LeCroy LSA1000, Operator'S Manual

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C – 2
Appendix C
Determining Time
Parameters
Once top and base are estimated, calculation of the rise and fall
times is easily done ( Fig.1 ). The 90 % and 10 % threshold levels
are automatically determined by the LSA1000, using the
amplitude (
ampl ) parameter.
Threshold levels for rise or fall time can also be selected using
absolute or relative settings ( r@level, fall@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
( )

=
−
Mr
i
i i
Tr Tr
Mr
1
10 90
1
Falling Edge Duration
( )

=
−
Mf
i
i i
Tf Tf
Mf
1
90 10
1
Where Mr is the number of leading edges found, Mf the number of
trailing edges found,
x
i
Tr the time when rising edge i crosses the x %
level, and
x
i
Tf
the time when falling edge i crosses the x % level.
Time parameter measurements such as width , period and delay
are carried out with respect to the mesial reference level ( Fig.
C
–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.
Determining Rise and
Fall Times