Comparing Key Performance Specs of Quartz Crystal and MEMS Oscillators

There are myriad applications today where MEMS (Micro-Electro-Mechanical) oscillators are preferred. In high-volume consumer products like mobile phones, toys, games, and automotive entertainment/navigation systems, for instance, MEMS devices provide a low-cost solution with acceptable performance levels. However, nothing we know of will ever offer the “Q” (stability and performance value) and the spectral purity of quartz, which makes them ideal for high-reliability applications.

This article compares MEMS and crystal oscillators, addressing the key areas that differentiate their quality and performance.

MEMS Oscillators Vs. Crystal Oscillators

MEMS oscillators require more complex circuitry to perform the basic function and to compensate for temperature variation and other variables. Despite their intrinsically more complex structure, MEMS are typically considerably less expensive than crystal oscillators. And with all the other functions being implemented employing emerging MEMS topologies, the adoption of low-cost MEMS oscillators is clearly going to increase.  Moreover, MEMS oscillators can handle shock and vibration adequately. On the downside, MEMS oscillators typically exhibit a low Q, which results in poor phase noise and jitter. Finally, MEMS device circuitry simply can’t handle space-related radiation exposure that crystal oscillators can tolerate.

Topologies Figure 1: MEMS oscillator

As shown in the block diagram in Figure 1, the MEMS oscillator is much more complicated than a crystal oscillator. It comprises a micromechanical silicon resonator and digital circuitry, including a phase-locked-loop (PLL) used to determine and control frequency.

In comparison, the basic quartz crystal oscillator is very simple. It depends heavily on its very high “Q” quartz crystal resonator as the sole frequency-determining element, as shown in Figure 2. Figure 3 shows the equivalent circuit of a quartz crystal oscillator.

Figure 2: Basic quartz crystal oscillator

Figure 3: Equivalent circuit of high "Q" quartz crystal resonator

Both MEMS and crystal oscillators (XOs) can be made more precise by using temperature compensation to achieve less than 1PPM stability or oven control to achieve PPB levels of stability. In common parlance, crystal oscillators utilizing these compensation methods are referred to as temperature-compensated XOs (TCXOs) and oven-compensated XOs (OCXOs), respectively. And while the “XO” is specifically used for crystal oscillators, these terms are often used to refer to similarly compensated MEMS.

Radiation Tolerance

MEMS oscillators are generally not considered to be radiation-tolerant, especially compared to quartz crystal oscillators. MEMS oscillators, due to their design and materials, are more susceptible to radiation damage than quartz crystal oscillators, which can tolerate higher radiation doses. Radiation can cause a shift in the resonance frequency of MEMS resonators, which can impact the accuracy and stability of the oscillator.

MEMS devices have been reported to fail at doses as low as 20 kRad, while crystal oscillators optimized for space applications are specified with a total Incident dose (TID) of 50 kRad (LEO) to 300 kRad for full-space applications. One limiting factor for MEMS in space applications is the radiation sensitivity of their phase-locked loop (PLL) and memory components. Quartz crystal oscillators also use PLLs, but for space applications, these circuits are quite expensive and go through extensive testing. MEMS manufacturers could use the same PLL circuits, but then they would lose much of their price advantage. The memory devices used in MEMS oscillators are also sensitive to radiation—perhaps more of a problem in MEMS than in the PLL circuits.

Timing Oscillator Noise Specs: Comparing Apples, Oranges—and Cherries

When dealing with oscillator noise specs, there is an issue of comparing oranges, apples and cherries.

When it comes to specifying and measuring phase noise and jitter in oscillators, there are many ways of doing so, which can produce very different apparent results. Quartz oscillators, for instance, always exhibit lower phase noise and jitter than comparable MEMS oscillators, since quartz has a much higher Q factor and, therefore, much lower close-in phase noise. 

SERDES 4-16A Vs. Unfiltered Noise Measurement 

A common method used by leading MEMS suppliers to specify jitter is to employ a SerDes 4-16A measuring protocol that uses a high-pass and low-pass filter to reduce much of the noise. (The SerDes 4-16A phase jitter method is often employed in high-speed data links.) If one were to use the SERDEs 4-16A method on a quartz oscillator, the quartz clock would have much lower jitter than the MEMS device. Rather than using this method, companies like Q-Tech use a very conservative and consistent measurement protocol that doesn’t filter out any noise. It also uses a wider bandwidth of 1 kHz to 20 MHz. When a MEMS device is measured using this method, its jitter will be worse than when using the SerDes 4-16A method—and will always be worse than that of a quartz oscillator.

Allan Variance

Allan Variance is a statistical measure used to quantify the frequency stability of clocks, oscillators, and similar devices over different time intervals. It is particularly effective at characterizing noise processes in frequency signals. Mathematically, Allan Variance is defined as:

where y_n represents the average fractional frequency over the n^th sampling period of duration Tau(τ), and the angle brackets denote the expectation value (time average).

Quartz oscillators like OCXOs (oven-controlled crystal oscillators) will have much better Allan Variance when measured using a Tau (observation time) of one second or longer.  A relatively long Tau correlates to the close-in phase noise, where quartz is so much better than MEMS. And it’s this close-in, short-term stability that Allan Variance was designed to measure and characterize.

Critical Low Phase and Jitter Applications

The bottom-line fact is that quartz oscillators of all types will always have lower phase noise, jitter and Allan Variance (as it is normally specified and measured using a long Tau) than a MEMS oscillator of the same type. Although achieving low phase noise, jitter and Allan Variance may not be as significant in some applications, in others it’s vital. Phase noise, for example, is especially important in radar and communications, and jitter specifications for digital-signal applications, such as high-speed data transmission and data conversion (DACs/ADCs). And, finally, Allan deviation is important for timekeeping and navigation applications, where stability and accuracy are essential. In all these applications, only the “cherry” specs provided by crystal oscillators will do.

The Role of Crystal Oscillators

There are an enormous number of applications where MEMS oscillators are being used. In some aerospace applications, where short-term performance (one and done) is important but long-term stability is not, MEMS might also be considered. And with all the other functions being implemented employing emerging MEMS topologies, the adoption of low-cost MEMS oscillators is clearly going to increase.  

So where does that leave the classic high-performance crystal oscillator? They are indispensable in high-performance applications where a “service call” is simply out of the question. Crystal oscillators are indispensable in space applications such as low Earth orbit (LEO) and geosynchronous orbit satellites, as well as in high-temperature applications like down-hole drilling. 

Other applications where quartz crystal timing devices are the better choice include military and avionics applications, such as missile guidance and navigation systems, where temperature and frequency stability are critical. Crystal oscillators are also found in high-definition video displays where exceptional performance is necessary. Another is driving high-frequency field-programmable gate arrays (FPGAs), where the lower jitter of quartz oscillators results in wider “eye” diagrams and lower Bit Error Rates (BER). And yet another is high-speed A/D converters, where crystal oscillators’ lower jitter translates to lower phase noise.

Conclusion

An important consideration in engineering design is delivering the required performance for the application at the lowest cost. The emergence of MEMS oscillators has enabled dramatic cost reductions in many digital consumer and commercial products. And while MEMS technology is making great inroads into oscillator applications where its lower cost offers an attractive solution, the world of ultra-high reliability is—and will remain—the realm of the venerable crystal oscillator.