Settling time measurements determines how long it takes for a waveform rise or fall to and remain, within a specified error band around the final value. While it is easy to see this by eye it is not easy for a scope to determine exactly when this occurs. By utilizing an all instance measurement and tracking threshold crossings until they no longer occur, we can measure the time from the reference event to the last crossing.


In Figure 1 we see a typical measurement of this type.

Figure 1:

A typical settling time measurement. How long does it take from the trigger event until the waveform remains within the threshold limits?

Here we have a waveform with a slowly decaying amplitude. The time@level parameter measures the time from the scope trigger to the threshold crossing. Here we use two instances of the parameter to establish upper and lower threshold levels. This parameter measures each threshold crossing instance so it retains a history of these events.

In Figure 2 we plot the track of time at level. The track function plots the parameter versus time and shows us a history of the parameter values.

Figure 2:

Using the track of time@level to see the history of parameter measurements over the whole waveform

Note that the track functions (one for each parameter) start a 0 and increase with every threshold crossing. When the waveform settles within the tolerance band outlined by the threshold levels, the track levels off with a slope of zero. This tells us when the signal is within the tolerance band.

It is easier to measure the duration of this if we have an edge, so we differentiate the track function. This is done in Figure 3.

Figure 3:

Differentiating the track function to see the edge

We can now apply the same time@level parameter to measure the duration of the settling time. We get two values, one for the upper threshold and another for the lower threshold crossing. The settling time is the greater of the two measurements.

Parameter math, in the form of the optional Min Max (Px, Py), determines the greater of the two parameters and yields the desired settling time measurement.

This application makes use of many of the LeCroy oscilloscopes analysis and measurement features to enable a relatively complex measurement.