X-TAL FACTS
Quartz, chemically crystalline silicon dioxide, SiO2 is one of many materials that is piezoelectric. It can be modeled as an electromechanical transducer meaning that it will transform mechanical energy to electrical energy. By applying mechanical pressure along certain axes electrical charges will develop along other axes. It is called the direct piezoelectric effect and was discovered 1880 by the brothers Pierre and Jaques Curie. Contrary it will also convert electrical charges to mechanical displacement. It is called the indirect piezoelectric effect and was predicted mathematically by Gabriel Lippmann 1881.In circuit diagrams we assign a symbol (on top of the circuit diagram) to represent the crystal. The crystal can be modeled electrically as a two pole with an equivalent circuit (under the crystal symbol) in the near vicinity of a mechanical mode. The real equivalent circuit is much more complex because of other unwanted modes but not shown here.
Apart from where the crystal has a mechanical resonance the crystal appears as capacitor with the impedance, response we find two points where the phase passes through zero. The lowest is at the series resonance frequency, In the oscillator we can tune the excitation frequency by various means so that it the Q-value will be loaded hence lower. The close in Phase Noise in the oscillator is directly proportional to the loaded Q-value. Also the motional capacitance influence the Phase Noise. The lower the better. If low Phase Noise is the objective the use of overtone in oscillators is preferred. However when C1 diminishes the tuning range reduces correspondingly. Therefore fundamental use is best for large tuning applications.Another way to improve the Phase Noise is to use the SC cut. There are other advantages using SC cut crystals. Improved transient response for temperature changes. A smaller amplitude – frequency effect is apparent enabling higher crystal current leading to a better S/N ratio hence lower Phase Noise floor. The back side is higher production cost and necessity to add a trap in the oscillator for the b-mode which is about 10 % higher in frequency. Coupling to other modes are also less prevalent for SC cuts.
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