MEMS Oscillators and Quartz Technology

From simple RC oscillators with an accuracy of about 30,000ppm to atomic clocks with an accuracy better than 0.001ppb, there are many clock options to meet different application requirements. For many years, bulk acoustic wave (BAW) crystal oscillators have been available to meet most requirements, providing accuracies in the 10ppm range. Less accurate options, such as SAW oscillators, ceramic oscillators, and IC oscillators, each have their own advantages for specific needs.

Quartz-based devices have long been the standard against which most other timing devices are compared. Quartz’s history as a stable, controllable, high-quality material for frequency selection and timing devices is widely recognized, and its frequency temperature response, aging rate, and jitter and phase noise characteristics are well documented.

Figure 1 Phase noise diagram

Recent introductions to MEMS-based oscillators are often accompanied by assertions that the technology will eventually replace quartz by offering lower costs, shorter design and production cycles, superior shock and vibration performance, and better signal quality. Despite these assertions, very little research has been done to help understand the characteristics of MEMS oscillators. This study seeks to provide a direct comparison of MEMS oscillators with traditional quartz resonator-based oscillators. For many years, bulk acoustic wave crystal oscillators have met most needs with their 10×10-6 accuracy range.

Research Methods

The results presented here are based on the application of typical industry measurement techniques for frequency control and represent commercially available technology at the time this research was conducted in 2008. Many electrical characteristics were evaluated, including frequency vs. temperature, phase noise/jitter, short-term stability, start-up time, current, and long-term stability (aging).

Due to space limitations, only phase noise/jitter data related to short-term stability are presented in this article. A PDF copy of the full study is available at www.pletronics.com/ple/pages/documentation/.

The above products are all CMOS level output, working at 3.0 or 3.3V, and the output frequency is 25~50MHz. The tested BAW quartz crystal oscillator is in fundamental harmonic mode at 25MHz and in third-order harmonic mode at 50MHz. The results given here are the test values ​​of typical devices or the average value of multiple devices of this model.

Phase Noise/Jitter

The measurement is performed using an Agilent 5052 signal source analyzer test system. This system measures the output signal levels other than the valid signal, and is also a reference for the actual main output level. The oscillator is operated in a fixed device with a 15pF load bypassed by the same capacitor, and powered by a low-noise Agilent linear power supply.

As shown in Figure 1, the higher phase noise levels indicate that MEMS oscillator technology is not an equivalent technology. The Agilent test system test results show that jitter is likely to be a problem in current communication and data transmission applications.

The phase noise plots provide a good illustration of the design and behavior of these devices using different technologies. The level of phase noise in the near-field (<1kHz) depends mainly on the Q value or choice of resonator, with quartz BAW resonators being much more selective than other MEMS devices.

(a) and MEMS2 resonant oscillator

(b) Short-term stability

Figure 2 MEMS1 resonant oscillator

The 1-100kHz section reflects the relevant information of the design. The MEMS oscillator uses a phase-locked loop (PLL) design in which the MEMS resonator is phase-locked by the VCO of the M/N synthesis loop. The phase noise level of the MEMS oscillator is the result of the PLL loop bandwidth, VCO selectivity and the Q value of the main resonator. The quartz resonator device operates at the output frequency and there is no additional noise signal from the PLL at the output.

Phase noise can be integrated over a defined frequency interval and converted from the frequency domain to the time domain to provide an rms jitter value, as shown in Table 1. This is the normal practice for calculating jitter for quartz crystal-based resonators, which typically have jitter performance that equals or exceeds that of the best oscilloscopes.