Holdover Performance Definition and Testing Methods

Holdover performance is a key indicator of a disciplined oscillator’s ability to maintain accurate timing after losing an external reference source such as GPS or BeiDou signals.

Definition

When external timing references become unavailable, the clock system relies on its internal high-stability oscillator—such as an OCXO—to continue operating independently while minimizing deviation from UTC (Coordinated Universal Time).

Key evaluation parameters include:

· Maximum Time Error (MTE): the largest time deviation during a specified holdover period.

· Frequency Offset: the frequency deviation at the moment the reference signal is lost.

· Frequency Drift: long-term frequency variation caused mainly by aging and temperature changes.

Testing Method

Holdover testing is conducted in a controlled environment using a highly stable UTC-synchronized reference source, such as a hydrogen maser, cesium clock, or GPS-disciplined rubidium clock.

The device under test is first locked to the reference source for calibration:

· DOCXO devices typically require 2–4 hours.

· DTCXO devices typically require about 3 minutes.

After calibration, the external reference signal is disconnected, forcing the device into holdover mode. A high-precision time interval analyzer continuously measures the time error between the device output and the reference source over periods such as 1 hour, 4 hours, or 24 hours.

Environmental temperature is monitored throughout the test, as temperature variation is a major factor affecting oscillator stability, especially for DTCXO devices.

Data Analysis

The recorded time error data is plotted as a holdover curve to evaluate:

· Maximum Time Error (MTE)

· Frequency Offset

· Frequency Drift Rate

For batch production testing, host computer software records per-second timing errors from onboard precision timing modules and calculates cumulative errors in real time to determine holdover performance.

In practical terms, holdover testing simulates the real-world loss of satellite signals and evaluates how long a clock system can maintain accurate timing independently. Under stable temperature conditions, a high-performance DOCXO typically shows a smooth, slow-changing holdover curve, while a DTCXO often exhibits periodic fluctuations caused by ambient temperature changes.

To know more: OCXO (GPSDO)