How to achieve perfect synchronization in data acquisition

In test and measurement technology, the term synchronization refers to the alignment of processes in terms of time. When two measuring devices run synchronously, the clocks in both devices run the same – no clock is lagging behind or running ahead. But why is synchronization important and how is it realized in practice?

Imagine your clock is running 10 minutes late, but you are not aware of it. What are the consequences? Maybe you miss your train or show up late for work. These issues arise because your clock is not synchronized with an accurate reference. If your clock were synchronous with the train clock or your work clock, you would not have these problems. Beyond this, everyday clocks are inherently imperfect and tend to drift over time. Therefore, one has to constantly adjust them to the world time in order not to drift too far away from it. To achieve this, most modern clocks regularly synchronize with more accurate clocks (e.g. atomic clocks).

The same applies to measurement technology. If measuring devices are not synchronized, significant problems can occur:

• Data recorded by different measuring devices does not line up or even drift apart.
• the start of the measurement does not take place at the same time for different devices. This means that one may miss an important event because it happened too early.
• Many analysis methods, such as the Fast Fourier Transform (FFT), require an accurate time measurement. If the time measurement is also inaccurate due to a lack of synchronization, this can lead to inaccurate analysis results.

An obvious and simple way to synchronize measurement devices is to use a trigger signal. The trigger signal can be sent from an external source, such as an atomic clock, and repeats at regular intervals. Since the individual measuring devices receive a synchronization signal at regular intervals, it is possible to avoid the time drift of different clocks (due to their inaccuracies).

Synchronization via a trigger signal from clock

But the problem with the simple trigger method is obvious: An absolute time indication (e.g. showing the world time UTC) is not possible, because we do not know the travel time of the trigger signal to the individual measuring devices. For instance, one measuring device can be 100 m further away from the origin of the trigger than the other. As a result, the trigger signal also takes longer to reach one measuring device than the other.

To avoid this, the so-called Precision Time Protocol (PTP) was introduced. Here, the Grandmaster Clock, a very precise clock, sends out the trigger signal at regular intervals. The other measuring instruments react to this signal and send their own time signal back to the Grandmaster Clock. Thus, by communication from both clocks, one can calculate the propagation time of the trigger signal, and account for it. Therefore, the hardware used is the only source of error. The more accurate the hardware, the more accurate the time synchronization.

Precision Time Protocol does require a wired connection, which is not always practical. If you want to synchronize clocks over long distances or over rough terrain, GNSS (global navigation satellite system) synchronization via e.g. GPS is an excellent alternative. You can read more about this in our blog post about satellite navigation.

In addition to PTP and GNSS, there are a number of other synchronization techniques, each with its own advantages and disadvantages. Examples include IRIG (Inter-Range Instrumentation Group) timecodes and PPS (Pulse Per Second) signals.