Load capacitance is the amount of external circuit capacitance in parallel with the crystal itself.
In this example, we see that the crystals parallel-resonance mode is always above the series resonance frequency and is characterized by inductive reactance. In parallel-resonance oscillation mode, the crystal’s inductance (motional inductance) is in parallel with the oscillator’s load capacitance, thereby forming an LC tank circuit. This LC determines the oscillator frequency.
When specifying a series-resonant crystal, you can ignore load capacitance. This is true because the crystal’s motional inductance and motional capacitance are the only LC components that determine oscillation frequency.
One of the detractors in a Quartz Crystals performance is anti-resonance. Our example shows that anti-resonance can occur when the net inductive component of the crystal model resonates with the crystal’s internal holder capacitance. Being aware of this anomaly can save you lots of headaches.
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Load Capacitance was last modified: June 30th, 2022 by