W
AVE
R
UNNER
X
I
S
ERIES
110
WRXi-OM-E Rev B
D
ETERMINING
R
ISE AND
F
ALL
T
IMES
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-5005, 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.
The time interval separating the points on the rising or falling edges is then estimated to yield the rise or fall time.
These results are averaged over the number of transition edges that occur within the observation window.
Rising Edge Duration:
Falling Edge Duration:
Where
Mr
is the number of leading edges found,
Mf
the number of trailing edges found,
the time when
rising edge
i
crosses the x% level,
and the time when falling edge
i
crosses the x% level.
Determining Time Parameters
Time parameter measurements such as width, period and delay are carried out with respect to the mesial
reference level (see Figure 2), located halfway (50%) between the top and base reference lines.
Time-parameter estimation depends on the number of cycles included within the observation window. If the
number of cycles is not an integer, parameter measurements such as rms or mean will be biased. However, only
the last value is actually displayed, the mean being available when statistics are enabled. To avoid these bias
effects, the instrument uses cyclic parameters, including crms and cmean, that restrict the calculation to an
integer number of cycles.
Figure 2
