Abracon MEMS: When to Select MEMS Clocking for Compact Designs

The term “Micro Electro-Mechanical Systems (MEMS)” refers to technology using the combined features of nano and micro-scale machining along with electronic circuitry. MEMS devices are typically etched out on wafers and constructed to such a small scale that they exhibit unique properties and features not achievable at larger geometries.

Despite some drawbacks, there are two ways that MEMS devices present improvements over standard quartz technology: miniaturization and resistance to shock and vibration. This article focuses on the benefit of small size. For example, MEMS have been extremely successful when used in compact pressure sensors, optical imaging mirrors, microphones and switching applications.

MEMS vs. Quartz Crystals

MEMS clocking technology has been more challenging to develop, mainly due to the precision required to develop the more compact devices. Whereas other sub-systems can observe accuracies of 1% or higher, clocking applications, in most cases, cannot tolerate 10,000ppm of error.

In contrast, the incumbent technology, quartz crystal blanks, is naturally self-compensated due to the crystal structure and achieves ±20ppm clocking accuracy very easily. Consequently, crystals have become the default clocking solution due to their inherent stability. However, using temperature compensation, it is possible for MEMS technology to match quartz crystal stability performance.

Abracon recently released an article highlighting the output frequency range achieved in a variety of package options for both quartz crystals and high-performance, low-jitter oscillators. The article discusses the unavoidable dependency between quartz blank dimensions and frequency of oscillation. It’s simply a matter of physics: Smaller crystal blanks will oscillate at a higher frequency. Thus, there is a size penalty in achieving lower frequencies.

For a wide range of compact footprints, frequencies lower than 8MHz are not achievable. A quartz crystal, such as the Tiny but Mighty™ ABM13W, fits within a 1.2mm x 1.0mm footprint and achieves a frequency range of 32MHz to 80MHz. The 1.6mm x 1.2mm ABM12W supports 24MHz to 52MHz. The larger ABM8W with a 3.2mm x 2.5mm footprint operates with a lower 10MHz to 54MHz frequency range. Finally, the 6.0mm x 3.5mm ABM7 supports no lower than 8MHz, while series like the 7.8mm x 3.1mm ABU can reach 6MHz.

Where MEMS Wins

So, then how can one obtain both small size and low frequency? Well, this is where MEMS devices truly shine. Output frequency flexibility is natural for MEMS oscillators, which can generate any frequency within their range. But the most unique benefit is achieving that frequency range within a very small footprint. Since MEMS are based on silicon micro-machined structures, the manufacturing readily scales to a smaller size without sacrificing range, stability or other specifications.

For example, Abracon’s AMPM and AMJM series of MEMS oscillators support 1MHz to 100MHz output frequencies in a variety of footprints – down to 1.6mm x 1.2mm. The AMJD and AMPD both support a selection of one or two output frequencies on a single device using a frequency select pin. The AMPDAFH-A13T supports 6.1298496MHz or 6.16791MHz outputs, depending on frequency select.

There is an advantage of having a selectable frequency range, especially one that reaches below 10MHz. This frequency addresses the needs of wireless charging controllers, which need to operate below 10MHz while remaining in small form factor. The typical limit for crystals lies somewhere between 1MHz and 6MHz, although any crystal operating at 1MHz will be quite large.

Therefore, many applications with sub-8MHz clocks requiring compact form factor should strongly consider MEMS oscillators. Space savings has become a strong consideration in some Industrial, automotive, medical monitoring and wireless charging designs.