Murata’s solution “Ceramic Resonator”
Highly reliable ceramic resonator “CERALOCK^®”, indispensable to ECU
Precision improved to disseminate the high-speed in-vehicle LAN

Mr. Mitsuhiro Yamada
Chief of product engineering section, piezoelectric components department 1, piezoelectric components division, Murata Manufacturing Co., Ltd.

Resonator is a key device to support the in-vehicle ECU (Electric Control Unit). Murata Manufacturing Co., Ltd. is a major supplier of resonators. “CERALOCK^®” is a ceramic resonator. It is an electronic device created with ceramic technology, one of the company’s strong points. For many years Murata has provided products that satisfy the strict requirements of in-vehicle equipments. Recently the company focuses on the development of high-precision products increasingly needed as high-speed in-vehicle LAN is widely spread.

A resonator is a source of clock signal indispensable to drive digital circuits. Especially, it is an important device essential to peripheral circuits of microcomputers functioning as the heart of an ECU. “Murata’s ceramic resonator (CERALOCK^®) is a device born from the company’s own piezoelectric ceramics technology. Murata has been working on the technology since 1950s. It has the second longest history to the ceramic capacitor in the company. (Mr. Mitsuhiro Yamada, Chief of product engineering section, piezoelectric components department 1, piezoelectric components division,Murata Manufacturing Co., Ltd.) Piezoelectric ceramics has a nature to generate electric energy when mechanically stressed. On the contrary, when electrically energized, it generates mechanical energy. A ceramic resonator is a device that takes advantage of the latter nature: when piezoelectric ceramics is vibrated by electrical energy, it generates a constant frequency signal that serves as a standard clock signal. Murata commercialized ceramic resonators back in the middle of 1970s. “It was around the time when products using microcomputers were released in the market one after another. After that, application field of microcomputer has broadened and the market for ceramic resonator has expanded as well.” (Mr. Yamada)

Fig-1. Product lineup for in-vehicle ceramic resonators (Click to enlarge)

The application of ceramic resonators to in-vehicle equipments took root in the 1980s when introduction of ECU to engine control system started. “Since then, as the number of ECU installed in vehicles increases, the demand for in-vehicle use ceramic resonators has been steadily increasing.” (Mr. Yamada) At present Murata carries 5 series of ceramic resonators covering 2 MHz—70 MHz frequency band as standard products for in-vehicle use (total frequency deviation is ±0.5%) (See Fig-1) In addition the company provides 3 series of high-precision products that reduced total deviation of oscillation frequency to ±0.3%. “Lately as the number of auto makers adopting high-speed in-vehicle LAN systems such as CAN-BUS increases, the needs for high-precision products are growing.” (Mr. Yamada)

Ceramic resonator equipped with many excellent features

Fig-2. Comparison of temperature characteristics of oscillation frequency between conventional product and high-precision product

As resonators used as clock signal source in digital circuit, not only a ceramic resonators but also a crystal resonators using crystal as piezoelectric material are used. “In the market of in-vehicle resonators, it seems that the ceramic resonators and the crystal resonators share the market equally.” (Mr. Yamada) Ceramic resonators have some excellent features as compared with crystal resonators. There are four main features. First, the electromechanical coupling factor that indicates a ratio of mechanical vibration energy to supplied electrical energy is large. That is, since the vibration energy of ceramic resonators is greater than that of crystal resonators when the same electrical energy was supplied, ceramic resonators are less affected when vibration or shock was applied or extraneous material was attached to a resonator.

Fig-3. Ceramic resonators have a faster rise time.(Click to enlarge)

Second, the rise time of ceramic resonators is far shorter than that of crystal resonators. (See Fig-3) “You can boot up a microcomputer from sleep mode in a very short time. In recent cars, a system interlocking several ECUs is employed. Therefore, the delay of one ECU can delay the entire system. Ceramic resonators are advantageous to avoid such a problem.” (Mr. Yamada) Third, ceramic resonators have a small footprint. “The size of a ceramic resonator is less than a quarter of a crystal resonator comparing those with the same oscillation frequency.” (Mr. Yamada) Fourth, since ceramic resonators can operate over a wide frequency range, it is easy to determine circuit constants of the oscillation circuit.

Although ceramic resonators are equipped with many excellent features over crystal resonators, they are inferior in terms of accuracy. “Many customers use these two types of resonators according to the required frequency accuracy and their design concepts. I think that is the reason the market is divided evenly. However, if the frequency accuracy is acceptable, ceramic resonators are of advantage in terms of the cost.” (Mr. Yamada)

Optimized for the in-vehicle use by the material

Fig-4. Structure of ceramic resonators
(Click to enlarge)

In general, in-vehicle components are required of higher reliability than general-purpose components. Especially, reliability against temperature, vibration and shock is highly important. To support such strict requirements, many design efforts are made in Murata’s ceramic resonators. (See Fig-4)

Murata’s ceramic resonator consists of a ceramic substrate with external electrodes, topped with a rectangular solid ceramic piezoelectric element, and sealed with a metal cap. Both ends of ceramic piezoelectric elements are glued with conductive adhesive. The electrodes to generate vibration are located at its center. By applying voltage to these electrodes, the center of piezoelectric element vibrates. In other words the piezoelectric element generates vibration in a space sealed by ceramic substrate and a metal cap. “The structure is strong enough against shock and vibration in nature as both ends of a resonator is fixed. Moreover, we particularly reinforced the joint parts that often cause problems at strict high-temperature tests or thermal shock tests for in-vehicle products.” (Mr. Yamada)

Another important technology used in Murata’s ceramic resonator is miniaturizing. The both ends of piezoelectric element in a resonator are normally connected to capacitors in order to stabilize vibration. In case of Murata’s ceramic resonator, capacitors are integrated in its ceramic substrate. “Since you can reduce the number of components with our ceramic resonator, it is advantageous for miniaturization. You can enhance the reliability as well.” (Mr. Yamada)

Improved accuracy for in-vehicle LAN

Murata’s in-vehicle ceramic resonator has already established many achievements in the market. “If we look at just the ceramic resonators, Murata occupies a large share in the market.” (Mr. Yamada) The company believes that the high-speed in-vehicle LAN such as FlexRay^® will become a driving force in the in-vehicle ceramic resonator market. “The resonators used in the control circuit of high-speed in-vehicle LAN are required the have an oscillation frequency with higher accuracy than ordinary. Therefore, we are focusing on improving the accuracy of our ceramic resonators further.” (Mr. Yamada) (See Fig-5) Murata has been making effort to improve accuracy of ceramic resonators since 1990 when practical application of CAN-BUS, a precursor of high-speed in-vehicle LAN, started. Initially, Murata responded to the demand by selecting appropriate products among the existing product line. However, in order to meet the requests for resonators with higher accuracy, the company reviewed the materials and manufacturing process. As a result Murata succeeded in achieving a high accuracy of ±0.3% in total frequency deviation. “We intend to continue improving the accuracy through material and manufacturing process improvement. This is something only the company like Murata can achieve, since we are full-scale manufacturer who produces from materials to manufacturing equipment. I think this is our strength.” (Mr. Yamada)

Fig-5. Trend of accuracy improvement

*CERALOCK^® is the trademark of Murata Manufacturing Co., Ltd.
FlexRay^® is the trademark of Daimler AG.