LeCroy DDA 7 Zi series, Operator'S Manual

Page: 472 / 544

WavePro 7Zi
471
WP700Zi-OM-E-RevA
Local Feature Concepts
Overview
The term "local feature computation" indicates that a parameter computed on a waveform is determined only by
information in the immediate vicinity of a specified feature of that waveform. The DDA defines a local feature as a
waveform peak followed by a trough, like this:
However, it is not the opposite: a trough followed by a peak. The diagram below shows a single local feature: the
first peak and the trough that follows it.
Peak-Trough Identification
The key to identifying peak-trough pairs is the ability to discriminate between real pairs and false ones. For
example, noise in a signal can be mistaken for a local feature, as here:
Similarly, ‘bumps’ in a waveform may also be mistaken for peak-trough pairs.
In order to avoid such misidentification, a hysteresis argument is provided for many local feature parameters. This
essentially enables you to set a voltage band, which a peak-trough pair must exceed in order not to be considered
noise or a "bump."
The hysteresis setting is also essential to the way peaks and troughs are identified by the DDA.
The search for local features extends from the left to the right parameter cursor. But first a peak must be found;
and a waveform must rise to at least the value of the hysteresis setting in order to be positively identified as a
peak.
This peak search starts with the first waveform sample, whose voltage value is used as an initial reference value
for locating the peak. If a following waveform sample is found to be higher than the first by an amount greater than
the hysteresis setting, a peak is said to exist. Any sample lower than the reference value, made prior to
determination of a peak’s existence, is used as a new reference point.