TECHNICAL INFORMATION

Published in "AEI June 2002"
Increasingly Tiny Crystals Bring Extra Functions to Mobile Sets

Crystals are essential to computers and communications because of their unique physical and behavioral properties. These properties also pose technical challenges.

The mobile communications industry gradually is resuming its normal pace of activity. The field had been in a lull phase since information technology reached a plateau in 2001. Lately, though, manufacturers have finished preparations to take mobile communications to the next stage. Thus the market is on the move again, as service providers and customers begin switching to the so-called third-generation mobile communications systems.

New Designs

These changes have given rise to certain trends, such as the competition among Global System for Mobile Communications (GSM) terminals emphasizing slim, compact and lightweight designs. In Japan, such competition already has swept through the sector for Personal Digital Cellular (PDC) terminals. Competition now will affect advanced wide-band codedivision multiple-access (W-CDMA) terminals. Here, the contest will center on advanced functionality, enhanced performance, rapid data transmission, and the ability to send and receive moving images. Even with these extra capabilities, users will not accept any gains in terminal size or weight. Therefore, engineers will have to continue shrinking the dimensions of the mobile phones and the electronic parts they use, even while upgrading the performance.
Amid these trends, Daishinku Corp. (which also uses the KDS brand) has cultivated the ultra-small 2520-format (2.5 * 2.0mm) monolithic crystal filter for first intermediate frequency (1st IF) devices. This product supports dense mounting for dual-band (800MHz and 1.5GHz) terminals, full-packet terminals, and other kinds of sets for Japan's PDC system. Its market introduction coincides with a number of recent trends in the development of monolithic crystal filters.

Monolithic Overview

he crystal in monolithic crystal filters exhibits excellent stability and a steep cutoff characteristic, which the filter then puts to full use. To make these filters, engineers divide an electrode in two, and install it on a single crystal blank. Its filter characteristic arises from acoustically coupling the resonance of the electrodes to each other. The thickness of the crystal blanks stands in inverse proportion to their frequencies, because the blanks use the thickness share of the AT cut. Accordingly, if the high-frequency of the fundamental mode must be 100MHz, engineers have to process the crystal blank to a thickness of just 16.7μm.
In the past, monolithic crystal filters held positions of unchallenged dominance among narrow band-pass filters, because of their cutoff characteristics and their stable temperature characteristics. In recent years, though, as high frequencies and broad bandwidths have come into demand, manufacturers have been switching from monolithic crystal filters to surface acoustic wave (SAW) filters. To address this competition, crystal manufacturers have pared the size of monolithic crystal filters, and have raised the fundamental modes to high-frequency levels.

From Small to Ultra-Small

Daishinku's 2520-format monolithic crystal filter is one of the smallest models yet, with dimensions of 2.5 * 2.0 * 0.9mm (Figs. 1, 2). It features a seamwelded seal. It offers a fundamental mode that is applicable to a range of 100 to 150MHz; the crystal blank is 11 to 16m thick. The resulting crystal chip is extremely small and thin.

Fig. 1: External dimensions of a
2520-format monolithic crystal filter		 Fig. 2: Internal structure of a
2520-format filter

To take the package cavity and the four bonding points into consideration, the engineers fabricated the crystal chip as a flat plate that measures 0.9 * 1.3mm. The original design called for a balance-balance characteristic, so the product has four support points. Engineers employed a rubber- elastic silicon conductive bonder for the support points, and for current passage to the electrodes.

Before processing the quartz material into a crystal blank, Daishinku also reviewed the quality and characteristics of the material. This included examination of the dislocation, the inclusion, the Q value, and other characteristics. The company then used the results of their review to grow a fine-quality crystal material, suitable for high-frequency use. After polishing the crystal blanks, the company used wet etching processes to fine-tune the frequencies. The engineers also sought an optimum design with satisfactory tradeoffs among the band-pass characteristic, the attenuation-band characteristic, the insertion loss, and the processing conditions.
Additionally, the engineers adopted silver (Ag) instead of aluminum (Al) because silver has a better aging stability, a smaller linear expansion coefficient, and greater specific gravity than aluminum, to handle energy confinement. They set the terminal impedance at 200Ω. Through such measures, Daishinku cultivated a monolithic crystal filter that can offer a 130.05MHz nominal frequency. The product also offers suitable equivalent constants (Table 1), temperature characteristics (Fig. 3), and waveform characteristics (Fig. 4).

  Fs Fa F1 F2
Freq.[MHz] 129.979158 130.009673 129.993089 129.995471
DF[ppm] -160.3 74.4 -53.1 -34.8
CI[Ω] 18.40 67.90 32.00 37.30
C1[fF] 3.06 0.76 1.60 1.53
L1[mH] 0.49 1.97 0.94 0.98
Co[pF] 0.95 0.34 0.63 0.60
Co/C1 311 442 391 395
Table 1: Equivalent constants

Fig. 3: Temperature characteristic Fig. 4: Waveform characteristics of products with the usual design

Taking Spurious Figures Seriously

  Most Japanese-made PDC handsets so far have incorporated a combination of a monolithic crystal filter and a ceramic filter to serve as the IF stage. These days, the increasing ressure to diminish the size and volume of these terminals has prompted manufacturers to shift to a combination of a monolithic crystal filter and an IC filter. This combination enables dense mounting.
However, IC filters exhibit a sharp characteristic, and have slower inclinations than ceramic filters. This tends to shift certain unwanted characteristics to the 1st IF filter, especially an attenuation inclination of F0 1MHz, and spurious response. F0 levels of 50kHz affect the selectivity of adjacent channels. Spurious response affects interference waves.
Daishinku's development team has worked to overcome these unfavorable effects. It is quite difficult to resolve problems with spurious response, especially because monolithic crystal filters use the thickness share of crystals. Spurious responses are an unnecessary vibration mode. Daishinku addressed the problem by trying to improve the spurious responses. In this way, the company worked to improve the attenuation characteristics, confining them within a limit of F0 1MHz.
Generation of 1, 1, 3-mode spurious response occurs at F0 400 to 600kHz. Using the technology it has built up over the years, Daishinku suppressed the spurious response, and moved the vibration to the high-frequency side. Specifically, the engineers applied a eighting unit to one electrode, and a weight-decreasing unit to the other electrode. At the sametime, by keeping the electrode dimensions within certain limits, the engineers avoided unwanted effects on the main vibration unit.

Because of this special electrode design, Daishinku was able to move just the 1, 1, 3-mode spurious vibration to the high-frequency side, and then to attenuate it (Fig. 5).
In this way, the company was able to design a product that exhibits a 130.05MHz nominal requency, and guarantee a vibration free of spurious response. This product offers a ubstantially improved spurious vibration within F0 1MHz, compared to the conventionally guaranteed attenuation characteristic.
Daishinku adopted the same approach to design for a three-pole product (Fig. 6). This improved the spurious response, again confining it within F0 1MHz. The three-pole product offers a terminal impedance of 250.//-1.5pF, and adopts the 2520 format. Despite its extremely compact dimensions, it exhibits excellent electrical characteristics.
Fig. 5: Waveform with no spurious response

Fig. 6: Waveform characteristic of the 130MHz DSF223SCF

New Development

Lately, Daishinku has been developing a four-pole product with external dimensions of 6.0 * 3.5 * 1.0mm, and featuring a fundamental mode of 183.6MHz (Fig. 7). It will serve as an IF filter for CDMA triple-band mobile phones, which now mainly use SAW filters for this purpose. The 183.6MHz product also must offer a sharp attenuation inclination.
These days, the company is working to cultivate three-pole products and other monolithic crystal filters. However, engineers know that neither a SAWfilter nor a three-pole monolithic crystal filter can meet the necessary specifications while also satisfying the characteristics for the selectivity of adjacent channels, intermodulation, and temperature.

Fig. 5: Waveform with no spurious response

Nevertheless, using its own technology, Daishinku has devised a single-package, four-pole product. Now the company is working out how to merge this technology with a high-frequency technology. Daishinku engineers have been able to cultivate encouraging levels of characteristics so far. However, there are a number of remaining technical challenges to meet before the product will be ready for practical use.
For instance, the use of a flat plate for the design base makes it necessary to devise a technology for processing extremely thin chips. There must be some appropriate way to handle extremely thin, lightweight chips, too. Right now, this is quite difficult. Finally, engineers will have to make additional improvements in the quality of the crystal material.

Conclusion

 Daishinku will tackle these issues one by one in its quest to offer a practical-use version of its product. Besides working on filters, it will focus on resonators and oscillators. Its aim is to develop products that work at higher frequencies than ever before, that are smaller and cover broader bands, and that adopt multiple poles.

About This Article

 The author, Hiroyuki Arimura, serves as Product Manager for the Filter Product Group at Daishinku Corp.