Glossary of Crystal Terms
Nominal Frequency
Nominal frequency is a desired center frequency of the crystal which is specified in KHz or MHz.
Operation Mode
Crystal unit could operate at Fundamental mode or Harmonic modes. The fundamental mode is always the preferred oscillating mode. Harmonic modes are overtone modes (e.g., 3rd, 5th, 7th overtone).
Frequency Tolerance
Frequency tolerance is the maximum allowable frequency deviation from a specified nominal frequency at ambient room temperature (25°C ± 3°C). It is expressed in parts per millions (ppm).
Frequency Stablility
Frequency stability is the maximum allowable frequency deviation of crystal unit over a specified operating temperature range. It is expressed in parts per millions (ppm).
Operating Temperature Range
Within the specified minimum and maximum temperature range which crystal unit will work normally.
Storage Temperature Range
With the specified minimum and maximum temperatures that the crystal unit can be stored without damage.
Equivalent Series Resistance(ESR)
The value of equivalent resistance of the crystal exhibits in the operating resonant circuit.
Load Capacitance(CL)
Load capacitance 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, do not need to specify load capacitance.
Shunt Capacitance(C0)
Shunt capacitance or Static capacitance is the value of capacitance including the electrodes, the holder and the crystal terminals.
Motional Capacitance(C1)
Motional capacitance is the capacitance of the nominal arm of the equivalent circuit.
Drive Level(DL)
Drive level is the amount of power dissipated in the crystal and is expressed in milliwatts(mW) or microwatts( μW). Excessive drive level will result in possible long-term frequency drift and unstable operation increased aging rates or crystal fracture.
Aging
Aging is the relative frequency change over a certain period of time and is typically expressed as a maximum value in parts per million per year (ppm/year). Typically, aging is computed within the first 30 days to 90 days and predicted exponentially over a longer period usually a year.
Insulation Resistance
Resistance between crystal’s leads, or between lead and case (metal case). It is tested with a DC voltage at 1OOV ± 15V and insulation resistance required minimum of 500 MΩ.
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.
Suprious Responses
It is possible for a crystal to vibrate at a unwanted resonance frequencies. These unwanted frequencies are referred to as spurious and they are usually above the operating mode, specified in dB max. or number of times of ESR value. The circuit R&D engineers should guard against spurious responses by ensuring that the oscillator feedback circuit achieves its highest gain at the desired operating frequency.