June 02, 2019 by Gaber Intihar
AEL Crystals, the UK’s largest supplier of frequency control components, has recently added MEMS oscillators from SiTime Inc. to their IoT range.
This technology promises reliable frequency generation while also allowing for the miniaturisation and lower power consumption of end products—the two key hurdles in the designing of ever-smaller devices.
Every electronic system needs a timing device. Traditionally, quartz crystals have been used to build electronic oscillator circuits because of their ability to produce a precise and constant frequency. But in recent years, the microelectromechanical system, aka MEMS oscillators have been developed to the point where they can now provide a reliable alternative that has many advantages over quartz-based solutions.
These oscillators use MEMS resonators to define stable frequencies, utilise sustaining amps to maintain those frequencies, and employ phase locked loops (PLLs) to output the desired frequencies. In order to achieve widespread adoption of MEMS oscillators, they are often available as 4 or 6-pin integrated circuits, which allows them to be integrated into standardised PCBs—without the need for any modifications.
A MEMS oscillator chip. Image courtesy of Wikimedia.
Because of how ubiquitous and well-established they are, quartz oscillators set the bar for MEMS oscillators. Until fairly recently, MEMS-based timing devices couldn’t compete with quartz solutions; nowadays, however, MEMS oscillators can match or even surpass their performance in certain key areas.
SiTime’s MEMS oscillators offer a variety of advantages:
Because of their fully encapsulated design, they are up to 30 times more reliable, which lowers production costs.
They can fit in a smaller package, which can reduce the PCB area by up to 65%. SiTime’s 1507 oscillators measure 1.5mm x 0.8mm and don’t require any additional external components. For comparison, the smallest quartz crystals measure 1.6mm x 1.2mm without the necessary additional components. When those are taken into account, the board area of the MEMS oscillator ends up being up to 3 times smaller, which is ideal when it comes to for building smaller devices.
MEMS oscillators are capable of driving multiple loads at once. This means that they can effectively replace up to 3 quartz crystals—therefore further reducing the costs and final board size.
Due to their design, MEMS oscillators don’t have any exposed PCB connections. That, coupled with short bond wires between the MEMS resonator and the CMOS IC, means that they have over 130 times lower sensitivity to EM energy than quartz solutions.
Given that MEMS resonators don’t require a crystal, their mass can be up to 3,000 times lower. This makes them far more resistant to shock and vibrations.
Because MEMS oscillators generate output frequencies by programming the PLL to different values, they are capable of a wide and accurate frequency range. Quartz oscillators, on the other hand, need to have a different crystal cut for every frequency.
SiTime product image of MEMS Oscillators. Image courtesy of SiTime.
IoT devices use multiple wireless protocols (Bluetooth, Z-wave, and so on) to communicate with other devices. This makes reliable frequency generation critical. MEMS timing solutions offer just that, but what makes them perfect for IoT, mobile, and wearable applications is their smaller size and lower power consumption (again, particularly compared to their quartz crystal alternatives).
In fact, one of SiTime’s products is the SiT8021: the smallest and lowest power Mhz oscillator that’s currently available. It has a size of 1.5 x 0.8 mm and a maximum operating power of 270 µA—and just 0.7 µA when in standby mode.
SiTime also offers real-time clock products, such as their SiT15xx range, which keep their 32 kHz clock always on, leading to further battery life improvements.
Ultimately, while quartz-based oscillators aren’t going to be completely replaced anytime soon (if ever), the future of IoT devices seems to belong to MEMS oscillators—thanks to their superior reliability, lower power consumption, and physical footprint.
