Published in "AEI July 2005"
Advanced Crystal Oscillators Meet Requirements of New Mobile Phones
Temperature-compensated crystal oscillator (TCXO) with voltage-control
functions, which is used as a reference for phase-locked loop (PLL), has been
employed as a crystal oscillator for mobile communication devices. It is used
mainly in the mobile phone market.
The past few years have seen the introduction and spread of multi-function mobile phones, including those that come with an application processor or camera module. Among these advanced models of mobile phones are units equipped with Simple Package Crystal Oscillator (SPXO) or Voltage Control Crystal Oscillator (VCXO). These crystal oscillators provide the clocks required for the mobile phones' various functions.
Crystal oscillators mounted on mobile equipment need to have low power consumption, low voltage operation, and must support miniaturization.
An SPXO module combines a crystal oscillator and an oscillation circuit
in one package. It provides a stable frequency signal regardless of the design
of the oscillation circuit or the dispersion of a circuit.
The first SPXO products used for surface-mounting applications were 7050-sized devices. Manufacturers then started developing products that are smaller than previous SPXO devices. At present, makers are mass-producing 2520-sized SPXO products (Fig.1).
|Fig.1: SPXO size, and area-volume ratio|
A surface mounting-type crystal oscillator has been miniaturized into less than 1/10 in volume ratio. However, several problems must be resolved before miniaturization can be accomplished. In the existing design technology, for example, when a crystal oscillator is miniaturized, the loss (resistance value) of the oscillator increases and the reliability of oscillation start decreases. Moreover, the unwanted mode resonance occurring when designing a compact oscillator must be suppressed. Design engineers need to develop or find a way to solve these issues.
Compatibility With High Frequency
An AT-cut mode is used for the crystal diaphragm (crystal prime plate)
of a 1MHz or higher-frequency oscillator. The resonance frequency of this diaphragm
is inversely proportional to its thickness.
A fundamental-wave oscillator of 40MHz, for example, is approximately 42mm thin. Therefore, low-priced products are limited in processing. In frequencies higher than 40MHz, the third oscillation mode in which the thickness that is three times as much as the normal one can be used has been employed. In this mode, however, when an oscillator is miniaturized, the loss increases and the reliability of oscillation start decreases. At the same time, the circuit's power consumption increases. In 3225-sized SPXO products, a prime plate based on a fundamental-wave mode crystal oscillation is employed. Engineers have developed a method for creating a high-frequency quartz oscillator, treated as a thin plate with high precision using a unique grinding technology. This method has enabled an oscillator of up to 150MHz (11mm in thickness) to be supplied at a low price.
A dedicated IC chip was developed to oscillate a subminiature crystal oscillation prime plate in the fundamental-wave mode under optimum conditions. This IC chip can oscillate a crystal oscillation plate using optimum power. Power consumption could be reduced to less than 1/5 of the conventional model by using the two newly developed parts (Fig.2). Furthermore, an operating supply voltage can be used in the range of 1.8V to 3.3V.
|Fig.2: Comparison of current consumption|
Use of High-Reliability Basic Structure
A wire bonding system, which is the conventional mounting structure, is used for 3225- and 2520-sized products (Fig.3). This structure was first established with 7550-sized products. It has the highest actual results as a crystal oscillator and is highly reliable. This structure also facilitated an assembly process, and miniaturized the parts to be stored, ensuring high performance and reliability. For example, a subminiature chip, featuring a scaled down IC process rule, is used to ensure a wiring area or a thin-type IC is obtained to realize a short model in 2520-sized products. The latest high-density mounting IC chip technology is used for any of the cases described above. Reliability can be secured by matching new IC technology and a wire bonding system with actual results. As a result, the facilities of the conventional model can be used for almost all assembly lines, and provide flexibility in production. Moreover, a little investment in the manufacturing plant can help control production costs.
|Fig.3: Internal structure common to four product size categories|
VCXO is a crystal oscillator that can finely adjust an oscillation frequency
by using a control voltage. It was previously used only in high-speed communication
or digital video applications. However, VCXO devices have been increasingly
mounted on camera-equipped mobile phones that require video data input and
output functions. At the same time, VCXO devices designed for these mobile
phones need to be miniaturized.
Efforts to develop miniature VCXO devices are not as advanced as for SPXO. However, 3225-sized products are presently available in the market and are used in high-function mobile phones. The fundamental-wave mode must be employed for the diaphragm used for VCXO.
Since the structure consisting of a crystal oscillation prime plate or package has many sections that are in common with SPXO, 3225-sized products can be designed by using a compact IC chip. Another problem that occurs during miniaturization of VCXO is that a compact oscillator also secures sufficient variable width. The variable width of 3225-sized product is approximately 70 percent of 5032-sized 27MHz products, but in DSV321SV, a variable value of ±100*10-6 (Fig.4) can be secured. In 74MHz products, a variable width value almost similar to those of 5032-sized products can also be secured (Fig.5). Optimizing the drive level for driving an oscillator and matching an IC chip and characteristics helped resolve this problem.
|Fig.4: VCXO size and frequency variable characteristics|
|Fig.4: VCXO size and frequency variable characteristics|
SPXO and VCXO products, sizes 3225 and 2520, have low-power operation, high-frequency compatibility, and high reliability. This basic performance (Tables 1 and 2) exceeds the performance of conventional 7050- and 5032-sized products. Such performance was realized using a high-precision fundamental-wave diaphragm and an oscillation circuit (IC chip) that oscillates the diaphragm under optimum conditions. For SPXO devices, 3225- and 2550-sized products have common specifications. An operating voltage can be used for 1.8V or higher. For an operating voltage of 3.3V, a frequency can be used for up to 150MHz. For VCXO, 3225-sized products can be used for up to 90MHz. Also, the low-current consumption for which the latest IC is used is given as a feature. Designing a high-frequency oscillator of more than 100MHz can be accomplished either by multiplying a crystal oscillator using a PLL circuit, or by using a SAW resonator. For PLL, however, jitter characteristics pose a problem, while with SAW, the problem is with the temperature fluctuation characteristics of a frequency.
Table 1: Characteristic specifications of DSO321SV and DSO221SV
Table 2: Characteristic specifications of DSV321S
About This Article
The author, Masashi Hirano, is the Manager of Section 1, Central Laboratory at Daishinku Corp.
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