| Crystal Unit Definitions |
| Crystal Unit |
| A case housing a thin piece of quartz crystal (silicon dioxide) or crystal strip with vacuum-evaporated metal electrode and terminals for connections. It is widely used as passive electronic component for mobile phones, wireless devices, telecommunication devices, personal computers and other digital equipments. |
| Frequency |
| The number of cycles of output waveform occurring per second. The unit of frequency is cycles per second, or Hertz, abbreviated Hz. |
| Fundamental Mode |
| The main mode of the crystal. It is also called first overtone. |
| Overtone Mode |
| Odd numbers assigned for frequencies in terms of specified oscillation mode. Standard third overtone mode, followed by fifth, seventh, ninth, etc. The frequencies are not exactly three, five, seven, or nine times the undamental frequency. |
| Frequency Tolerance |
| This refers to the allowable deviation from the nominal frequency in parts per millions (ppm), at room temperature,usually +25º C. |
| Frequency Stability |
| This refers to the maximum allowable frequency deviation compared to the measured frequency at 25º C over specified temperature range, e g, -10ºC ~ + 70ºC. |
| Equivalent Series Resistance |
| The value of impedance the crystal exhibits in the operating resonant circuit. |
| Shunt Capacitance |
| Shunt capacitance (C0) is the capacitance between the crystal terminals. It varies with package, usually it is smaller in SMD (4pF typical) and is 6pF in leaded crystals. |
| Load Capacitance |
This refers to external capacitance to the crystal, and the amount of capacitance measured or computed across the crystal terminal on the PCB. Load capacitance need to be specified when
the crystal is used in a parallel mode. If the application requires a “series” resonant frequency crystal, load capacitance is not a factor and
do not need to be specified. Load capacitance is calculated as follows: |
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| Insulation Resistance |
| Resistance between crystal's leads, or between lead and case (metal case). It is tested with a DC voltage at 1 OOV ± 15V and insulation resistance is 500 MΩ(min.). |
| Drive Level |
| The amount of power dissipation experienced by the crystal in the oscillation circuit. The power is a function of the applied current and usually expressed in Milliwatts or Microwatts. Excessive drive level will result in possible long-term frequency drift and unstable operation increased aging rates or crystal fracture. The drive level may be calculated by the following equation: |
| Power = (I rms2 * RL) |
Where I is the rms current through the crystal unit and R is the maximum resistance value of the specific crystal unit in question. This
equation is simply “ Ohms Law” for power. Measurement of the actual drive level in an operating oscillation circuit maybe accomplished by
temporarily inserting a resistor in series with the crystal unit. The resistor must be of the ohmic value as the unit. The voltage drop across
the resistor may then be read and the current and power dissipation calculated. The resistor must then be removed. As an alternative way of
measuring the drive level, a current probe may be used at the output lead of the crystal unit, space permits. The method is described as below
Fig7. |
|
Where
RL = loaded resonance resistance
R1 = resonance resistance of crystal unit
Iq = current flowing to crystal unit
C0 = shunt capacitance
CL = load capacitance |
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| Figure 7.Drive level measurement |
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| Aging |
| This refers to the cumulative change in frequency over a certain period of time. This rate of change of frequency is fastest during the first 45 days of operation. Many Interrelated factors are involved in aging, some of the most common factors are: |
(1) Excess drive level,
(2) Internal contamination,
(3) Crystal surface change,
(4) Wire fatigue
(5) Various thermal effects
(6) Frictional wear, etc… |
| All these problems can be minimized by proper circuit design incorporation low operating temperature, min. drive levels and static pre-aging. |
| Spurious |
| It is also possible for a crystal to vibrate at a frequency that is not related to its fundamental or overtone frequency. Such unwanted frequencies are referred to as spurious. Spurious are usually above the operating mode, specified in dB max. or number of times of ESR. Frequency range must be specified. |
| Operating Temperature Range |
| Temperature range within which crystal units operate under specified conditions. |
| Mode of Vibration |
| It is a piezoelectric effect of quartz crystal. The mode of vibration of quartz crystal varies with crystal cuts such as Thickness-shear for AT cut and BT cut, or Length-width flexure for tuning fork crystals (+2º) X cut. The most popular cut is the AT-cut, which offers a symmetrical frequency shift over a wide temperature change. |
| Change of Load Capacitance and Pullability |
| The pullability of a crystal refers to a crystal operating in the parallel mode and is a measure of the frequency change as a function of load capacitance. Pullability is important to the circuit designer who wishes to achieve several operating frequencies with a single crystal by means of changing in values of load capacitance. |
| When a crystal is operating at parallel resonance (Fs
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| The same crystal with frequency at third-overtone mode will have much less pulling because its motional capacitance Cl' is approximately 1/9 of Cl at fundamental. |
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| Frequency pullability of a fundamental vs. Its 3rd overtone crystal. The oscillating mass of the quartz crystal corresponds to the motional inductance Ll while the elasticity of the oscillating body is represented by the motional capacitance Cl. |
