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Increasing Features and Reliability in Wireless Mobile Devices

Thu, 09/07/2006 - 11:45am

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Remember when the mobile phone was called a bag phone? This was a major accomplishment at the time. Then manufacturers began the miniaturization of the phone and they became smaller than a cigarette pack. The size reduction drove the market for new phone sales. People changed phones every two years so they would have the smallest device. Phones had reached the smallest practical size; any smaller and they would not be as user-friendly. Then a new marketing tactic surfaced to drive new phone sales - the addition of non-phone features.
Customers are now driven to purchase new phones with many new features including email access, digital cameras, video recorders, GPS, Digital Video Broadcasting, RFID and other value-added functions. All engineering marvels. Now all of these, once upon a time stand-alone devices, are being rolled into one feature-rich device along with Bluetooth™ connectivity. Designers are faced with putting all of this together without increasing size while improving reliability.

Semiconductor providers have done an excellent job of producing multifunction devices, thus many of these features are achieved using one IC. While one can consider the IC as the brains of the mobile phone, one can also consider the crystal device as the heart beat of the phone. The crystal is critical to the system providing the frequency stability needed to keep the phone locked in on the proper frequency.
Reduced Size
Figure 1. The shrinking of the 7 × 5 mm package to a 2.5 × 2.0 mm package has resulted in an 86% savings in board space (5 mm vs. 35 mm with 2.0 × 1.6 mm packages coming in the future.
For many years the frequency device standard size was a 7.0 × 5.0 mm VCTCXO. Inside the device, a ± 20 ppm stability quartz crystal (䔮 ppm at room temperature, 䔮 ppm over the temperature range — other small crystal tolerances such as aging are ignored for this article, but should be considered during design) is transformed into a ۬.5 ppm oscillator. When used in a PLL, it provides the necessary accuracy for the RF transceiver frequency. However, the 7 × 5 mm size is a lot of real estate for the shrinking mobile phone’s PCB. VCTCXO manufacturers have embarked on a steady progression of size reduction. Today, manufacturers are producing VCTCXOs in 2.5 x 2.0 mm packages that deliver comparable performance to their bulky ancestors. This represents an 86% savings in board space (5 mm2 vs. 35 mm2) while reducing the height from 1.8 mm to 0.55 mm. This miniaturization is a result of the transformation from discrete temperature compensation to a single chip IC solution.
Reduced Cost
Figure 2. Oven Driven Crystal: This diagram illustrates a possible response of when too much drive level is applied across the crystal
The 2.5 × 2.0 mm VCTCXO is about one third the price of its 7.0 × 5.0 mm larger predecessor. How does one further reduce the cost of a single chip IC VCTCXO solution? When one considers that the frequency control supplier is buying an IC to compensate the crystal so it can be suitable for use with the mobile phone chipset silicon, the answer is simple. The mobile phone chipset producers only need to outsource the packaged quartz resonator with the 䔸 ppm accuracy. The compensation circuitry will be incorporated with the mobile phone chipset.
Challenges
Figure 3. Reference Crystal: A basic mobile phone block diagram utilizing a reference crystal.
Some applications have tight phase noise requirements; some applications require tight RF accuracy (۪.08 ppm for CDMA) with short PLL lock times. Many of these applications still require the full VCTCXO. For applications that can source the stand-alone quartz crystal, there are many challenges. Some problems to look out for include:

1)No oscillation or slow start-up This can be avoided by improving the negative resistance of the circuit. The crystal supplier can provide the minimum negative resistance required for reliable oscillation.

2)Center frequency offset The crystal manufacturer produces a given frequency for a specified capacitive load. If the actual CL of the application is different than what is specified, the result is a frequency offset error. While this can be negated during the frequency compensation stage, the frequency offset error should be minimized to maximize design margin.

3)Insufficient frequency compensation during phone assembly The designer has a 䔸 ppm crystal. At the end of mobile phone assembly, the following tolerances can be compensated for by adjusting the capacitive load of the crystal: a)䔮 ppm room temperature inaccuracy; b)Minor frequency shifts such as those that occur during the assembly process; c)Frequency shifts due to tolerances of the oscillator components.

4)Insufficient frequency compensation during phone operation The designer has to compensate for several frequency inaccuracies during the phone’s operation: a)䔮 ppm of the crystal’s temperature stability; b)The temperature tolerances of the oscillator components.

5)Insufficient resolution of frequency compensation When the designer sets up digital compensation algorithm, the frequency steps must be well within the RF transceiver frequency tolerance (۪.1 ppm for GSM). 6)Activity dips The designer must be careful to not put too much power across the crystal. If this is done, spurious response within the crystal will be activated and there will be an instantaneous frequency jump. Consult the crystal manufacturer for maximum power (or drive level) for reliable oscillation.

Figure 4. Early Involvement: It is important to take advantage of the frequency component suppliers’ expertise during all phases of design, especially during the early stages when proper decisions can help improve reliability.
The design engineer should work very closely with the crystal manufacturer to ensure that the tight frequency accuracy is maintained over all conditions. In order to address potential problems at the earliest possible stages, crystal manufacturers should work closely with chipset manufacturers to mate the best crystal product with a particular chipset. This will lay a good foundation when the mobile phone designer works with the crystal manufacturer. Hopefully, only minor adjustments to the crystal will be needed due to layout changes, PCB differences, etc. These factors can change the capacitive load which will create a frequency offset.

It has been a great challenge to design crystals with sufficient frequency pullability, which is the amount of frequency shift per capacitive load shift. This pull is directly proportional to the motional capacitance of the crystal. For obvious reasons, the industry trend has been to go to smaller crystals. Today 2.0 × 1.6 mm crystals are being sampled. However, smaller crystals have smaller motional capacitance. Increasing the frequency will result in increased pull. Traditionally, the GSM phone designer used a 13 MHz reference; today 26 MHz and 52 MHz are being used. As an added benefit, each time the frequency is doubled, the number of harmonics is halved; this may lessen the interference with the RF frequency. The PLL multiplication factor is reduced, which may improve phase noise.

In conclusion, to achieve the maximum performance of the Tx/Rx signals, it is imperative that the design engineer pays close attention to the performance of the reference oscillator. Working closely with the frequency control supplier, will improve the reliability of the wireless product, thus creating a satisfied end customer.

Ken Hennessy is engineering manager for NDK America Inc., 701 Crystal Parkway, Belvidere, IL 61008; (815) 544-7917; www.ndk.com.

Glossary of Terms

CDMA— Code Division Multiple Access GSM— Global System for Mobile Communication PLL— Phase Locked Loop VCTCXO— Voltage Controlled Temperature Compensated Crystal Oscillator

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