Crystal Loading Capacitance Calculator
Calculate the load capacitance seen by a crystal and the frequency-pulling effect of off-spec loading. Verifies C1/C2 + stray match the crystal's datasheet CL.
How to Use
- Enter C1, C2 (the two external load caps) and Cs (stray — typically 3-7 pF per pin).
- Enter the crystal's spec CL (from datasheet) and pulling sensitivity (typically 10-30 ppm/pF).
- Tool computes actual load capacitance and frequency offset in ppm.
Show Work
Formulas
History of Crystal Loading
The practice of specifying crystal CL (load capacitance) dates to RCA's 1940s standards for AT-cut quartz oscillators. Setting CL correctly ensures the crystal oscillates at its nominal series-resonance frequency - deviations of 1-2 pF shift the frequency by 10-30 ppm, enough to fail cellular baseband carrier accuracy specs. Modern MCU application notes (STMicro AN2867, Microchip AN826) devote chapters to load-capacitance calculation and PCB layout rules for HSE/LSE crystal pins.
About This Calculator
Enter the two load caps C1 and C2, stray capacitance Cs (typical 3-7 pF), the crystal\'s datasheet CL, and pulling sensitivity. The tool computes actual loading CL_actual = (C1·C2)/(C1+C2) + Cs, mismatch from spec, and the resulting frequency offset in ppm.
For tight accuracy, add a trimmer cap in series with C1 (or C2). Tune while monitoring frequency with a counter — 0.1 pF resolution trimmers give sub-ppm adjustment. For MCU crystals with ±20 ppm spec tolerance, CL match within ±1 pF is usually sufficient. Everything runs client-side.
Frequently Asked Questions
What is CL?
Load Capacitance — the capacitance the crystal is specified to oscillate at its nominal frequency. Typical values: 12.5 pF for watch crystals, 18-20 pF for MHz crystals, 32 pF for some legacy parts.
Frequency pulling?
If actual CL differs from spec, the crystal oscillates off-frequency by a predictable amount: Δf/f ≈ C_m / (2·(C₀ + CL)²) · ΔCL. Datasheets quote 10-30 ppm/pF typical sensitivity.
Trimming?
Add a trimmer cap (3-30 pF variable) in series with C1 or C2 to adjust oscillation frequency to spec. Measure with a frequency counter while trimming — 0.1 pF resolution is achievable.
Common Use Cases
MCU Clock Trim
Adjust trimmer for crystal to hit exact ppm accuracy for UART Baud rate matching.
RTC Drift Compensation
Characterize CL vs frequency offset to enable digital compensation.
RF Reference
Fine-tune crystal reference oscillator to exactly 10.000 MHz for GPS-disciplined references.
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