AN-004: OCXO Warm-Up and Power Design

Application Note AN-004 · By KCCS Engineering Team · Updated 2026 · 6 min read

Why Warm-Up Time Matters

When an OCXO powers up, the oven must heat the crystal package to its setpoint (typically 70–80 °C). During this warm-up, the frequency is nowhere near its rated accuracy. A 10 MHz OCXO might drift several ppm before the oven stabilizes. For systems that need valid timing within seconds of power-on, warm-up time is a critical design constraint.

Typical Warm-Up Numbers

Designing the Power Supply

The most common OCXO design error is undersizing the power supply for warm-up inrush. A 100 mA steady-state OCXO can draw 600 mA during warm-up. If the 3.3 V rail shares a fuse or current limit with other loads, the OCXO will never reach the oven setpoint, or the supply will brown out.

Best practice:

- Size the 3.3 V or 5 V rail for the warm-up current, not just steady-state.

- Add a bulk capacitor (100 µF or more) close to the oscillator pin to handle the peak.

- Power the OCXO on before the rest of the digital board if fast valid timing is needed.

- If your system runs on battery, the KOP low-power series (33 mA steady-state, 20 s warm-up) is designed for exactly this use case.

Fast Warm-Up vs Low Power — The Tradeoff

Fast warm-up and low steady-state power are partially in conflict. A smaller oven heats faster but also loses heat faster, requiring more steady-state power. A larger oven holds temperature better but takes longer to heat. KOP12 is optimized for fast warm-up (20 s) at the cost of higher steady-state current. KOP27 prioritizes 33 mA steady-state for battery-powered use.

Do Not Cycle Power on an OCXO

Frequent power cycling is hard on the oven heater and shortens oscillator life. If your system has sleep modes, keep the OCXO powered on even when the rest of the board sleeps. The steady-state current is small compared to the reliability and stability benefit.