Crystal Oscillator (XO/TCXO)
Timing
A complete clock source in a can: crystal, drive circuit and output buffer together. Feed it power and it outputs a clean square wave — no fussing with load capacitors like a bare crystal.
Schematic symbol
Crystal / resonator
How crystal oscillator (xo/tcxo) appears in a circuit diagram.
Types & variants
Key specs
Frequency
HzFrom 32.768 kHz to hundreds of MHz.
Stability
ppm±50 ppm XO, ±2 ppm TCXO, ppb-level OCXO.
Supply voltage
V1.8 / 2.5 / 3.3 V families — check before wiring.
Output type
CMOS for MCUs; LVDS/LVPECL for fast serial links.
Markings
Frequency and part code printed on the lid; pin 1 marked with a dot.
Standard values
32.768 kHz (RTC), 8/12/16/25 MHz (MCUs, Ethernet), 24.576 MHz (audio).
How to choose
Bare crystal is cheaper if your chip has an oscillator amp; buy a packaged XO when it doesn't or when you need guaranteed startup. TCXO for GPS/radio, OCXO for lab-grade stability.
Pinout & package
4 pads: Vcc, GND, OUT, and enable/NC. Pin 1 = enable on most 3225/5032 SMD packages.
Example circuits
- Clocking an FPGA or Ethernet PHY
- TCXO reference for an SDR receiver
- 32.768 kHz XO feeding an RTC
Common failures
Wrong supply voltage kills them; marginal decoupling causes jitter; cheap XOs drift with temperature far past their spec.
How to test
Power it and probe OUT with a scope or frequency counter — expect a clean square wave at the marked frequency. A multimeter in DC mode reading ~Vcc/2 hints it's oscillating.
Substitutes
Bare crystal + your chip's internal oscillator, or a MEMS oscillator (same footprint, better shock tolerance, slightly more jitter).