Quartz crystal
A quartz crystal is a passive resonator used in an external oscillator circuit. Frequency, load capacitance, equivalent series resistance and drive level must match that circuit. Body size alone does not establish an interchangeable part. For example, the cited NDK NX3225GD sheet covers 7.98–12 MHz options and requires a frequency and specification string alongside the series name.
Quartz is a piezoelectric material: applying voltage across two crystal faces causes a mechanical stress, and applying mechanical stress generates a voltage. A thin crystal blank with metal electrodes on both faces has a sharp mechanical resonance — the AT-cut at 16 MHz literally vibrates 16 million times per second. Place it in a Pierce oscillator (an inverting amplifier with two load capacitors to ground forming a 180° phase shift), and the circuit oscillates exactly at the crystal's resonant frequency because that's the only frequency at which the loop has zero net phase shift and gain greater than one. The load capacitance value (— e.g. 18 pF or 20 pF in the datasheet —) sets where on the crystal's narrow resonance curve the oscillator locks; getting it wrong shifts the frequency by tens of ppm.
In plain terms
Like a perfectly-tuned tuning fork frozen in time: tap it electrically and it rings forever at exactly one frequency, set by the way the crystal was sliced — just like an A-440 tuning fork rings at exactly 440 Hz because of its precise length and shape.
Why designers use it
- Set the master clock of a microcontroller — an internal RC oscillator drifts by 1–3% over temperature, but a quartz crystal holds within ±10–30 ppm (0.001–0.003 %).
- Provide a 32.768 kHz tick to a real-time clock chip (DS3231, PCF8523) so a coin-cell-backed RTC keeps wall time within seconds per month.
- Reference frequency for USB, Ethernet, Wi-Fi radios, and GPS — every protocol has a ppm spec the crystal must meet or the link fails.
- Anchor frequency for radio transmitters, RTC dividers, and frequency-counter circuits: the crystal’s thermal stability turns a noisy circuit into a precision instrument.
Best for
- MCU clocks
- RTC tick
- USB / Ethernet refs
- Radio LO
Key specifications
- Frequency range: 32.768 kHz – 200 MHz
- Initial tolerance: ±10 – ±50 ppm
- Temperature drift: ±10 – ±100 ppm over −40 … +85 °C
- Load capacitance (CL): 8 pF – 22 pF typical
- Equivalent series resistance: 10 Ω – 100 Ω
When not to use it
- When board space is at a premium and ±50 ppm is good enough — a MEMS oscillator or ceramic resonator wins on size and shock robustness.
- When the application sees shock or vibration that could crack the quartz blank — use a MEMS oscillator (DSC, SiTime) for automotive / industrial.
- When the design needs a single-chip drop-in clock source — a packaged crystal oscillator (XO/TCXO) integrates crystal + amplifier and saves the load-capacitor design work.
Common mistakes
- Sizing the load capacitors to the crystal's CL spec without subtracting board stray capacitance (≈3–7 pF) — the oscillator runs 10–50 ppm fast.
- Routing long traces from the crystal to the MCU oscillator pins — the parasitic capacitance shifts frequency and the antenna effect couples digital noise into the loop.
- Mixing up a series-resonant and parallel-resonant crystal in the same Pierce circuit — a fundamental-mode parallel crystal works; a 3rd-overtone series crystal won't start in the wrong topology.
- Putting the crystal next to a switching regulator or noisy GPIO — the oscillator's low loop power makes it picky about EMI on the load caps.
Where you will find it
- Every Arduino Uno board has a 16 MHz HC-49 crystal between pins 9 and 10 of the ATmega328P, with two 22 pF C0G load capacitors to ground: that crystal sets the timing of every <code>millis()</code>, <code>delay()</code>, and UART baud rate.
- A digital wristwatch runs from a 32.768 kHz tuning-fork crystal (the "watch crystal") that drives a 15-bit binary counter to produce exactly one tick per second — the same circuit topology Seiko introduced in 1969 in the Astron, the first quartz watch.
- A USB-to-Serial adapter (FT232RL, CH340) uses a 12 MHz quartz crystal because USB Full-Speed requires its 1.5 ppm reference precision; an internal RC oscillator wouldn’t meet the spec and the host would mark the device as faulty.
A short history
The piezoelectric effect of quartz was discovered by the Curie brothers in 1880, and the first quartz crystal oscillator was built by Walter G. Cady at Wesleyan University in 1921. Bell Labs engineer Warren Marrison built the first quartz clock in 1927, three orders of magnitude more stable than the best pendulum clocks of the era. The AT-cut, invented at Bell Labs in 1934, became the dominant high-frequency cut because its frequency varies by only a few ppm over a wide temperature range. The 32.768 kHz tuning-fork cut was commercialised by Seiko in the 1969 Astron wristwatch, which sold for the price of a small car and changed timekeeping forever. ECS Inc, NDK, Epson Toyocom, Abracon, Kyocera, and Citizen are the major modern manufacturers; the family-part-numbers (ECS-, NX, ABM, FA, CX, CS) are still the catalogue prefixes you see on Digi-Key today.
Good to know
- A 32.768 kHz watch crystal is chosen because 32,768 = 2¹⁵, so a single 15-bit binary counter divides it down to exactly one Hz — perfect for a digital clock.
- The Curie brothers (yes, Pierre Curie who later married Marie) discovered the piezoelectric effect of quartz in 1880, 47 years before the first quartz oscillator was built.
- Sub-ppm-stability TCXOs (temperature-compensated crystal oscillators) wrap the crystal with a tiny heater and a calibration table burned into ROM at the factory — a $5 chip can hold 0.5 ppm across the entire automotive temperature range.