In addition to overcoming the higher native frequency challenge for quartz crystals and traditional MEMS, we also studied known quartz limitations such as activity dips, aging, vibration sensitivity, etc.
One of the known issues for crystal oscillators is activity dips that can cause intermittent failures. These failures affect both the frequency and the resistance (i.e., the Q) of crystal resonators. Activity dips are usually caused by interfering modes (e.g., by high overtone flexure modes) and are strongly influenced by the crystal’s drive level and load reactance. These activity dips are not present for MEMS oscillators since the MEMS resonators are designed to suppress undesired modes over temperature and process variations that can impair crystal-based oscillators.
Shock and vibration sensitivity
In addition, as part of the improved reliability, MEMS oscillators demonstrate enhanced semiconductor-grade shock and vibration resistance. Standard quartz devices are fragile since the crystal is housed within a metal or a ceramic package, allowing the crystal to be fractured by a shock of 50-100g. Manufacturers have to then implement specific storage, packing and shipping protocols for crystal devices to avoid careless handling.
pMEMS oscillator resistance to shock and vibrations were evaluated. The devices were easily able to survive more than 1500G of shock and 20G of vibration test as shown in figure 6.
Figure 6: pMEMS devices passed military vibration and shock testing devices to long-term aging (i.e., frequency drift) tests Click on image to enlarge
The small size of pMEMS resonators results in better reliability, i.e., less mass results in better vibration/shock sensitivity. In addition to meeting military grade shock / vibration specifications, pMEMS devices also remain functional aftershock tests of 70,000G.
Frequency stability: long-term frequency stability was also measured by subjecting these plastic QFN packaged pMEMS devices to long-term aging (i.e., frequency drift) tests.
In these tests, plastic packaged pMEMS resonators were placed together with quartz devices in temperature-controlled chambers. At 25°C, pMEMS devices have a frequency variation of less than ±2.5 ppm over a period of 21 months, which is better than the typical quartz performance of ±5 ppm – see figure 7.
Figure 7: Aging measurements of 10 pMEMS resonators over 21 months at 25°C
In addition, when tested at 125°C, pMEMS devices show less than ±3 ppm frequency drift over 4500 hours, which is significantly better than the typical quartz measurement of ±10 ppm – see figure 8.
Figure 8: Accelerated aging measurements of 10 pMEMS resonators over 4500 hours at 125°C. (measurement accuracy ±2ppm)
Finally to demonstrate the performance of these high-performance sub-ps jitter (12kHz – 20 MHz) pMEMS oscillators in the real world, three applications were implemented for networking, FPGA and storage applications as shown below.
All of these pMEMS application demonstrations have been traveling at various shows around the world –see figure 9.
Figure 9: pMEMS oscillators demonstrated in three different applications
Click on image to enlarge
Other advantages of MEMS-based products 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. IDT’s MEMS oscillators support low-voltage differential signaling (LVDS) and low-voltage positive emitter-coupled logic (LVPECL) outputs at frequencies of up to 625MHz, which is required in most communications, networking and high-performance computing applications.
A new class of pMEMS oscillators has been introduced for high-frequency, low phase noise (<1.0 ps) timing reference applications. The high performance, as well as compactness and stability of the pMEMS devices, has proven this technology to be a cost-effective and more reliable next-generation replacement for quartz crystal oscillators for the high-frequency frequency reference applications.
About the author
Harmeet Bhugra is a managing director at IDT and is responsible for the vision, growth and general management of the MEMS business - www.idt.com
Courtesy of EETimes Europe
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