit is also crucial.

The 'Q Factor' That Determines the Performance of Crystal Units

The Q factor Quality Factor) is widely recognized as an indicator of a crystal unit's performance in terms of phase noise and jitter. It represents the sharpness of resonance at a specific frequency. A higher Q factor indicates fewer unwanted oscillations at frequencies other than the target.

In an ideal clock signal, the frequency spectrum appears as a sharp single line, as shown in Figure 1. However, in reality, it includes additional components near the main frequency, as illustrated in Figure 2. These unwanted frequency components are referred to as phase noise.

The Q factor of a crystal unit is influenced not only by post-process designs such as machining, sealing, and packaging, but also by the intrinsic quality of the quartz crystal itself. In fact, the raw quartz material has its own Q factor, separate from that of the finished crystal unit.

A key parameter that correlates with the Q factor of the crystal material is the alpha (α) value, which represents the infrared absorption coefficient of synthetic quartz. A lower α value indicates higher transparency, meaning less infrared absorption. It is well known that a lower α value corresponds to a higher Q factor.

Therefore, using high-quality quartz material is essential for realizing high-precision clock performance.

At NDK, we maintain vertically integrated manufacturing starting from the growth of quartz crystals.

Learn more about our world-class synthetic quartz production here:

Producing the Ultimate Synthetic Quartz Crystals｜Synthetic Quartz Crystal｜Products｜NDK - NIHON DEMPA KOGYO CO., LTD.

For more information on "N-Grade" and "N-Grade EX," our ultra-high-purity products that meet the highest grade in all categories of the JIS C 6704 quality standard for synthetic quartz, please visit:

High Purity Synthetic Quartz Crystal "N-Grade"｜Synthetic Quartz Crystal｜Product Lineup｜Products｜NDK - NIHON DEMPA KOGYO CO., LTD.

Conclusion: The quality of crystal units and clock jitter

1. In digital audio playback, clock jitter is a slight deviation in sampling timing and is a key factor that can degrade audio quality. It may cause issues such as waveform distortion and reduced spatial expression.
2. Jitter is caused not only by circuit noise but also by the clock source itself, such as the crystal oscillator or the crystal unit.
3. The Q factor, an indicator of the performance of crystal units, is directly related to phase noise and jitter characteristics. A higher Q factor results in lower noise components.
4. To achieve a high Q factor in a crystal unit, the crystalline quality of the raw quartz material is essential. The higher the purity of the crystal, the more precise and higher-performing the resulting crystal unit becomes.

NDK Product Lineup

NDK offers the following products related to the high-precision crystal units discussed in this column.

■High-Precision Crystal Units（MHz range）RC-8・NC-18C

This model features low jitter and excellent low phase noise characteristics, achieved through the use of N-Grade* high-grade, high-purity synthetic quartz grown in-house by NDK, which ensures a high Q factor and low alpha value.
It is ideal for use in OCXOs for communication infrastructure, as well as in high-end audio equipment, professional sound systems, and measuring instruments.

Features

• High frequency stability and high reliability
• SC-cut overtone mode
• Uses high-grade, high-purity synthetic quartz: N-Grade*
  Inclusion density: Class Ia, Infrared absorption coefficient: Class A (based on JIS C 6704)
• High stability, high precision, and low aging
• Excellent short-term stability and low phase noise near the carrier
• Frequency range：5 MHz to 50 MHz

Product details

RC-8｜Crystal Unit｜Products｜NDK - NIHON DEMPA KOGYO CO., LTD.

NC-18C｜Crystal Unit｜Products｜NDK - NIHON DEMPA KOGYO CO., LTD.

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