Chapter 2. Overview

Temperature training improves the ability of the Güralp Fortimus accelerometer to maintain accurate sample-timing in the absence of an external time reference.

The Fortimus derives its sample-timing from a voltage-controlled crystal oscillator (VCXO). The precise operating frequency of such an oscillator can be changed by modifying an input voltage. In the Fortimus, the input voltage is derived from the output of a digital-to-analogue convertor (DAC). When an external timing source, such as a GNSS receiver or PTP grand-master, is used, the digitiser firmware varies the input to the DAC in such a way as to keep the sample clock synchronised to external time.

All crystals change their natural frequency when the temperature changes. There are times when an external time source is not available and the sample clock is free-running; if, at such times, temperature variations changed the frequency of the VCXO, the sample timing would be adversely affected. The Fortimus contains a feature to prevent this from happening.

The behaviour of the crystal with respect to temperature is not linear and the temperature response of any individual crystal cannot be predicted. However, the frequency-temperature relationship of a crystal remains relatively stable over time so, if the relationship can be measured, the firmware can adjust the input of the DAC whenever the temperature varies in order to annul any temperature-related variation in clock frequency.

The Fortimus firmware contains a temperature compensation table which maps temperature values to correction values which must be subtracted from the input to the DAC in order to stabilise the frequency as the temperature varies. This table is populated by synchronising the unit to a GNSS time-source, placing the firmware into “Temperature training” mode and varying the ambient temperature. The Fortimus will measure the internal temperature using its own thermal sensor, adjust the DAC input until the VCXO is synchronised, compute the required offset and store it in the table. This process is repeated across a wide range of temperatures until the table is populated.

This document describes the process.