TECHNICAL INFORMATION

Published in "AEI October 2006"
Daishinku's Crystal Units Meet Tough Car Requirements

The rush of electronics in automobiles calls for extremely reliable parts. Daishinku has stepped up design of its crystal units to address this requirement.

These days, more and more automobiles are integrating electronic components. Car electronics have become indispensable components for automotive safety and comfort. Among others, crystal devices, crafted for providing stable frequencies, are essential parts in various electronic products including mobile phones, digital cameras, game machines and high-definition TVs. They also serve in various applications in automobiles, especially in the engine control unit and other safety-related systems as well as in information and communication systems such as car navigation. Statistically, the utilization ratio of crystal devices per vehicle is rising.
 In general, crystal devices for car-mounted equipment serve in extreme and more severe conditions than in other applications. Accordingly, they are required to ensure higher reliability. Automobile constructors and automotive equipment makers often request to parts manufactures to supply smaller devices so that they can fully implement high-density mounting and packaging through modularization of electronic equipment and provision of unit components.
  The following paragraphs will discuss a list of things to consider in designing crystal oscillators for car-mounted equipment, as well as a general description of small and high-performance surface-mounted crystal units.

Table 1: List of crystal units for car-mounted equipment

List of crystal units for car-mounted equipment

 

Table 2: Frequency range and specified CI-values

Frequency range and specified CL-values

 

Internal structure of crystal units
Fig.1: Internal structure of crystal units

Points to Consider

There are three points to consider in designing a crystal oscillator circuit. First is checking the load capacitance value. Second is guaranteeing a sufficient negative resistance (oscillation margin) to prevent non-oscillation. Third is limiting the level of drive to a relatively low level to prevent frequency failure.

Load capacitance
The frequency matching for a crystal unit is carried out in relation to the load capacitance specified for production as reference value. If the frequency of the crystal unit is not identical to the equivalent capacity of the circuit, there will be no frequency matching between the oscillator and the circuit. In such a case, the designer has one of the two options: adjust the equivalent capacity of the circuit to adapt it to that of the oscillator or modify the load capacitance of the oscillator to make it match the equivalent capacity of the circuit.

Negative resistance
This is a very important check item. To ensure a stable oscillation of a crystal unit, the negative resistance (oscillation margin) for the oscillating circuit must be sufficiently high. If the negative resistance is not sufficiently high, on-going oscillation may stop. This negative resistance is measured by connecting a resistor in series to the crystal unit being considered, reading the oscillation waveform and gradually raising the resistance of the connected resistor until the oscillation is stopped. The resistance value just before the oscillation is stopped is recorded as the negative resistance. A crystal oscillator for use in car- mounted equipment must have a sufficiently high negative resistance, taking into account a wide service temperature range and possible differences in resistance among surrounding circuit devices. Daishinku Corp. recommends a negative resistance for a crystal unit at least five times the specified highest value of equivalent series resistance for a crystal unit in ordinary service conditions and at least 10 times the same highest value in car-mounted service conditions.

Level of drive
If a crystal unit operates at an excessive level of drive, the oscillation can provoke connection with unnecessary oscillation modes, thus making the oscillation frequency unstable. Under most unfavorable conditions, the crystal blank is damaged and the oscillation is lost. A correct level of drive must be selected.
 The level of drive is generally measured with a high-frequency current probe. Daishinku recommends 300μW of less for the fundamental frequency and 500μW or less for the 3rd overtone frequency ( the actual correct level of drive depends on the kind of oscillator and the usable frequency).
 Daishinku regards these three considerations as indispensable check items for applications of a crystal unit to car-mounted equipment. The company provides customer support service for optimization of circuit design through circuit examination depending on customers' requirements.

Frequency variation (ppm) with changing load capacitance (CL) CL=12pFreference
 
Pulling sensitivity at 27MHz
Fig.2: Pulling sensitivity at 27MHz

Size Reduction, Lower Set Frequency

A designer of oscillating circuits must choose a sufficiently high negative resistance for an oscillating circuit. In this respect, the designer will have an advantage if the crystal unit under consideration has a lower equivalent series resistance (CI: crystal impedance). However, as equipment makers often require size reduction for electronic parts, the crystal blank to be mounted should come in smaller dimensions and the CI-value cannot further be lowered easily. If the crystal blank is smaller, the blank central zone with greater oscillation displacement becomes closer to the blank circumferential mounting zone, thus the latter hindering the former from oscillating correctly.
 Daishinku achieved improvements in package design and sealing process and developed an original technology for expanding the inner clearance of a package and mounting a crystal blank maximized in size in the package. The first product developed based on this technology is the 3.2 × 2.5 mm DSX321G crystal unit the full-scale production of which started in 2002. Daishinku applied the same technology to an oscillator of 5032 size (5.0 × 3.2 mm): the DSX 530GA crystal unit that was set for full-scale production in 2003.
 With the older process, the lowest supported frequency had been limited to 12MHz. The new technology lowered it to 7MHz and made it possible to support lower frequencies. Fig. 1 shows the internal structures of the abovementioned products.
 The DSX321G crystal unit has a usable frequency range of 9.8MHz to 55MHz and the DSX530GA crystal unit 7MHz to 54MHz. These are both industry-highest values (Tables 1 and 2).

Pulling Sensitivity

The pulling sensitivity increases proportionally with the size of the driving electrode. In other words, a crystal blank in greater dimensions will improve the pulling sensitivity.
 It is generally difficult to ensure a wide pulling sensitivity since size reduction is required in products. Daishinku's technology allowed a crystal blank of greater dimensions to be designed for a wider range of pulling sensitivity.
 Fig.2 shows the pulling sensitivity characteristics of various oscillators at 27MHz. The DSX530GA has a pulling sensitivity characteristic similar to that of Daishinku's previous model, SMD-49( 11.0 × 4.6 × 4.2 mm, 12 times larger in volume than the DSX530GA).
 In general, the pulling sensitivity characteristic may affect the amount of frequency variation in a VCXO circuit with variable frequency function; greater pulling sensitivity characteristic stands for greater variable frequency. The characteristics of VCXO circuits are very important because this kind of circuits are widely used in car navigation systems, high-definition TVs and other digital image handling applications.

Improved Shock Resistance

 Daishinku's crystal units meet the reliability requirements defined in the AEC-Q200 car-mounted electronic parts standard. The company examined the mounting scheme for the crystal blank to find a solution that will meet more stringent requirements for shock and vibration resistance in consideration of more severe service conditions.
 Daishinku found that the one-end mounting scheme was less advantageous than two-end mounting scheme and succeeded in enhancing the shock resistance of the device. Fig. 3 shows technical data obtained during a comparative test of a two-end mounting scheme and a one-end mounting scheme. These data are obtained from a marginal comparison test, and the products based on a one-end mounting scheme largely meet the requirements stated in the AEC-Q200.
 In 2002, Daishinku started the production of the DSX840GK crystal unit( 8.0 ×　4.5 mm) which has a two-end crystal element mounting mechanism. Then in 2005, the company began production of the DSX530GK crystal unit ( 5.0 × 3.2 mm) which has the same crystal blank mounting mechanism.

Test conditions : A load of 100g is applied to the specimen, and the specimen is made to drop onto a concrete surface 20 times in six directions from heights of 120cm and 150cm,respectively.
Results :
DSX530GK 9.84375MHz(two-end mounting scheme),
DSX530GA 9.8437MHz(one-end mounting scheme)

Data obtained during a comparative test of a two-end mounting scheme

Fig.3: Data obtained during a comparative test of a two-end mounting scheme and a one-end mounting scheme

Perspective

 Efforts now center on reducing costs of crystal units for car-mounted electronic equipment. At the same time though, these crystal units should meet stringent reliability requirement Thus, any cost increase must be limited to lowest level as compared to oscillators for ordinary non-automotive electronic equipment.
 Daishinku's best solution in this respect has been to use the existing (but improved) designing methods and manufacturing processes, mobilize its accumulated technologies for product size reduction and lowering the usable frequencies in order to develop products that endure severe service conditions expected for car electronics. The company has successfully addressed these requirements and came up with reliable products. It will continue keeping track specific market needs and step up development activities for further product size reduction, lower frequencies, smaller CI-value and higher product reliability.

About This Article:
The author, Yoshiki Maeda, works as Engineer at Section 1, Engineering Dept., Daishinku Corp