RFMEMS Oscillator vs. Crystal Oscillator: Key Differences
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Both MEMS (Micro-Electro-Mechanical Systems) oscillators and crystal oscillators are electronic oscillators used to generate precise and stable clock signals in various electronic devices. However, they differ significantly in their structures, working principles, and characteristics.
What is a MEMS Oscillator?
A MEMS oscillator is a type of electronic oscillator that utilizes MEMS technology to generate stable and precise clock signals. It finds applications in a wide range of devices, providing accurate timing references.
Figure 1: SI-based MEMS oscillator circuit and its structure
As shown in Figure 1, a MEMS oscillator circuit uses a silicon resonator as the source of oscillation. It often incorporates a Phase-Locked Loop (PLL) circuit to correct frequency mismatches that can arise due to manufacturing tolerances and temperature drift.
The fundamental working principle of a MEMS oscillator involves the mechanical vibrations of tiny microstructures integrated onto a semiconductor substrate. These microstructures, which can be in the form of beams, resonators, or cantilevers, are fabricated using microfabrication techniques similar to those used in IC manufacturing. When electrical signals are applied to these microstructures, they vibrate at a specific resonant frequency, generating an oscillating signal.
Some prominent manufacturers of MEMS oscillators include SiTime Corporation, Microchip Technology, Abracon, IQD Frequency Products, Vectron International, Rakon, and TXC Corporation.
What is a Quartz Crystal Oscillator?
A quartz crystal oscillator leverages the piezoelectric properties of a quartz crystal to generate stable and precise clock signals. It is widely used in various electronic devices, such as communication systems, computers, and space systems, to provide accurate timing references.
Figure 2: Crystal oscillator circuit and its structure
Figure 2 illustrates a typical crystal oscillator circuit and its structure. As shown, it utilizes a simple oscillator circuit with a quartz crystal operating in its fundamental mode.
At the heart of a quartz oscillator lies the quartz crystal itself, which operates based on the piezoelectric effect. When a voltage is applied across the crystal, it mechanically deforms and generates vibrations. It vibrates at a specific resonant frequency determined by its dimensions and properties. An electrical signal is applied through metal electrodes attached to the crystal, sustaining the oscillation.
Major manufacturers of quartz crystal oscillators include Epson Toyocom, TXC Corporation, Vectron International, Murata Manufacturing Co. Ltd, NDK, Bliley Technologies, CTS Corporation, Citizen Watch Co. Ltd., and Abracon.
Difference Between MEMS Oscillator and Crystal Oscillator
The key differences between MEMS and quartz crystal oscillators lie in their characteristics, structures, and working principles. The following table summarizes these differences:
| Specifications | Quartz Crystal Oscillator | MEMS Oscillator |
|---|---|---|
| Structure | Simple | Complex (resonator, PLL, temperature compensation, calibration) |
| Size and integration | Larger size, may not be as integrable | Small size, can be integrated onto chips |
| Aging and drift | Very low aging and drift, suitable for precise applications | Some drift due to mechanical aging, temperature variations |
| Cost | Higher manufacturing costs due to crystal precision | Cost-effective manufacturing |
| Frequency determining element | Very high Q quartz crystal resonator | PLL (Phase Locked Loop) |
| Frequency flexibility | Fixed frequency based on crystal’s resonance properties | Flexible frequency designs. |
| Working principle | Utilizes piezoelectric effect in quartz crystals | Utilizes micro-structured mechanical structures |
| Phase Jitter (ps) / Phase Noise | Lower | Higher |
| Power Consumption (mA) | Lower | Higher |
| Oscillator start up time | Low | High |
| Frequency accuracy and stability | Excellent accuracy and long term stability, Exceptionally stable over time, No frequency jumps (i.e. less variation) | Good accuracy and stability; may not match crystal, Frequency jumps observed (i.e. more variation) |
| Efficiency and reliability | More | Less |
Conclusion
In conclusion, both MEMS and quartz crystal oscillators serve specific purposes in various applications.
MEMS oscillators find use in high-volume, low-cost clock applications where stringent phase noise requirements are not paramount.
Quartz oscillators are preferred in space applications, radar systems, noise-sensitive circuits, and other RF and microwave applications where high precision and stability are crucial.
While MEMS oscillators offer advantages in terms of size and integration, they may not provide the same level of frequency accuracy and long-term stability as quartz crystal oscillators.
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