RF and Antennas
Model class: Exact ideal relationship
Crystal load-capacitor selector
Calculate symmetric external load-capacitor values from a crystal's specified load capacitance and board stray capacitance.
Interactive engine
Start with the stated conditions.
Values stay in this browser. Choose a representative scenario, then calculate deliberately.
Example ready
Calculate to inspect the result.
The result will identify the direct answer, assumptions, and any warning that changes the next decision.
How much does each input matter?
Each row holds every other input fixed at its current value and moves one input by ±10%, using the same model as the main result. It identifies which input is the binding lever, not a full tolerance stack.
Assumptions to check
- The entered crystal-load-capacitance values represent the stated operating condition.
- This exact ideal relationship is evaluated in the declared lumped or first-pass model.
- A crystal-load-capacitance calculation is not a component qualification or safety approval.
What this crystal-load-capacitance calculation establishes
Calculate symmetric external load-capacitor values from a crystal's specified load capacitance and board stray capacitance. The useful result is the stated electrical quantity and the decision it supports, not an unstated claim about a finished product. This engine keeps the governing relationship visible so an input, unit, condition, or model boundary can be reviewed before a value becomes a component or layout choice.
The two load capacitors in parallel combination must equal the specified load capacitance minus stray capacitance; the symmetric solution doubles that combined target. Treat the number as a first-pass result for the declared operating point. When a source, load, temperature, frequency, waveform, component tolerance, or measurement condition changes, repeat the calculation at the relevant corner rather than assuming the nominal answer persists.
Worked decision context
CL = 12 pF and Cstray = 2 pF require equal C1 = C2 = 20 pF ideally. That example verifies the equation and illustrates the scale of the result, but it does not select a part by itself. Compare the result with available values, ratings, tolerance bands, and the receiving circuit or physical environment before implementation.
Use the primary output to identify the binding constraint. If it leaves little margin, document which input dominates and use selected-part data, a higher-fidelity model, simulation, or measurement. This is especially important when a small numerical difference changes a thermal, timing, noise, or reliability decision.
Limits and validation handoff
This gives the symmetric solution only. An asymmetric board layout with unequal stray capacitance on each pin needs the two-capacitor equations solved separately per pin. The calculation does not silently include omitted parasitics, installation conditions, manufacturing variation, or product policy. Those conditions can be decisive even when the arithmetic is exact for the selected model.
Record inputs, units, model assumptions, and the intended decision with the result. Verify the leading risk against the selected component data sheet and a representative measurement when the circuit has consequential energy, high voltage, safety, compliance, or reliability requirements.
Common mistakes
- Treating the crystal-load-capacitance result as a guaranteed operating limit rather than a first-pass exact ideal relationship estimate.
- Mixing a data-sheet value measured under one condition with this crystal-load-capacitance calculation performed at another.
- Selecting a component before checking the boundary this calculation names: this gives the symmetric solution only. An asymmetric board layout with unequal stray capacitance on each pin needs the two-capacitor equations solved separately per pin.
Model limit and handoff
Keep the entered crystal-load-capacitance conditions with the calculation, then validate the binding limit using the selected component, physical implementation, and representative operating corner.
FAQs
Is this crystal-load-capacitance result sufficient to approve a design?
No. It applies exact ideal relationship reasoning to the entered crystal-load-capacitance values and names the checks that still need selected-part data, a higher-fidelity model, or measurement. This gives the symmetric solution only. An asymmetric board layout with unequal stray capacitance on each pin needs the two-capacitor equations solved separately per pin.
What does this crystal-load-capacitance calculator assume that could make the result wrong?
The two load capacitors in parallel combination must equal the specified load capacitance minus stray capacitance; the symmetric solution doubles that combined target. If the entered values do not match the real operating condition, the result no longer describes the actual circuit.
Where should this crystal-load-capacitance result go next?
Compare this crystal-load-capacitance result with Oscillator and clock accuracy budgets, then use the stated next decision below the calculator to move from this first-pass number toward an implementation.