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11. What will be the effect of spurious frequencies?

When signal level of spurious mode gets as strong as the main mode, the oscillator may run on the spurious mode instead of the main mode. Such a phenomenon is called mode hopping. Spurious mode is usually defined as either a resistance ratio or dB suppression to the main mode. A resistance ratio of 1.5 or 2.0 to that of the main mode is needed to avoid mode hopping for most oscillators. This would be approximately equivalent to a -3dB to -6dB signal suppression over the main mode.

12. What happens if I operate a crystal over its maximum drive level spec?

An over-drive crystal may cause its frequency and resistance to change, in many cases, to a higher value. This would mean changes in crystal electrical characteristics. Sometimes activity dips could thus happen. It could also result in a broken crystal wafer due to too much power over-drive for too long an interval of time.

13. What is an activity dip and do I need to worry about them?

Activity dips are symptoms of discontinuity in frequency or resistance of a crystal over its operating temperature range. Sometimes it is also referred to as "non-linearity". Depending on the real circuit implementation, different circuit designs may tolerate different levels of crystal activity dips.

14. Why don’t HC-49S crystals pull as much as HC-49U crystals?

Pull-ability of a crystal usually has to do with the electrode size which forms on the crystal blank. A bigger size crystal blank of course can accommodate a larger electrode. HC-49S has a smaller dimension in blank than HC-49U. Larger electrode would typically provide a wider frequency pulling range when crystal is placed in series with a given load capacitance in the oscillation circuit.

15. What is trim sensitivity (T.S.)?

Trim sensitivity is the incremental frequency change of a crystal for an increment change in load capacitance. It is often expressed in ppm/pF. A typical mathematical approximation for trim sensitivity shows T.S. changes as CL varies: T.S.= C1 / [ 2 (Co+CL) ]

16. What are the motional and shunt capacitances of a crystal unit?

Motional capacitance (C1):
It is the capacitance residing in the motional (series) arm of the ideal crystal equivalent circuit model.

Shunt capacitance (C0):
It is the static capacitance between the crystal electrodes, together with the stray capacitance of the mounting system

17. What are the differences between AT-cuts and AT-strip cuts?

Please also refer to FAQ No. 6 for explanation on AT-Cut. AT-strip cut is usually referred to the rectangular crystal blanks which have the AT-Cut angle.

18. What is the difference between a "crystal" and a "strip resonator"?

A strip resonator is a crystal in which an AT-strip cut blank is used and mounted. A strip resonator is more sophisticated in its electrical characteristics than a crystal that utilizes a round blank. More skills and cautions are required in the design for a strip resonator to achieve the desired electrical characteristics

19. What is load capacitance?

Crystal by its function is to be placed and work in an oscillation circuit for generating a desired oscillation frequency. When a crystal sits in an oscillation circuit, it sees a "load capacitance" at the two terminal leads of the crystal. Such a load capacitance is the equivalent capacitive effect of the entire oscillation circuitry that appears at or presents to the crystal. Thus, the nominal spec frequency of a crystal is often defined as FL which stands for "load resonant frequency" at a given capacitance value. This capacitance value is to reflect the actual "load capacitance" presented to the crystal when it is placed and work in a real oscillation circuit. A crystal with zero (0) load capacitance number has its resonate frequency designated as Fr, series resonant frequency

20. What are piezoelectric characteristics of a quartz crystal unit?

Quartz is a device that carries the piezoelectric characteristics. The piezoelectric characteristics of a quartz crystal is briefly explained below: If a piezoelectric quartz crystal has electrodes plated on opposite faces and if a potential is applied between these electrodes, forces will be exerted on the bound charges within the crystal. If the crystal is properly mounted, deformations take place within the crystal, and an electromechanical system is formed which will vibrate at a resonant frequency when properly excited.



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