ABOUT CRYSTAL

 

QUARTZ CRYSTALS

 

Synthetic quartz is composed of silicon and oxygen (silicon dioxide) and is cultured in autoclaves under high pressure and temperature. Quartz exhibits piezoelectric properties which generate an electrical potential when pressure is applied on the surfaces of the crystal. Conversely, when an electrical potential is applied to the surfaces of a crystal, mechanical deformation or vibration is generated. These vibrations occur at a frequency determined by the crystal design and oscillator circuit. Under proper conditions, quartz can be used to stabilize the frequency of an oscillator circuit.

 

CENTER FREQUENCY

 

The specified reference frequency of the crystal and is typically specified in megahertz (Mhz) or kilohertz (Khz)

 

FREQUENCY TOLERANCE OR CALIBRATION ACCURACY

 

The amount of frequency deviation from a specified center frequency at ambient temperature (referenced at 25C).This parameter is specifed with a maximum and minmum frequency deviatio, experssed in percent(%) or parts per million(ppm). This deviation is associated with a set of operationg conditions including. Load, Capacitacne and Drive Level.

 

FREQUENCY STABILITY

 

The amount of frequency deviation from the ambient temperature frequency over the operating temperature range. This deviation is associated with a set of operating conditions including: Operating Temperature Range, Load Capacitance, and Drive Level. This parameter is specified with a maximum and minimum frequency deviation, expressed in percent (%) or parts per million (ppm). The frequency stability is determined by the following primary factors: Type of quartz cut, and angle of the quartz cut. Some of the secondary factors include: mode of operation, drive level, load capacitance, and mechanical design.

 

OPERATING TEMPERATURE RANGE

 

The maximum and minimum temperatures that the crystal device can be exposed to during oscillation. Over this temperature range, all of the specified device operating parameters are guaranteed.

 

SHUNT CAPACITANCE (C0)

 

The static capacitance between the crystal terminals. Measured in picofarads (pF), Shunt Capacitance is present whether the device is oscillating or not (unrelated to the piezoelectric effect of the quartz). Shunt Capacitance is derived from the dielectric of the quartz, the area of the crystal electrodes, and the capacitance presented by the crystal holder.

 

MODE OF OPERATION

 

The mode of operation of a quartz device is one of the factors that will determine the frequency of oscillation. For AT cut quartz crystals, overtone modes are at odd frequency harmonics. For example, a crystal may operate at its fundamental frequency of 10Mhz, or at odd harmonics of approximately 30Mhz(Third Overtone), 50Mhz(Fifth Overtone), and 70Mhz(Seventh Overtone).

 

DRIVE LEVEL 

 

A function of the driving or excitation current flowing through the crystal. The drive level is the amount of power dissipation in the crystal, expressed in microwatts or milliwatts. Maximum power is the most power the device can dissipate while still maintaining operation with all electrical parameters guaranteed. Drive level should be maintained at the minimum levels necessary to initiate proper start-up and assure steady state oscillation. Excessive drive level can cause poor aging characteristics and crystal damage. 

 

AGING

 

The systematic change in frequency with time due to internal changes in the crystal and/or oscillator. Aging is often expressed as a maximum value in parts per million per year (ppm/yr). The rate of aging is typically greatest during the first 30 to 60 days after which time the aging rate decreases. The following factors effect crystal aging: adsorption and desorption of contamination on the surfaces of the quartz, stress relief of the mounting and bonding structures, material outgassing, and seal integrity. 

 

STORAGE TEMPERATURE RANGE

 

The minimum and maximum temperatures that the device can be stored or exposed to when in a non-oscillation state. After exposing or storing the device at the minimum or maximum temperatures for a length of time, all of the operating specifications are guaranteed over the specified operating temperature range.

 

 

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