Frequency control is critical in wireless, feature-rich devices. So, the smallest package, at the lowest prices, wins.
By Ken Hennessy , NDK Co., Ltd.

Remember the mobile telephones of twenty years ago? The receiver was about the size of your typical landline hand receiver, while the rest of the electronics were encased in a container that could double as a shoebox. Imagine walking through the airport today and seeing everyone carrying those monstrous artifacts.

However, since then, advances in technology, particularly in the semiconductor and battery arenas, have reduced the size of mobile devices to make them practical for the masses. Today's added functionality include features such as GPS, digital cameras, web access and soon, functional multimedia.

To enable these next-generation features and function, frequency control will be more critical than ever. for these wireless feature rich devices.

Three years ago the market was paying over $3 for a bulky 7.0 × 5.0 × 1.6 mm VCTCXO. Today the mobile industry is looking for a VCTCXO with one-sixth the size in a 3.2 × 2.5 × 1.1 mm at one-third the price. Looking ahead, the market has its sights set on an even smaller 2.5 × 2.0 × 1.0 mm VCTCXO package.

Selection of the optimal VCTCXO can be difficult. When new packages are introduced, there is a cost premium associated with them to recover engineering and capital equipment costs. However, this price greatly reduces over a relatively short time. In selecting the optimal VCTCXO, one needs to consider the size/cost tradeoffs for the life of the product, instead of at the introduction of the product.

NDK has developed miniature VCTCXOs for wireless mobile applications that address these design requirements.

The latest stage of the size (as well as cost) reduction scenario is to design and incorporate the VCTCXO circuitry within an ASIC, where the only external frequency device necessary would be a tight tolerance crystal. The crystal must also be packaged with a form factor and cost to compliment the VCTCXO. As the crystals become smaller and smaller, crystal manufacturers face tough challenges in maintaining the pullability necessary to achieve the tight frequency accuracy required for mobile devices.

NDK has also developed small crystals for wireless mobile applications. The latest series (2.5 × 2.0 × 0.6 mm) is available in a fundamental frequency range of 20 MHz to 80 MHz with tight temperature stability and a frequency tolerance of 10 ppm to 100 ppm over the operating temperature range of – 30°C to +80°C. For the next generation of crystals, expect them to realize a size reduction of at least an additional 20%.

Finally, incorporating the temperature compensation into the ASIC functionality is no small task especially for frequency pulling. If the added facet of the design delays the end product release, then one would have a tough time justifying the use of this design technique. The design engineer should work very closely with the crystal manufacture to ensure that the tight frequency accuracy is maintained over all conditions.

NKD Co., Ltd.