Using crystals in embedded applications - Part 4

Ceramic resonator (vs) Quartz Crystal (vs) RC Oscillator (vs) Silicon Oscillator:

• Ceramic resonator and quartz crystal are mechanical resonant devices where as oscillator works on electrical phase shift principle (where R-C are the phase shift circuit).
• Power consumption of Ceramic is more than quartz crystal. 
• Ceramic resonators have good rise time than quartz crystals.
• Ceramic resonator and quartz crystal operate on the same principle as the electrical signal applied to them causes mechanical vibrations. 
• Ceramic resonators are more rugged than crystal.
• Quartz crystals are very stable and maintain their frequency under extreme conditions. Quartz crystal may not drift from it's frequency even when the PCB stray capacitance is high. Also, at quick varying temperatures, the quartz crystal remains stable.
• Ceramic resonators are made of ceramic material which are not as stable as quartz. These are piezoelectric in nature.
• Comparatively, Quartz costs more than ceramic resonators. 
• Ceramic requires much higher load capacitance required to quartz crystal.
• An oscillator is addition of feedback to the crystal along with amplification such that oscillations happen. Take the case of micro-controller, on the external clock pins we add a crystal and combined with internal circuitry it forms a oscillator.
• Oscillator is immune to EMI and humid conditions as they come packaged with all components for frequency generation.
• Temperature co-efficient which is a important material property is low for crystals and resonators.
• Ceramic resonators are available in miniature packages also.
• Ceramic resonator can sometimes be of compound material which helps change the characteristics of the crystal as per the material.
• The main disadvantages of oscillators is package size and cost and sensitive to vibration,  

Note: Some micro-controllers have internal capacitors, in this case, there is no need to have external capacitors.

Considering the disadvantages of ceramic resonator compared to Quartz, where do they find application?

Check the age old radio circuits you find resonators. Also, they are used in cost critical application and applications where stability is not important criteria.

What is the main selection criteria for any clock input?

Accuracy is the important criteria. Depending on the application for which the circuit is used, clock must be chosen properly. For example, applications like USB, SATA, PCIe requires a very stable clock with less deviation from the desired frequency.

Note: There are R-C oscillators internal to some ics which have very less stability but is a very cheap implementation. Micro-controller internal RC oscillator is an example.