1 Introduction 

Oscillators have been made from quartz crystal resonators connected to an ***og circuit that drives the resonator to vibrate at a specific frequency. Now, there is an alternative – MEMS oscillators – and these devices outperform quartz oscillators in noisy environments. The drive toward higher speed telecom and mobile applications places greater demands on the clock source. Additionally, more complex electronics and higher clock frequencies necessitate that the clock device continue to perform well in noisy environments. This paper shows results of comparative experiments that were conducted on quartz and MEMS oscillators. The data demonstrate that MEMS oscillators outperform quartz in realistic environmental conditions. Oscillator vendors provide data sheets for each product stating performance parameters such as frequency stability, jitter, and phase noise. While data sheets are a good indicator for selection of timing devices, the user must also evaluate how these devices perform in real-life environmental conditions. Testing under conditions seen in the real operating environment provides valuable information about true component performance. The performance of oscillators subjected to environmental stressors, such as EMI, vibration, and noise from power supplies or other system components, will degrade as compared to datasheet conditions. Ultimately, environmental stressors may reduce the reliability and lifetime of a device. It is important to consider the performance of oscillators under realistic, noisy, harsh conditions when selecting a timing device. 

2 MEMS Advantages 

MEMS oscillators have some inherent advantages over quartz oscillators that allow them to perform reliably in a variety of environments. Resonators are fully encapsulated in silicon and enclosed within a micro-vacuum chamber. The combination of very ***all mass of the resonator and its stiff silicon crystal structure makes them durable and extremely resistant to external stresses such as shock and vibration. Additionally, optimally designed ***og circuits in the oscillator deliver high performance in electrically noisy conditions. 

Most quartz oscillator vendors are experts in manufacturing resonators, but not necessarily in circuit design. They usually outsource the ***og circuit and must purchase die that are designed to work with a variety of crystals rather than being optimized for the specific resonator. 

3 Environmental Stressors 

Several factors in the operating environment can negatively impact oscillator performance, degrading phase noise and jitter.

3.1 Power Supply Noise 

One major source of noise in any system comes from power supplies. Most of this noise is filtered out by passive filters and decoupling capacitors that are placed on the power supply input of the oscillator. However, some noise remains, which may increase the jitter on the output clock, and can negatively impact system timing margins. This noise is amplified not only when the power supply itself is switched on, but when other devices on the board turn on or off during system operation. On-board issues, such as inadequate power supply filtering or ground bounce, also affect noise and jitter. The Power Supply Noise Sensitivity (PSNS) is a specific parameter that is used in the design circuits and provides an indication of how robust a circuit is to noise from the power supply. The results of testing show that MEMS’s PSNS is much better than some competing quartz devices, which performance is good according to data sheets. The question is how they perform in the presence of system-generated noise.

A test setup that includes a power supply and waveform generator, as shown in above picture, is a good method to generate conditions that closely mimic the environment of a real system. The waveform generator adds system noise at a specified voltage and frequency to measure the effect of power supply noise on the oscillator jitter. The plot below shows integrated phase jitter as a function of power supply switching noise frequency for 50mV of peak-peak power supply noise, comparing results for quartz oscillator . As the plots indicate, MEMS oscillator jitter is lower at nearly all noise frequencies. The reason for this is advanced PSNS built into MEMS oscillator circuitry to protect the oscillator from power supply-induced jitter. 

3.2 External EMI Noise 

Another important noise source to consider is externally generated EMI noise that impacts the oscillator performance (as opposed to EMI signals that are emitted by a clock source). Power supplies, power lines, lightning, computer equipment, and electronic components are all potential sources of externally generated EMI, which can be coupled into the system through radiation. EMI is a major concern in applications such as passive optical networks (PON), cellular base stations and many products that are used in outdoor environments where large electromagnetic sources are present. EMI is also a concern in dense electronic boards with multiple switching power supplies, since oscillator components can be placed close to these power supplies. Inbound EMI can change the clock jitter and, in catastrophic cases, even the operating frequency of clock devices, negatively impacting the functioning of any system that depends on the clock signal for reliable performance. Phase jitter and phase noise increase significantly in the presence of incoming EMI, and attempts to filter out the noise reaching the oscillator are not always successful. Another approach is to design clock devices that successfully reject EMI. Testing oscillators in the presence of a power amplifier mimics the effects of real-world EMI exposure. Measurements of integrated phase jitter (12 kHz to 20 MHz) taken with and without a nearby 9kW switching power supply turned on demonstrate the effect of EMI on oscillator jitter. The data show that MEMS built-in circuitry successfully maintains low jitter of 0.5 ps in the presence of external EMI radiation. Quartz oscillators included for comparison perform fairly well without external EMI, with phase jitter levels comparable those claimed on published data sheets. The performance with EMI is quite different, with jitter rising to levels that may be unacceptable for many applications. These results emphasize the importance of understanding the relationship between performance and operating environment. 

3.3 Vibration 

Many electronic products are subjected to substantial vibrational forces during use. This is especially true of mobile, portable devices carried around in pockets or backpacks. Electronics in mobile GPS units, industrial equipment, or aerospace applications may undergo higher levels of vibration. Even stationary products may experience vibration from a nearby fan or other equipment. Quartz oscillators may show significant sensitivity to vibration because of the mechanical assembly and packaging used. MEMS resonators that are inherently more resistant to vibration-induced performance degradation for two reasons. First, the moving section of the silicon resonator has much ***aller than a quartz resonator. This reduces the force applied to the resonator from the vibration-induced acceleration. Second, the silicon MEMS resonators are very stiff structures that resist movement caused by vibration forces.Vibration can degrade oscillator performance by inducing an electrical signature at the same frequency as the mechanical vibration. This is observed as spurious phase noise, or noise spurs, at a specific frequency. This phase noise is translated into a frequency modulated (FM) noise and normalized to the carrier frequency for 1g vibration acceleration. The result is expressed in part-per-billion/g (ppb/g) as a function of vibration frequency. Because oscillator response depends on the direction, severity and frequency of the vibration, it is important to measure sensitivity to vibration-induced noise under a variety of vibration conditions. To measure vibration sensitivity, oscillators were subjected to vibration in the x- and z-axes using the vibration fixture shown below. Because of resonator symmetry, vibration in the y-axis has essentially the same effect as x-axis vibration and was therefore not measured. The vibration frequency ranged from 10 Hz to 2 kHz to mimic vibration frequencies experienced in the field. The data show the oscillator sensitivity at various vibration frequencies for an acceleration value of 5g. These plots demonstrate that MEMS oscillators exhibit lower levels of vibration spurs than other oscillators. This holds true for both x and z directions and at most tested vibration frequencies and acceleration rates up to 50g acceleration.

4 Reliability 

One method of quantifying the reliability of a component is predicting mean time between failure (MTBF). For semiconductor components, this is the inverse of the failures in time (FIT) rate, expressed as the number of failures statistically expected after 1 billion operating hours. The higher the MTBF, the longer the expected lifetime of the device and therefore the more reliable the device. Low values of FIT rate indicate a low number of expected failures and high reliability. MEMS Oscillator is calculated FIT by subjecting oscillators to stress testing at elevated temperature and voltage for an extended period of time. Stressing thousands of oscillators for cumulative test time of over two million device hours resulted in no failures. Based on these results, MEMS oscillators have a calculated reliability rate of less than 2 FIT, corresponding to a MTBF of at least 500 million hours. showing that semiconductor-based oscillators as a class are more reliable than quartz oscillators and that MEMS is over 10 times better than the most reliable quartz oscillators.

It is important to consider oscillator response in real-world conditions. Testing demonstrates MEMS oscillators is better than Quartz Oscillator.