In spite of their popularity, there are some significant limitations to crystal resonators and crystal oscillators. The most basic limitation is that typically, a crystal resonator cannot resonate at higher than 75 MHz in fundamental mode. To overcome this limitation for high frequencies, there are three alternatives:
* Use overtone mode – such as 3rd overtone, 5th overtone, etc. In this case, the oscillator operates at a frequency that is the third or fifth harmonic of the fundamental frequency. The disadvantage of this mode is that the costs are higher, and startup is not guaranteed over the entire temperature range.
* Use Phase Locked Loops to multiply the frequency – this is a very popular mode for MEMS-timing companies, but is not very popular with the crystal-based companies as they do not have analog circuits expertise. The benefits of this mode are that the intelligence is now in the electronics, leading to more features and higher performance at lower cost.
* Use Surface Acoustic Wave (SAW) technology – in which the crystal device operates differently than a standard AT-cut crystal.
Below is a comparison of the key parameters between SAW oscillators and MEMS Oscillators
* Frequency Range- Each device has to be cut to operate at a unique frequency
* Frequency Stability - Generally, 50 PPM, occasionally 25 PPM
* Package & Size – Large, 5032 and 7050 packages only
* Power Consumption- Typically more than 40mA
*1.8V Operation - Limited availability
* Aging - ± 3 PPM per year
* Shock & Vibration sensitivity – Sensitive, can break easily. Typical shock resistance is 2,000 to 5,000G
* Available Add-on features – Varies with part and frequency. Very few standard options available.
* Frequency Range- Programmable up to 800 MHz
* Frequency Stability - Can support as good as 0.5 PPM over industrial temperature range
* Package & Size - Small 2520 and 3225 in addition to 5032 and 7050 packages
* Power Consumption- Typically more than 20mA
* 1.8V Operation – Easily available
* Aging -Typically less than ± 1 PPM per year
* Shock & Vibration sensitivity – Extremely robust – 50,000G shock
* Available Add-on features - VCXO, SSXO (spread-spectrum) available in all frequencies and voltages, programmable drive
As can be seen from the above comparison, MEMS oscillators can offer superior characteristics compared to SAW crystal oscillators for high frequency applications.