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.
El holdover es la precisión de cronometraje del oscilador después de perder su referencia externa (GNSS 1PPS, PTP, u otro reloj maestro). El oscilador interno sigue funcionando por sí solo, pero su frecuencia deriva debido a cambios de temperatura, envejecimiento y su ruido inherente. El error de tiempo acumulado crece con el tiempo.
Los catálogos típicamente citan el holdover como un error de tiempo máximo sobre una ventana definida, por ejemplo « ±1.5 μs en 24 horas ». No es un único número — es el error acumulado en el peor caso bajo las condiciones especificadas (temperatura fija, o rango de temperatura en el peor caso).
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 |
Si su aplicación requiere holdover sub-microsegundo en un rango amplio de temperatura (gabinete exterior, vehículo, plataforma aérea), no confíe en la spec de 25 °C del catálogo. Pregunte al fabricante el número de holdover sobre el rango de temperatura real que verá. La serie KCCS KDO27 está especificada a ±1.5 μs/24 h a temperatura constante; entre -40 °C y +85 °C el número cambia, y podemos proporcionar la cifra en el peor caso bajo solicitud.
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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