TECHNICAL INFORMATION

Published in "AEI October 2003"
Crystal Devices Allow High-Density Mounting on Portable Terminals

Introduction

Portable terminals for mobile communications adopt an increasing number of parts as they incorporate different capabilities other than voice communication - games, Internet connection, and image capture. The adoption of 800 MHz and 1.5 GHz bands in Japan for Personal Digital Cellular (PDC) terminals has further accelerated the trend for smaller devices and more densely mounted parts. These two bands were adopted to secure radio channels that would meet the requirements for high-rate communications for exchanging large data. Daishinku Corp. supplies temperature compensated crystal oscillators (TCXO),
key devices for the radio signal units in portable terminal for mobile communications. In 2002, the company started designing activities for peripheral circuits of a portable terminal apart from its product development activities for smaller devices. Among others,
Daishinku was engaged in the development activities for a TCXO that incorporates additional capabilities (multifunction TCXO module) and finally started shipping this product.
This report features the DSA536MA, a multifunction TCXO module specifically designed for domestic PDC terminals and the DSV321ST, a crystal oscillator that has a frequency shift function for camera module. The latter, in particular, will contribute to denser device mounting in portable terminals that incorporate camera function.

Multifunction TCXO module
– DSA536MA		 Crystal oscillator for camera module
– DSV321ST

Multifunction TCXO Module

Integration is being accelerated for the parts of portable terminal. In many cases, the existing IC incorporates peripheral circuits. In any case, the most important thing is the advantage of the IC's integration of peripheral capabilities. That is the case with the multifunctionality of a TCXO. The main development concept of the DSA 536 MA was contribution to smaller PDC portable terminals.

Selection of Integrated Functions

If additional functions are added to a TCXO, definite functions to be incorporated for making a smaller terminal must be selected. Daishinku's attention was directed to the radio signal unit in which a TCXO module is located. The company finally selected the following three functions to be integrated (Fig. 1).

Fig.1 : Block diagram of multifunction TCXO module

(1) Two-port Output (TCXO output)
TCXO output is usually used not only for the PLL synthesizer of the radio signal unit but also as clock frequency for the logic circuit for system control. This newly developed TCXO module has one output port each for the radio signal unit and the logic circuit. One isolation buffer amplifier is incorporated for each of them. And 12.6 MHz and 14.4 MHz are covered. These are the frequencies generally used for PDC applications.
(2) Second Local Frequency Output
The TCXO module incorporates a reference frequency (second local) generation circuit that is required at the radio signal unit for converting the intermediate frequency (second IF). This module has an IC that incorporates a PLL circuit capable of multiplying by 9 for generating the required reference frequency. Linked with the intermittently operating signals of the radio signal unit, the PLL circuit has an externally controllable(enable/disable) terminal. This contributes to lower power consumption during stand-by for incoming call. The PLL's output frequency is 129.6 MHz (when the TCXO output is 14.4 MHz). The TCXO module consumes +5.0 mA (max.) during "enable" status and +1.8mA during "disable" status (Fig.2).
Fig.2 : Output signal of 2nd local frequency(example)

(3) Temperature Sensor Output
The temperature sensor provided at the radio signal unit is used for the temperature compensation for the gain at the radio unit's power amplifier. It could be used for temperature compensation for LCD contrast.

External View And Structure

The DSA 536 MA has a module size of 5.0 * 3.2 * 1.2 mm max. ("5032" size) (Fig.3).

Fig.3 : External view

For this module, a flip chip mounting technique was used to compensate for the bigger size of the IC chip that incorporates new functions. Moreover, the ICs are mounted on a special pattern of layout. The I/O terminals have a BGA structure, which was adopted to compensate for the increase in size of the module due to the increased number of I/O terminals.
It should be noted that the BGA terminal assembly is a totally lead-free device for
which only lead-free solder is used (Fig.4).

Fig.4 : Internal structure

Advantages of Multifunctionality

The module reduces its packaging area by 55 percent and its power consumption by 55 percent compared to a configuration of the same functions with external circuits in a "3225" size (3.2 * 2.5 mm) package. If all the functions mounted on this module are efficiently used, portable communications terminals will further become smaller and consume less power.

Crystal Oscillator With Frequency Shift Capability

The DSV321ST is a crystal oscillator capable of shifting the oscillation frequency by more than -100 ppm (increment from "L" level to "H" level). It has a "3225" size (3.2 * 2.5 * 1.2 mm max.). This is a product developed as clock for various application circuits such as camera module. A clock for application circuits can be formed in two forms: the existing clock (TCXO for example) is converted (the leading of the signal up to the application circuit is an important task) or a dedicated clock is provided near the application circuit.

Problems Related to Camera Functions

If functions of an application (e.g. camera) are added to the original functions of portable terminal, such functions usually affect the radio performance. This inconvenience occurs when the radio frequency of the portable terminal matches one of the higher harmonics of the operational clock frequency of the application circuits. Among others, the performance of the receiving unit that processes weak signals deteriorates. Since such problems result from the operational clock of the application circuits, the radio performance is not affected while the application circuits are not operating. Practically, an inconvenience has been observed: while the functions of camera are used, telephone call cannot be received.

Avoiding Radio Performance Deterioration

To reduce the noise generated in the application circuits, the circuits of the radio unit might be shielded with metal plates and an anti-EMI part. However, it is not realistic, if not possible, to add a noise prevention part because of the demand for smaller and multifunction portable terminals composed of densely packaged parts. Thus, an alternative is to provide a clock capable of shifting the frequency that is dedicated to the application circuits. This clock controls the operational clock frequency and higher harmonics to avoid deterioration of the original performance of portable terminal (Fig. 5). This solution attracts many designers' attention. Operating together with the radio unit control software, this clock generates the signals to control the operational clock frequency. Thus, the noise in the application circuits can be reduced by a minimum effort and smaller and lower-cost portable terminals will be manufactured.

Fig. 5 : Improved characteristics of the radio unit with the frequency shift function (reference)

Perspective

In its development activities for portable terminals, every manufacturer is making efforts to produce smaller products, integrating as many functions as possible in a single IC. The products we have described only concern a part of the entire industry segment. Customers will have to find their own best solutions in the repertoire of products. Daishinku has proposed multifunction TCXO modules designed for PDC. The company intends to propose other small TCXO modules that have advantages for all communication standards (PDC, GSM, W-CDMA, and CDMA2000). For its DSA536MA, Daishinku is considering the possibility to substitute an LCC structure for the BGA structure of the terminal assembly since the former is easier to handle during mounting. This will be a user-friendly evolution with easier-to-handle products incorporating smaller ICs and having the same functions and dimensions. The functions of the crystal oscillators that have a frequency shift function are linked with the customers' system designing. Therefore, they are partly different from the previous crystal devices. This report has only presented the aspect of this technology as camera module. It could be successfully applied as a solution to noise problems in any application circuits. At present, the company is supplying a 26.057MHz product for PDC camera module and studying the possibility to apply the technology to W-CDMA. Daishinku will not only focus its efforts on smaller products. It will also continue its activities for new trendy products designed according to customers' viewpoint.