KCCS
Precision Timing
By KCCS Engineering Team · Updated 2026 · 7 min read
A GPS-disciplined oscillator (GPSDO) locks an internal OCXO to the 1PPS signal from GNSS satellites. As long as the satellites are visible, the output is traceable to UTC. The real engineering question is what happens when the signal is lost — a scenario called holdover. This note explains how holdover is specified, what drives the error budget, and how to choose the right oscillator for your outage tolerance.
Holdover is the timekeeping accuracy of the oscillator after it loses its external reference (GNSS 1PPS, PTP, or another master clock). The internal oscillator keeps running on its own, but its frequency drifts due to temperature changes, aging and its inherent noise. The accumulated time error grows over time.
Datasheets typically quote holdover as a maximum time error over a defined window, for example "±1.5 μs over 24 hours". This is not a single number — it is the worst-case accumulated error under the specified conditions (fixed temperature, or worst-case temperature range).
1. Temperature stability of the OCXO. This is usually the dominant term. If the equipment room changes by 5 °C and the OCXO has 0.1 ppb/°C sensitivity, the frequency shifts by 0.5 ppb. Over 24 hours that accumulates to about 43 ms of timing error — far worse than any aging specification. This is why holdover numbers quoted at constant temperature are not the same as holdover numbers across the actual operating range.
2. Aging rate. A typical OCXO ages at 1E-10 per day (0.1 ppb/day). Over a 24-hour holdover that is about 8.6 ms. Lower-aging OCXOs (1E-11/day) reduce this by 10×.
3. Initial accuracy and calibration offset. The oscillator starts holdover with a small residual frequency error from the last disciplined state. A well-designed GPSDO leaves less than 1E-12 offset at the moment of loss.
4. Warm-up and re-acquisition behavior. When the GNSS signal returns, the oscillator needs to re-lock. Fast re-acquisition (3 minutes to within 10 ppb) reduces the exposure window.
| Class | Holdover (24 h, 25 °C) | Typical Use |
|---|---|---|
| TCXO-based | ±1–5 ms | Consumer GPS, simple sync |
| Low-cost OCXO (KDO5/9/11) | ±0.1–1 μs | Cost-sensitive base stations |
| High-performance OCXO (KDO27/65) | ±0.1–1.5 μs | PTP grandmasters, PMU, financial |
| CSAC (KCSA35) | ≤5 μs over years | GNSS-denied, portable reference |
| CPT (KCPTA50) | ≤1 μs over 24 h, ±5E-11 over temp | Outdoor / wide-temp disciplined |
If your application requires sub-microsecond holdover across a wide temperature range (outdoor cabinet, vehicle, airborne platform), do not rely on the 25 °C spec on the datasheet. Ask the manufacturer for the holdover number across the actual temperature range you will see. KCCS KDO27 series is specified at ±1.5 μs/24 h at constant temperature; across -40 °C to +85 °C the number changes, and we can provide the worst-case figure on request.
For truly unattended operation where GNSS might be unavailable for days (underground, ocean bottom, indoor testing), a CSAC or CPT clock replaces the OCXO entirely. Their holdover is not temperature-dependent in the same way, and they do not need an oven.
Tell us your holdover window, temperature range and power budget — we will spec the right KDO series or atomic clock.
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