Crystal Load Capacitance Calculator
Calculate actual load capacitance seen by a crystal oscillator, estimate frequency error from spec, and find recommended external capacitor values.
Formula
Reference: IEC 60444 / Crystal manufacturer application notes
How It Works
Crystal load capacitance is a critical parameter in oscillator circuit design that determines the precise frequency of crystal oscillators. The actual load capacitance (CL_actual) represents the total capacitive load seen by an oscillator crystal, which includes both parallel capacitors and any stray capacitance. Accurate capacitance matching is essential to maintain the crystal's specified operating frequency and minimize frequency drift.
Worked Example
Problem: Design an oscillator with a 32.768 kHz crystal, specified load capacitance of 12.5 pF, and 2 pF of stray capacitance. Calculate the actual load capacitance and potential frequency error. Solution: 1. Set C1 = C2 = 2 * (CL_spec - Cstray) = 2 * (12.5 - 2) = 21 pF 2. Calculate CL_actual = (C1 * C2) / (C1 + C2) + Cstray = (21 * 21) / (21 + 21) + 2 = 11.5 pF 3. Calculate frequency error = (11.5 - 12.5) / (2 * 12.5) * 1,000,000 = -40 ppm
Practical Tips
- ✓Use capacitors with tight tolerance (±5% or better)
- ✓Minimize PCB trace lengths near crystal
- ✓Consider temperature coefficients of capacitors
- ✓Use symmetrical layout for parallel capacitors
Common Mistakes
- ✗Ignoring stray capacitance in load calculations
- ✗Unbalanced parallel capacitor selection
- ✗Failing to match specified load capacitance
- ✗Not considering PCB layout parasitic capacitance
Frequently Asked Questions
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