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The critical parameters of a crystal resonator are the frequency, maximum resistance, load capacitance, tolerance, stability, mode of operation, and package. Since the frequency of the crystal will vary depending on the capacitive reactance in series with the crystal, the designer must specify the load capacitance value to calibrate to the crystal frequency. Typical values are between 9 to 32 pF; 18 and 20 pF being the most common. If a parallel mode crystal is selected it is calibrated to the desired frequency at one of the inductive regions of the crystal’s reactance curve. Since it is a region, the designer must identify exactly where in the region the crystal is to operate. The exact point in the region is controlled by the value of the load capacitance. However, if a series mode crystal is chosen then no Load Capacitance needs to be specified since it is a point of operation and not a region. The selected operating frequency of the crystal will determine whether the resonator will need to operate in the fundamental or overtone mode. A fundamental crystal is designed and calibrated to the desired frequency on the lowest major resonant response. Typically, for frequencies operating over 40 MHz an overtone crystal must be selected.
