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EETE OCT 2014

ANALOGUE DESIGN Expanded options for the system clock By Craig Bakke When faced with the decision of what to use for the clock source in an electronic design, the traditional assumption has been that the only realistic choice for any application that needs even moderate accuracy is a quartz-based oscillator. Quartz has dominated the market for decades but its limitations are opening up opportunities for new technologies. Quartz oscillators rely on the material’s piezoelectric properties. The crystal distorts when an A typical rectangular MEMS resonator structure using aluminium nitride for the moving mass. electrical pulse is applied to it. Conversely, its distortion leads to the generation of an electrical signal. These two aspects of the piezoelectric effect are used in oscillators to induce resonance such that the crystals oscillate within a narrow range of frequencies. The key to controlling the frequency and range of frequencies lies in size and shape of the crystal. The higher the desired frequency, the smaller the crystal, but as the crystals are made smaller they become more fragile and harder to make reliably. Maintaining tight frequency control involves the use of a number of external components and often leads to precise rules over how the devices are placed on a board and how close they can be to other parts of the system. To provide a stable clock signal, the oscillator circuitry needs to be protected from Block diagram of the Si50x CMEMS oscillators. on-board interference. High-frequency data buses can inject switching noise into the oscillator feedback loop, disturbing the relatively weak signal produced by the quartz crystal. In general, the simplest way to protect the oscillator is to keep most signal traces and components outside an ‘exclusion zone’ of several square centimetres on the PCB. One issue that quartz clocks face is frequency shifting caused by the high temperatures encountered during soldering. Crystal-based products exposed to solder reflow, during which temperature excursions may reach more than 200°C for tens of seconds, can experience frequency shifts of up to ±5ppm. Quartz devices are also vulnerable to shock and vibration because of the way in which the crystal needs to be held within the package. As a result, the trend for miniaturisation in electronics as well as demand for more rugged devices is building demand for other options. Silicon semiconductor technology has moved into the position where it can provide an effective alternative. Microelectromechanical system (MEMS) technology provides one way to harness semiconductor-grade silicon for clock generation, with an increasing number of products arriving on the market based on this approach. The core component of the MEMS clock generator is a mechanical resonator etched from a silicon wafer, tuned to vibrate at a target frequency. Masses of various sizes and shapes, including discs and multibeam structures, have been used to create the vibrating element, which is generally set into motion using electrostatic interactions. The mass of this resonator is extremely small – often on the order of one ten-billionth of a gram. The small mass can be used to generate high frequencies that often require cuts to quartz crystals that are difficult and expensive to make. The MEMS resonator can be constructed using a variety of silicon-based materials, such as single-crystal silicon, polysilicon or polysilicon-germanium (poly-SiGe). The resonators are susceptible to the adsorption of moisture. A single layer of water can push the operating frequency down by hundreds of parts per million as the mass of the resonator increases. To prevent adsorption, MEMS timing devices need to be sealed against the environment. Their resonating frequencies will also drift significantly with temperature – the materials become softer as temperatures rise leading to a temperature coefficient of up to −40 ppm/°C. However, MEMS products are typically less vulnerable to temperature shifts caused by reflow soldering. For example, Maxim Integrated’s MEMS-based real-time clock products experience a shift of less than ±1ppm following reflow, five times lower than equivalent quartz products. Being much smaller, MEMS products exhibit much greater immunity to shock and vibration than quartz-based components. To deal with the problem of temperature-related frequency shifts, the output of the MEMS resonator is typically compensated electronically. This can be done in a similar manner to the techniques used for temperature-compensated crystal oscillators (TCXOs). One digital technique is to provide an electronic reference oscillator, based on a transimpedance amplifier, that is fed to a fractional-N synthesiser and phase-locked loop. The output of the reference oscillator is measured during calibration at room temperature and compared to the target frequency. The output is then measured at another temperature and these results stored in local memory. As temperature shift is normally monotonic, only a few points are needed to provide voltage offsets that are used to drive the frequency synthesiser or PLL to provide the required frequency output. To improve the performance of MEMS resonators, aspects of quartz and MEMS have been brought together, using piezoelectric effects rather than mechanical resonance. IDT, for example, manufactures piezoelectric MEMS-based oscillators. In these devices, the cantilevered beam is coated with a thin piezoelectric film made from materials such as aluminium nitride or zinc oxide. The combination of structures leads to strong electromechanical coupling, providing low resistance Craig Bakke is Product Manager at Digi-Key - www.digikey.com 42 Electronic Engineering Times Europe October 2014 www.electronics-eetimes.com


EETE OCT 2014
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