LeCroy DDA-3000A, Operator'S Manual

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WM-OM-E Rev I
Parameter Calculations
Parameters and How They Work
Determining Top and Base Lines
Proper determination of the top and base reference lines is fundamental for ensuring correct
parameter calculations. The analysis begins by computing a histogram of the waveform data over
the time interval spanned by the left and right time cursors. For example, the histogram of a
waveform transitioning in two states will contain two peaks (see Figure 1). The analysis will attempt
to identify the two clusters that contain the largest data density. Then the most probable state
(centroids) associated with these two clusters will be computed to determine the top and base
reference levels: the top line corresponds to the top and the base line to the bottom centroid.
Figure 1
Determining Rise and Fall Times
Once top and base are estimated, calculation of the rise and fall times is easily done (see Figure 1).
The 90% and 10% threshold levels are automatically determined by the DDA-5005A, using the
amplitude (ampl) parameter.
Threshold levels for rise or fall time can also be selected using absolute or relative settings (r@level,
f@level). If absolute settings are chosen, the rise or fall time is measured as the time interval
separating the two crossing points on a rising or falling edge. But when relative settings are chosen,
the vertical interval spanned between the base and top lines is subdivided into a percentile scale
(base = 0 %, top = 100 %) to determine the vertical position of the crossing points.