The following terms are frequently used in connection with Quartz Crystal technology
1. Nominal Frequency
The actual frequency specified by the customer or end user at the time of ordering.
2. Frequency Tolerance
The maximum permitted variation of the Working Frequency from the Nominal Frequency due to a number of causes.
The three primary causes are:
Adjustment Tolerance or Calibration Tolerance The Calibration Tolerance is the allowable variation in frequency at the calibration temperature that must be taken into account due to inaccuracy in manufacture and acceptable differences between test sets used in manufacturing and testing.
For example ±10 ppm @ 25°C or ±.001% @ 25°C
In well-designed equipment it should be possible for this Adjustment Tolerance to be "trimmed out" with the aid of variable capacitors or inductors included in the oscillator circuit.
Temperature Tolerance or Temperature Stability The maximum allowable variation of the Working Frequency from its value at Nominal Temperature over a specified Temperature Range with reference to the measured frequency at the calibration temperature. The variation is usually quoted as a ± figure expressed either as "parts per million" (ppm) or as a percentage of the Nominal Frequency over the Operating Temperature Range.
For example ±10 ppm @ -10 to +60°C or .001% -10 to +60°C
Ageing A change in frequency and other characteristics following the manufacture of a crystal occurring with the passage of time. This change is very minimal with modern well-manufactured units.
3. Operating Temperature Range
The range of temperature as measured on the holder, over which the crystal unit will function within the specified tolerance.
4. Operable Temperature Range
The range of temperature as measured on the holder, over which the crystal unit will continue to function though not necessarily within the specified tolerance.
5. Calibration Temperature
The actual temperature at which a crystal unit is to be calibrated. This is usually 25° C unless specified otherwise by the customer i.e. for oven use
6. Mode of operation
The mode of operation will be fundamental, third, fifth, seventh or ninth overtone and either series or parallel resonant
Oscillation without an external capacitance, at the lower of the two frequencies at which the impedance of the crystal is real. Series resonance is indicated by minimum impedance and zero phase shift across the crystal terminals.
Parallel Resonance or Antiresonance
Oscillation with a capacitance being in series with the crystal at the lower of the two frequencies at which the impedance of the combination is real. Parallel resonance is indicated by the maximum impedance across the crystal terminals.
7. Crystal Unit Equivalent Circuit
The electric circuit which has the same impedance as the crystal in the frequency region of resonance. It is represented by an inductance, capacitance and resistance in series, this being shunted by the capacitance between the terminals of the crystal. The parameters of the series branch of inductance, capacitance and resistance are frequently represented by the symbols Lm, Cm and Rm respectively. The parallel capacitance is usually represented by the symbol Co. See Diagram here.