Automatic Top-Base - HP 54501A Programming Reference Manual

When
any
of
the standard measurements are requested, the
HP 54501A
first determines the
top-base voltage levels at
100%-0%. From
this
information,
it can
determine the
other important voltage values
(10%,
90%, and
50%)
needed to make the measurements.
The 10%
and
90%
voltage
values are used in the risetime and falltime measurements
as
well
as
in all other edge measurements. The 10%
and
90%
values are also
used
to determine the
50%
value.
The 50% voltage
value is used
for
measuring frequency, period, pulse width, and duty cycle.
Automatic
Top-Base
Top-Base
is the heart of
most automatic measurements.
It is
used to
determine Vtop and Vbase, the
0%
and
100%
voltage
levels at
the top and
the bottom
of the waveform. From this information the oscilloscope can
determine
the
10%, 50%, and
90%
points, which are
also
used in most
measurements.
The
top or
base
of
the waveform
is not necessarily the
maximum or minimum voltage present
on the waveform.
Consider
a
pulse
that has
slight
overshoot.
It
would be wrong to
select
the
highest point of
the waveform
as
the top since the
waveform normally
rests
below the
perturbation.
Top-Base performs
a
histogram on the waveform and finds the most
prevalent point above and below the waveform midpoint. The
most
prevalent point is one that represents greater than approximately
5%
of
the total display points
(501)
and
is considered to be either the top or
base.
If no point accounts for more
than
5% of
the total, then the top
is
chosen
as
the absolute maximum and the base is
chosen
as
the absolute
minimum.
EdQG
Definition
Both rising and falling edges are defined
as
transitional edges that
must
cross
three thresholds.
A
rising edge must cross the lower threshold in
a
positive direction
(defining
it
as a
rising edge),
cross
the mid threshold (any number of
crossings, both positive and negative are
permissible)
and then cross the
upper threshold without
any crossing
of the lower threshold.
Algorithms
A-2
HP 54501A
Programming