Almost every synchronous digital system — an MCU, an Ethernet PHY, a USB hub, a wireless SoC — needs a stable clock. In most designs that clock starts with a quartz crystal. Crystals look like commodity parts, but choosing the wrong one produces intermittent failures that are painful to debug months later. This guide covers the parameters that actually decide whether a crystal works in your circuit, and the mistakes we see most often.
The four numbers that define a crystal
Beyond the nominal frequency, four parameters determine behaviour.
- Frequency tolerance — the deviation from nominal at 25 °C at the time of manufacture, quoted in ppm. ±10 ppm is common for general use; ±30 ppm is acceptable for many MCU clocks.
- Frequency stability — how much the frequency drifts over the operating temperature range, also in ppm. A crystal can be ±10 ppm at room temperature and still move ±30 ppm across −40 to +85 °C.
- Load capacitance (CL) — the capacitance the crystal is designed to see across its terminals. This is the single most commonly mismatched parameter.
- ESR (equivalent series resistance) — sets how much gain the oscillator circuit must supply. Too high an ESR relative to the oscillator's negative resistance and the circuit simply will not start, particularly at low supply voltages.
Load capacitance: the classic failure
A crystal specified at CL = 12 pF is cut to run at its nominal frequency when it sees 12 pF. The two external capacitors C1 and C2, in series, plus the stray capacitance of the PCB traces and IC pins, form that load:
CL ≈ (C1 × C2) / (C1 + C2) + Cstray
With C1 = C2 = C and roughly 3–5 pF of stray capacitance, a 12 pF crystal needs about 15–18 pF capacitors, not 12 pF. Getting this wrong pulls the frequency by tens of ppm — enough to push a UART, USB or wireless link out of specification.
Package and footprint
SMD crystal packages are named by their footprint in tenths of a millimetre. The common sizes are 5032 (5.0 × 3.2 mm), 3225 (3.2 × 2.5 mm), 2520 and 2016. Smaller packages save board area but generally have higher ESR and less power handling, and they are more sensitive to PCB strain. For legacy through-hole designs and some industrial products, HC-49U and HC-49S packages remain in demand.
Practical selection checklist
- Confirm the frequency, and whether the IC expects a fundamental or third-overtone crystal.
- Match CL to your capacitor network, including stray capacitance.
- Check ESR against the oscillator's negative resistance, with margin across temperature and at your lowest supply voltage.
- Keep the crystal and its capacitors close to the IC, away from heat sources and board edges.
- Respect the drive level; over-driving a small crystal ages it quickly and can fracture the quartz.
Crystal oscillators and clock parts we stock
| Part | Type / package | Typical use | Link |
|---|---|---|---|
| X322516MRB4SC | Crystal, 3225 (3.2 x 2.5 mm), 16 MHz | Considered where EPSON TSX-3225 class devices are specified | X322516MRB4SC |
| X503227MSB4SI | Crystal, 5032 (5.0 x 3.2 mm), 27 MHz | Considered where EPSON FA-20H class devices are specified | X503227MSB4SI |
| Si52147-A01AGMR | Clock generator / PCI-Express clock | Multi-output clock distribution | Si52147-A01AGMR |
Browse the full range in filters, crystals and oscillators. Prices move with quantity and market conditions, so send us your frequency, package, CL and quantity for a real-time quote.
Frequently asked questions
What is load capacitance and why does it matter?
Load capacitance (CL) is the capacitance the crystal expects to see across its terminals. If your circuit presents a different value, the oscillation frequency is pulled off centre. Match the crystal's CL to the two external capacitors plus stray board and pin capacitance.
Can I drop a 3225 crystal into a 2520 footprint?
No. Package codes are physical dimensions in tenths of a millimetre: 3225 means 3.2 x 2.5 mm, 2520 means 2.5 x 2.0 mm. A smaller package will not bridge the pads reliably. Check the land pattern and height clearance before substituting.
What is the difference between frequency tolerance and frequency stability?
Tolerance is the deviation from nominal at room temperature (usually 25 C) at the time of manufacture. Stability is how much the frequency drifts across the operating temperature range. Both are quoted in ppm and both count toward your total error budget.
Do you supply YXC crystal oscillators?
Yes. We stock YXC parts including the X322516MRB4SC and the X503227MSB4SI, which are commonly considered where EPSON TSX-3225 and FA-20H class devices are specified. Send us your frequency, package and CL requirement for a quote.
About us & trademarks: JTDZ Tech (Shenzhen Jingteng Electronics Technology Co., Ltd.) is an independent distributor of electronic components. We are not an authorised distributor or representative of the manufacturers named in this article. Manufacturer and part names are used for indicative purposes only, to help identify the components discussed; all trademarks remain the property of their respective owners. Always verify specifications against the official datasheet before design-in or purchase.
Recommended next steps
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