Engineering Reference
Crystal, Stability and PPM Parameters Reference
Lookup for crystal load capacitance, ESR, drive level, equivalent-circuit terms, frequency tolerance, temperature stability, aging, ppm error, drift, and error budgets.
- Reading Time
- 12 min
- Format
- Crystal parameter lookup
- Updated
- September 29, 2026
Crystal Datasheet Parameters
| Parameter | Symbol | Unit | Meaning | Boundary |
|---|---|---|---|---|
| Nominal frequency | f0 | Hz | Specified reference frequency under stated load and conditions | Not guaranteed at every temperature, load or drive level |
| Load capacitance | CL | pF | External parallel-resonance load condition associated with the marked frequency | Includes the effective series combination of load capacitors plus stray capacitance |
| Motional capacitance | Cm | fF | Equivalent-series branch capacitance | Not the external load capacitance |
| Motional inductance | Lm | H | Equivalent-series branch inductance | Derived model parameter, not a discrete inductor |
| Motional resistance / ESR | Rm | Ω | Loss in the motional branch near resonance | Oscillator negative resistance needs adequate margin |
| Shunt capacitance | C0 | pF | Holder and electrode capacitance parallel to the motional branch | Influences antiresonance and pullability |
| Drive level | DL | µW or mW | Power dissipated in the crystal under specified operation | Excess drive can shift frequency or damage the resonator |
| Frequency tolerance | — | ppm | Initial frequency offset at stated reference conditions | Separate from temperature stability and aging |
| Temperature stability | — | ppm | Frequency variation over a stated temperature range | Requires range and reference definition |
| Aging | — | ppm/year | Long-term change under stated conditions | Not necessarily linear or identical each year |
| Pullability | — | ppm or ppm/pF | Frequency change obtainable by changing load capacitance | Depends on crystal equivalent parameters and circuit parasitics |
| Activity / spurious modes | — | level or ratio | Unwanted resonant responses | Oscillator design must avoid mode selection problems |
Pierce Load-Capacitance Model
| Quantity | Relationship | Boundary |
|---|---|---|
| Series combination | Cseries = C1C2 / (C1 + C2) | Only the external capacitor combination |
| Effective load | CL ≈ Cseries + Cstray | Stray includes pins, package, board and input capacitance |
| Symmetric capacitors | C1 = C2 = 2(CL - Cstray) | Requires CL > Cstray |
| Frequency error | ppm = (fmeasured - fnominal) / fnominal × 10⁶ | Signed convention must be preserved |
Stability Error Terms
| Contributor | Meaning | Combination boundary |
|---|---|---|
| Initial tolerance | Manufacturing offset at reference conditions | Usually worst-case bounded |
| Temperature stability | Frequency excursion across temperature | Depends on range, cut and compensation |
| Aging | Long-term resonator and package change | State interval and direction assumptions |
| Load error | Difference between specified and actual effective load capacitance | Includes board and pin parasitics |
| Supply sensitivity | Frequency pulling from supply/bias change | Circuit-specific |
| Calibration residual | Error remaining after trim | Measurement and trim resolution matter |
| Measurement uncertainty | Reference and counter uncertainty | Do not hide it inside device tolerance |
| Short-term noise | Phase noise, jitter or Allan-deviation behavior | Not represented by a static ppm bound |
PPM Relationships
| Need | Relationship | Boundary |
|---|---|---|
| PPM to percent | % = ppm / 10,000 | 10 ppm = 0.001% |
| Frequency error | Δf = fnom × ppm / 10⁶ | Signed or magnitude convention must be stated |
| Measured error | ppm = (fmeas - fnom) / fnom × 10⁶ | Reference accuracy affects result |
| Time drift | Δt ≈ elapsed time × ppm / 10⁶ | Constant fractional-error approximation |
| Worst case | sum of absolute bounded contributions | Conservative deterministic bound |
| RSS | √Σ(ppmᵢ²) | Statistical estimate only with justified independence |
Canonical Calculation Anchors
| Case | Result | Interpretation |
|---|---|---|
| C1 = C2 = 18 pF, stray = 3 pF | 12.000 pF effective load | 9 pF series plus 3 pF stray |
| Target CL 12 pF, stray 3 pF | 18.000 pF each | Symmetric first-pass capacitor choice |
| 20 ppm at 16 MHz | 320 Hz | 20 ppm measured error |
| 10 ppm for one day | 864 ms | Constant fractional-error approximation |
| 10, 15 and 5 ppm worst case | 30 ppm | Guaranteed-style magnitude sum |
| 10, 15 and 5 ppm RSS | 18.708 ppm | Not a guaranteed bound |
| 100 ppm | 0.01% | One part in ten thousand |
Common Errors
- Confusing external load capacitance with motional capacitance.
- Omitting pin and PCB stray capacitance.
- Assuming marked frequency is exact at every load.
- Treating series and parallel resonance as identical.
- Ignoring crystal ESR and startup margin.
- Exceeding the specified drive level.
- Adding statistical errors as guaranteed worst-case limits.
- Using RSS without independence assumptions.
- Treating aging as perfectly linear.
- Confusing ppm offset with phase noise or jitter.
- Using a low-accuracy counter to validate a tighter oscillator.
- Assuming resonance alone guarantees loop startup.
Support reference
FAQ
What does crystal load capacitance mean?
It is the effective external capacitance condition under which a parallel-resonant crystal is specified to operate at its marked frequency.
How do two Pierce load capacitors combine?
A first-order estimate is CL = C1C2/(C1 + C2) + Cstray. Pin, package and PCB parasitics must be included in the stray term.
Is crystal load capacitance the same as motional capacitance?
No. Load capacitance is an external circuit condition; motional capacitance is part of the crystal equivalent circuit.
What does ppm mean for frequency?
One ppm is one part per million, or 0.0001 percent. Frequency error in hertz equals nominal frequency multiplied by ppm divided by one million.
How many hertz is 20 ppm at 16 MHz?
The magnitude is 320 Hz, so a symmetric 20 ppm range is 15,999,680 Hz to 16,000,320 Hz.
Can ppm errors always be added?
Worst-case bounded contributors may be summed by magnitude. RSS is a statistical estimate that requires independent, suitable distributions and is not a guaranteed limit.
What is crystal ESR?
It is the effective motional resistance near resonance. The oscillator must supply enough negative-resistance margin to overcome crystal and circuit loss.
Why does crystal drive level matter?
Excess motional power can cause nonlinear frequency shift, aging acceleration or damage. Verify the crystal's maximum drive and circuit measurement method.
Does resonance guarantee oscillator startup?
No. The active loop must provide correct phase and sufficient gain or negative resistance across startup conditions.
Is static ppm accuracy the same as jitter?
No. Ppm is a frequency offset or range measure. Jitter and phase noise describe short-term timing fluctuations.
Can a Pierce calculator predict exact frequency pulling?
Not without the crystal's full equivalent parameters and accurate parasitics. A simple load-capacitance model is a first-pass design check.
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