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Parallel Resonance Frequency (Fp)
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Series Resonance Frequency (Fs)
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Required Capacitor C1
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Required Capacitor C2
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Load capacitance (CL) is the total capacitance seen by the crystal in the oscillator circuit. It determines the operating frequency and must match the crystal's specified load capacitance for optimal performance and frequency stability.
The external capacitors are calculated using the formula: C1 = C2 = 2 × (CL - Cs), where CL is the load capacitance and Cs is the stray capacitance. Both capacitors are typically equal in value for symmetric circuits.
Incorrect load capacitance can cause frequency drift, poor frequency stability, difficulty starting oscillation, or complete failure to oscillate. The crystal will not operate at its specified frequency, affecting circuit timing accuracy.
Series resonance frequency (Fs) is the crystal's natural frequency without external capacitance. Parallel resonance frequency (Fp) is slightly higher and occurs when the crystal operates with its specified load capacitance in the circuit.
Stray capacitance from PCB traces, component leads, and IC pins adds to the total load capacitance. It must be accounted for when calculating external capacitor values, typically ranging from 2-5 pF in most circuits.
Common load capacitance values are 12.5 pF, 18 pF, and 20 pF for most crystals. Watch crystals (32.768 kHz) typically use 12.5 pF, while higher frequency crystals may use 18-20 pF loads.
Frequency stability ensures accurate timing in digital systems, proper communication protocols, and reliable clock generation. Poor stability can cause data errors, timing violations, and system malfunctions in sensitive applications.