TECHNICAL INFORMATION

Published in "AEI July 2006"
Daishinku Updates Give Crystal Oscillator Lead Role in Mobilecom

 Temperature-compensated crystal oscillators (TCXO) with voltage-control functions are key devices in mobile phones. Models today operate at the phase-locked loop (PLL) frequency.
 As more and more functions are supported by mobile phones, such as wireless communication capability, camera, music playback and lately, one-segment broadcasting, manufacturers are mounting application processor, camera module and tuner module at high density. Accordingly, the use of simple-packaged crystal oscillator (SPXO) or voltage-controlled crystal oscillator (VCXO) has been increasing the clock signal for the processor or module. For crystal oscillators mounted on the processor or module, performance requirements include low current consumption, low voltage operation, and high frequency compatibility. There are also requirements for miniaturized units.
 To meet these needs, a crystal oscillator was developed. This was the result of a dedicated oscillation circuit and the high-precision quartz oscillator technology of an AT-cut fundamental wave mode.

Miniaturization Trend

 SPXO is a product that has in one package a quartz oscillator and an oscillation circuit. A user can easily obtain a stable frequency signal without requiring any circuit design. The compatibility with surface mounting began with 7050-size products. After that, however, miniaturization requirements emerged, and manufacturers started mass-producing 2520-size products. Miniaturization of less than one-tenth was realized in capacity ratio.
 In developing 3225-size products, Daishinku Corp. adopted a fundamental wave oscillation mode and developed a dedicated chip. Apart from addressing miniaturization requirements, the company also improved performance such as jitter characteristics and low current consumption.

Use of Fundamental Wave Oscillation

 An AT-cut mode is used for the crystal oscillation plate (crystal element plate) of more than a megahertz band. The resonance frequency of this oscillation plate is inversely proportional to the thickness. For example, in the case of 40MHz fundamental wave oscillator, thickness was slashed to about 42um. In the past, about 40um was considered as a limit in thin-plate processing. In a frequency of more than 40MHz, a triple-wave oscillation mode in which triple thickness can be used was employed.
 In the triple-wave oscillation mode, however, a loss increases when a crystal oscillation plate becomes smaller (Fig.1). Thus, there is a weak point that the reliability of oscillation start deteriorates and the current consumption of a circuit increases. For this reason, in all frequencies, it was required to employ a crystal oscillation element plate in the fundamental wave mode. The method that realizes a high-frequency crystal oscillation plate, which is used as a thin plate, in high precision through unique grinding technology, was developed, and a fundamental wave-compatible area could be expanded from 40MHz in the past to 150MHz (11um in thickness). Moreover, an oscillation plate area can be reduced to less than one-third of the conventional one. This contributes to the miniaturization of products.

Loss comparison between fundamental wave and triple wave in compact oscillator
Fig.1:Loss comparison between fundamental wave
and triple wave in compact oscillator

Low Power Consumption

 A dedicated chip was developed to oscillate a subminiature crystal oscillation plate in the fundamental wave mode under optimum conditions. This IC can oscillate a crystal oscillation plate by the optimum electric power. Additionally, using a more minute process, chip area was reduced by 56 percent.
 With the two newly developed parts, power consumption could be reduced to less than one-fifth of the conventional model (Fig.2). The operating supply voltage can also be used from 1.8V to 3.3V.

Current consumption reduced to one-fifth
Fig.2: Current consumption reduced to one-fifth

Very Reliable Structure

 With the miniaturized oscillation plate and chip, a wire bonding method was employed in common with five sizes up to the minimum 2520-size DSO221SR (Fig.3). The 7050-size products adopted such structure. This structure has actual results as a crystal oscillator and is high in reliability.

Internal structure of five-size common products in SR series

Fig.3: Internal structure of five-size common products in SR series
(wire bond mounting + seam sealing)

Common Parts

 This time, the latest IC and oscillation plate were employed in the SR series of SPXO and standardized in structure, parts, and performance (characteristics). Accordingly, facilities used for the conventional model can be used for almost all assembly lines, and production can be flexibly used by minimum plant investment.

3225-Size and 90MHz-Compatible VCXO

 VCXO is a crystal oscillator that can fine-tune an oscillation frequency. Until now, VCXO was used for only high-speed communication or digital video. However, the demand for VCXO has been increasing with the spread of HDTV and Digital Terrestrial Television (DTTV).
 With the start of one-segment broadcasting, mobile phones will be handling digital video data. Accordingly, typical samples in which VCXO is mounted are on the rise. Moreover, requirements emerged for miniaturized VCXO.
 VCXO was delayed in miniaturization as compared with SPXO. In 1999, Daishinku started the mass production of the first 3225-size VCXO (DSV321S) in the industry. Advanced mobile phones have been using VCXO. A fundamental wave mode having characteristics that are large in the amount of a frequency change must be employed in the oscillation plate used for VCXO.
 In addition, miniaturizing VCXO causes the problem that sufficient variable width must be secured even in a compact oscillator. For 27MHz, a variable amount of ±100*10-6 (capability value) was realized in DSV321SV. For 74MHz, the DSV321SV realizes a variable amount of ±130*10-6 (capability value) (Fig.4).
 The corresponding frequency is expanded to 90MHz. In 3225-size products, VCXO is considered as a series whose corresponding range is widest in the industry.

Frequency variable characteristics of 3225-size VCXO
Fig.4: Frequency variable characteristics of 3225-size VCXO

Summary

 The SR series of SPXO are high-performance products with sizes up to 2520 and whose parts and structure were standardized. For the VCXO line, 90MHz-compatible DSV321SV, which is said to be the smallest in the industry, was introduced.
 Common features include low-power operation, high-frequency compatibility, and high reliability. A high-precision fundamental wave oscillation plate and an oscillation circuit (IC) that oscillates the oscillation plate under optimum conditions paved way for the realization of these features. Tables 1, 2, and 3 highlight representative specifications.
 For SPXO, all five sizes carry common specifications. The SPXO can operate at a minimum of 1.8V. For 3.3V operation, a frequency can be used up to 150MHz. The way to design a high-frequency oscillator of more than 100MHz is to multiply a crystal oscillator output frequency by a PLL circuit or use a surface acoustic wave (SAW) resonance.
 However, other problems that need to be tackled are in jitter characteristics when a PLL circuit multiplies a crystal oscillator. Another issue is the temperature fluctuation characteristics of a frequency and miniaturization when a SAW resonator is used.
 In Daishinku's product line, the performance of a single crystal is utilized to the maximum. In future, the company will release products based on the performance (miniaturization, power saving, and high frequency) required for mobile communication equipment by combining the processing technology of a high-frequency AT-mode crystal oscillation plate that is the company's forte and the peripheral technology that boosts performance.

Table 1: Size and name of SPXO and VCXO

Size and name of SPXO and VCXO

 

Table 2: Specifications of SPXO and SR series

Specifications of SPXO and SR series

 

Table 3: Specifications of DSV321SV

Specifications of DSV321SV

About This Article
 The author, Masashi Hirano, is Manager of Sec.1, Central Laboratory at Daishinku Corp.