|The amount of power
dissipated by the oscillating crystal unit. Usually expressed in terms of
milliwatts (mW), and is usually specified in terms of current through the
resonator or power dissipated by the resonator. The latter is preferable.
The drive level of a crystal is a function of the reactance of the input
and output capacitance of the inverter or microprocessor and all other external
components including the crystal. To calculate drive level, "ohm's law"
for power is used.
Drive level should be held to a minimum to avoid problems with stability,
aging, nonlinear coupled modes and other nonlinear effects. However, the
phase noise performance of an oscillator can be increased by increased drive
level, so a compromise is sometimes in order. The maximum power dissipation
is specified for each type of crystal in the oscilent specifications. Drive
level is most critical in tuning fork crystals because of the comparably
lower maximum drive levels. Circuits utilizing tuning fork crystals should
be designed in such a way that avoids over driving the crystal.