Relationship between audio equipment and crystal oscillator
Sound quality and clock phase noise


 Sound essentially consists of analog signals, whose processing is associated with the problems of attenuation, noise and deterioration. These issues are addressed by passing the original sound through an analog-to-digital converter (ADC), and the resulting data can be distributed on CDs or via networks as digital sound. These data are then processed using a digital-to-analog converter (DAC) in the end-user's digital audio device and output as analog sound.

 In the digitalization of analog signals, sampling(*1) is carried out at a certain frequency. To reproduce sound with the highest fidelity possible, higher sampling frequencies(*2) and bit rates(*3) are required. Today's high-resolution sound sources are characterized by sampling frequencies and bit rates superior to those used for CDs, enabling digitalization for true high-fidelity audio reproduction.

Digital sound source sampling frequency bit rate
CD sound source 44.1kHz 16bit
Hi-Res. sound source 96kHz 24bit
192kHz 24bit
384kHz 24bit

Phase noise and jitter

  Faithful reproduction of high-resolution sound sources requires precise digital signal processing and analog sound output with reduced deterioration of the sound source in the digital audio device. This conversion accuracy depends on the noise characteristics (i.e., frequency components outside the target frequency) of the clock frequency of the audio device used.

  The clock frequency spectrum for a circuit with zero noise has the form of a straight line (Figure 1, right). However, real-life spectra are modulated by noise, and are characterized by an extra frequency component nearby (Figure 2, right) known as phase noise.

  The phase noise of a clock frequency influences DAC and makes the time interval irregular.
This phenomenon is called jitter (see the figure below).

Accurate low-noise clock sources are necessary

  In digital audio devices, the phase noise of the master clock influences DAC due to jitter, thereby impeding high-fidelity audio reproduction. To enhance sound reproducibility, a crystal oscillator for a master clock with superior phase noise characteristics (i.e., low jitter) is necessary.

  Phase noise is expressed as frequency component levels measured outside a crystal oscillator's original frequency, and is based on the component level of the original frequency. Offset frequency is the departure from the original frequency, and is normally measured in the range of 1 Hz - 1 MHz.

  Frequency stability (the characteristic by which frequency does not change over an extended period) is generally seen as an important property of crystal oscillators. However, audio devices require short-term rather than long-term stability. Against such a background, SPXOs(*4), which have a frequency stability of about ±30 - ±100 ppm, are commonly used for master clocks. High-end audio systems may feature OCXOs(*5) instead for even higher-quality sound.

Low Phase Noise Crystal Oscillators for digital audio equipment

  NDK mass-produces crystal oscillators with low phase noise for master clocks that enable faithful sound reproduction. The company's NZ2520SD SPXO (a small surface-mounted oscillator with low phase noise) is widely used by audio device manufacturers and enthusiasts.
  In 2015, NDK also developed the DuCULoN® (Dual Crystal Ultra Low Noise OCXO), which is a world leader(internal data, June 2015) in the market for audio-device crystal oscillators with low phase noise characteristics.

  The following are examples of phase noise in NDK crystal oscillators for audio equipment:

examples of phase noise


NH47M47LA Picture

NZ2520SD Picture

Type of crystal oscillators OCXO
(Oven Controlled Crystal Oscillator)
(Simple Packaged Crystal Oscillator)
Package Size 47.2×47.0×28.5mm 2.5×2.0×0.9mm
Model Name NH47M47LA NZ2520SD
Nominal Frequency 45.1584MHz, 49.152MHz 11.2896MHz, 12.288MHz,
22.5792MHz, 24.576MHz,
45.1584MHz, 49.152MHz
Supply Voltage [VCC] DC+5V±5% DC+3.3V±10%
[Voven] DC+5V±5% ---
Current Consumption [ICC] Max. 60 mA Max. 6.0mA (During Standby Max. 10μA)
[Ioven] Max. 740 mA (at Start) ---
Max. 210 mA (+25°C when Stable) ---
Operating Temperature Range 0 to+50°C -10 to +70°C
Frequency Tolerance Max. ±5×10-6 Max. ±30×10-6
(Overall Frequency Tolerance)
Output HCMOS / Bipolar Driver / Sine (option) CMOS
Catalog NH27M27LA Catalog NZ2520SD Catalog

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Please visit Chip One Stop Inc.(Electronic components and semiconductors online shopping site) to purchase NDK products.

♦ DuCULoN(Accessory : Instruction manual)

Model Name Output Specification Number Frequency Purchase
NH47M47LA Bipolar Driver NSA3647C 45.1584MHz

♦ DuCULoN(Accessory : Instruction manual) and Premium Case

Model Name Output Specification Number Frequency Purchase
NH47M47LA Bipolar Driver NSA3647F 45.1584MHz

♦ DuCULoN(Accessory : Instruction manual) and PC board

Model Name Output Specification Number Frequency Purchase
NH47M47LA Bipolar Driver NSA3647I 45.1584MHz

♦ Option Products

Option Products Specification Number Purchase
Premium Case NSA3647Y
PC board NSA3647Z

About Export Trade Control Order in Japan

DuCULoN®(Model name:NH47M47LA) is a listed item in the attached list No.1 (Categories 7- item 2) regarding the Export Trade Control Order of Japan.
In case of export from Japan, export licenses are required in advance.

[ Glossary ]

(*1) Sampling A process of quantizing analogue signals at certain intervals
(*2) Sampling frequency The number of conversions performed each second to digitalize analog signals. The sampling frequency for CDs is 44.1 kHz, which represents 44,100 conversions per second.
(*3) Bit rate Amount of information processed per second.
(*4) SPXO
(Simple Packaged Crystal Oscillator)
A type with basic elements and no temperature compensation or temperature control. Small SPXO products used for clock generation purposes are categorized as crystal clock oscillators.
(*5) OCXO
(Oven-Controlled Crystal Oscillator)
A type that provides high accuracy based on a structure that maintains the temperature around the crystal unit in the oscillator using a thermostat chamber. Commonly used in communication base stations for mobile phones and other communication systems and measuring instruments.

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