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Posted on 02 August 2019

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Piezoelectric MEMS Oscillators for Industrial Frequency Control

An accurate oscillator lies at the ticking heart of all digital electronics, from consumer multimedia products to industrial automation and monitoring systems, networking and communications infrastructure, and rugged military equipment. Historically, engineers have exploited the resonant properties of quartz to support accurate timing. Unrelenting pressure to increase reliability and performance, smaller dimensions and power consumption, and speed up of logistics are driving timing specialists to develop new materials and technologies.

By Harmeet Bhugra, Integrated Device Technology, Inc.

 

Piezoelectric MEMS (pMEMS) resonators and oscillators are part of an emerging generation of silicon-based frequency-control devices that deliver a number of advantages such as semiconductor-grade shock and vibration resistance. In addition, semiconductor feature sizes enable large numbers of devices to be fabricated simultaneously on a single wafer, delivering economies of scale. Also, the devices can be housed in low-cost plastic packages, taking advantage of semiconductor-scale integration targeting smaller package footprints providing the opportunity for board-space savings and ongoing miniaturisation. To address oscillator challenges IDT developed ultra-small CrystalFree™ resonators in tiny wafer-level packages (Figure 1).

Miniaturised pMEMS resonators combine enhanced reliability and cost-effectiveness with the benefit of passive operation requiring no power source

Other advantages of pMEMS oscillators include natural compatibility with surface-mount assembly processes and short lead times; this enables suppliers and users (electronic manufacturers) to hold smaller device inventory with reduced risk of supply shortages. As with a quartz crystal, the pMEMS resonator requires no power source, but also has lower insertion losses. In addition, the MEMS resonators do not suffer from activity dips that can impair the reliability of quartzbased devices.

The performance of commercial MEMS oscillators has been advancing steadily over several product generations, moving closer to that of traditional quartz-based devices. The market for the devices - initially driven by small startups, according to IHS iSuppli - is expected to move into a higher gear as established timing companies such as IDT apply greater development resources to further improve performance and extend functionality by improving the design of the oscillator chip. iSuppli’s director and principle analyst for MEMS and sensors, Jérémie Bouchaud, has predicted that MEMS oscillators will increase their share of the $4 billion frequency-control market from its relatively low base of $13.5 million in 2010 to $205 million in 2015. This corresponds to a compound annual growth rate (CAGR) of 72.3%.

With the introduction of its 4M series of oscillators in May 2012, IDT introduced the world’s first pMEMS oscillators to deliver frequency accuracy and jitter performance needed for high-performance communications, consumer, cloud computing, and industrial applications.

In the design of pMEMS resonators for the 4M series, IDT has combined the strong electro-mechanical coupling of a piezoelectric material with the stability and low damping of single-crystal silicon to create a convenient and cost-effective oscillator family offering high reliability and performance levels. Figure 2 shows a simplified functional block diagram of the 4M MEMS oscillator.

The latest pMEMS oscillators such as the 4M series deliver improved performance and feature integration

Jitter performance of sub-ps jitter for the 4M oscillators is comparable with typical crystal oscillators, and frequency accuracy is within ±50ppm. The devices support low-voltage differential signalling (LVDS) and low-voltage positive emitter-coupled logic (LVPECL) at frequencies of up to 625MHz, meeting the stringent requirements of most communications, networking and high-performance computing applications. The oscillator operates from a supply voltage in the range 2.5V to 3.3V. Specified aging stability of ±5ppm makes the devices suitable for use in application areas such as telecom and Internet infrastructure.

The inherent properties of pMEMS technology also allows for higher native resonant frequencies than traditional quartz-based devices, enabling these oscillators to provide higher performance at a competitive price without sacrificing critical specifications. Taking advantage of the small die size and wafer-level packaging, 4M series devices are available in a smaller 5mm x 3.2mm (5032) plastic package, and also offer the choice of a 7mm x 5mm (7050) variant matching the established quartz-crystal form factor.

An oscillator evaluation board (figure 3) helps engineers start new designs and quickly understand how to achieve the best performance from this new type of device.

An evaluation board assists design with 4M series pMEMS oscillators

IDT also offers pMEMS clock generators such as its CrystalFree™5V series, which combines pMEMS oscillator technology and phaselocked- loop (PLL) techniques to generate high-quality clock frequencies for consumer, computing and embedded applications. By integrating pMEMS technology, these devices eliminate the need for an additional external frequency source, thereby helping improve reliability, reduction in board space and lower system cost. In fact, pMEMS technology enables many novel functions to be integrated in the same package, such as a clock multiplier or divider or a multiple-output configuration.

A further benefit of pMEMS technology is that the oscillator output frequency can be programmed rapidly before the device leaves the factory, with no need for time-consuming or costly fine-tuning. This enables suppliers such as IDT to offer custom frequencies at a competitive price, even at low minimum-order quantities.

Robust Industrial Frequency Control

With the most recent improvements in MEMS oscillators, as seen in the 4M pMEMS family, the technology is now ready to deliver the levels of accuracy and stability needed for precision industrial applications. With changes in the industrial electronics sector, such as widespread adoption of handheld devices offering greater flexibility for workers and helping increase productivity, the high resistance to shock and vibration inherent in pMEMS oscillators is topical and interesting to equipment designers.

Standard quartz devices, in which the crystal is clip-mounted within a metal housing, are regarded as fragile components. A shock of around 50-100g is capable of fracturing the crystal; thus the crystal manufacturers publish storage, packing and shipping guidelines for crystal components that stipulate avoidance of careless handling such as dropping or throwing containers or assemblies, or using excessive force when inserting the components into assemblies.

Crystal manufacturers have developed high-shock products that offer significantly increased resistance to mechanical stresses. Among the techniques employed, minimising the dimensions of the crystal and the size of features such as tuning fork tines effectively reduces mass and internal shear forces. In addition, improvements to mountings and ceramic package technology make further contributions to improving survivability. Today’s high-shock crystals are able to survive impacts significantly over 10,000g, with the best achieving over 100,000g for use in extremely demanding military applications.

In contrast, MEMS oscillators offering semiconductor-grade robustness provide high shock resistance in the order of 50,000g without recourse to special construction or packaging techniques. This provides designers with an economical yet robust timing solution, offering long-term reliability in numerous equipment types such as drives for heavy-duty drilling or cutting equipment, motion controls, electronics in transportation systems such as rail traction drives or brake controllers. Designers of automotive electronics, also, can take advantage of the high mechanical robustness offered by pMEMS devices to build modules capable of withstanding continuous high levels of vibration experienced even during normal use by the vehicle owner.

Conclusion

Overall, sub-ps jitter pMEMS oscillators are delivering greater freedom of choice for designers and driving the pace of progress towards smaller, more accurate, competitively priced timing solutions. Ultimately this will allow designers to specify the exact type of device that best suits the end application; choosing from crystal oscillators, CMOS silicon-based oscillators, or piezoelectric MEMS devices to satisfy requirements such as cost, frequency, accuracy, stability, mechanical resilience, size, power consumption and availability.

 

 

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