Advantages of MEMs Timing Technologies Part 1

Learn how the MEMs resonators are less sensitive to shock and vibration than quartz resonators in this illustrated demonstration. You can also get a closer look at how the newest family of MEMs oscillators in the SiTime’s Elite Platform™ have several architectural advantages over quartz resonators.

Hello. It's Brett Hanebrink with another product training module for SiTime.

Today we're going to cover the advantages of MEMS technologies which is a two-part series. Let's go ahead and get started with Part 1. The mass of MEMS resonator is about 1-1,000 to 1-3,000 that of a quartz resonator depending on the particular quartz resonator blank. This means that MEMS resonators are inherently less sensitive to shock and vibration than quartz resonators, and this fact would be demonstrated on the next two slides which will show phase noise and frequency shift due to vibration.

This graft shows the phase noise comparison of a quartz resonator versus a MEMS resonator, while subjected to a random vibration profile of 7.5 Grms acceleration and vibration frequency from 15 Hz to 2 kHz. Phase noise measures noise in the frequency domain and lower values are therefore better.

As can be seen from the phase noise plot the quartz plot in red shows about 20 dB increase in noise resulting from the random vibration across the frequency range of 15 Hz to 2 kHz mentioned previously. By contrast the MEMS oscillator plot in green shows no discernible increase in noise from the vibration. An increase in noise at 20 dB can result at lower data throughput due to increase in bit errors and in the worst case dropped communication links.

The above plot shows the oscillator response to sinusoidal vibration profile of magnitude 4 G from 15 Hz to 2 kHz. This is measured in units of parts per billion frequency shift per G of acceleration. The MEMS has 3 to 100 times lower sensitivity to sinusoidal vibration than quartz-based oscillators depending on vibration frequency and THUNDER.

The newest family of MEMS oscillators from SiTime named Elite has several architectural advantages over quartz resonators. Central to this architecture is the use of two MEMS resonators: the TempFlat resonator which has a relatively flat frequency versus temperature response and provides reference frequency to the PLL, and the TempSense resonator which has a fairly steep but linear frequency response of -7 ppm per degree C which makes it a great temperature sensor.

These two resonators show the same die substrate with less than 200 μm separation thus ensuring the best possible thermal coupling between them. This level of thermal coupling between the temperature sensor and resonator is not possible on quartz-based oscillators because of the physical separation between the quartz resonator and temperature sensor which resides on the IC containing the oscillator circuit.

But DualMEMS along with the high bandwidth temperature, the digital converter enables the best possible frequency stability in the presence of fast thermal transients. And quartz oscillators, the CMOS die where the temperature sensor resides is mounted on the pack of substrate and connected to the package leads and crystal connections via bond wires.

This physical separation between the temperature sensor and resonator causes significant thermal lag and therefore more frequency stability error during fast thermal transients. There's little that can be done to improve this thermal lag because the temperature sensor cannot be placed on the quartz resonator. By contrast the Elite family of SiTime MEMS oscillators have a dual resonator construction with the TempSense MEMS sharing the same die substrate with the TempFlat with 200 μm separation.

With this close proximity and sharing the same die substrate, the thermal coupling between the temperature sensor and resonator is very tight resulting in very small thermal lag. To complete the MEMS oscillator construction, the DualMEMS resonator is stacked on top of the CMOS die which contains the CMOS circuit elements including sustaining amplifier, TDC and fractional PLL. This is a good place to close out Part 1. 

Please view Part 2 as we begin to show some added benefits to our new Elite products.

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