Tektronix 2221 - Glitch Catching and Missing Pulse Detection

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П
Basic
Applications -
2221
Operators
Anti -
Aliasing
90
O
10
( A ) NON
STORE
(
GLITCH NOT
VISIBLE ) .
n
П
п
In the
event
that an
alias
is
suspected
,
three
things
may
be
done to
determine
whether the
display is of
an
alias . The
first is to
switch back to
NON -
STORE
mode to
determine if
the
input
signal is
higher
in
frequency
than the
apparent signal being displayed ( see Figure 6-248 ) . Ensure
that this
display
is
being
triggered as
indicated by the
TRIG'D
LED
being
illuminated .
The second is
to use
either
the
ACCPEAK
or
the
PEAKDET
storage
modes (
PEAK
DET is
the
default
mode
for SEC /
DIV settings
from 5 s
per
division to 20
us per
division
) ,
which hold the
maximum
and
minimum
points
being
acquired .
PEAKDET
storage
mode
holds
the
maximum
and minimum
points
acquired
in
a single trigger cycle , and ACCPEAK accumulates the
maximum
and
minimum
points
acquired
over
many
trigger
cycles
.
Since the
maximum
and
minimum
points
of
the
alias
waveform do
not occur at
exactly
the
same
point in
relation to
the
trigger
each
time
, the
display
soon acquires
maximum
and
minimum
amplitude
levels
in
every
storage
address
and
the
top
and bottom
of
the
alias
display
become
flat
lines (
see
Figure
6-24C
) .
90
10
( B )
ACCPEAK
STORE
MODE (
GLITCH
DISPLAYED ) .
4998-27
Figure 6-25 .
Glitch display ,
ACCPEAK
Store
mode .
Missing Pulse
07
Third , if an
alias is
detected ,
the
SEC /
DIV
switch
may
be set
for a
faster
sweep
rate
so
that
the
number of
sam
ples per
cycle of
the input
signal
is
increased
.
However
,
at
sweep
speeds
of 2
us per
division
and
faster
, the
sampling
rate is
not
increased
;
and if an
alias
signal is
still
present
at 5
us per
division , the
frequency
limit of the
digital
circuitry
has
been
exceeded for
nonrepetitive
signals .
When
the SEC /
DIV
switch
is
set
for
sweep
speeds
faster
than 5 us /
div ,
Repetitive
Store
acquisition
mode
and
AVERAGE are
selected
.
On
repetitive
signals , the
random
phase
between
successive triggers
and the
time
-
base
clock
suppress
aliased
waveform
displays as a
result
of
the
increased
effective
sample
rate .
ACCPEAK
mode
is
useful for
finding an
intermittent
pulse
in
a
pulse
train
.
The
puise
may
either be
missing or
present
erratically . In
either
case
, the
change in
amplitude
levels is
displayed
as a
completely
filled
in
pulse
(
see
Fig
ure
6-26 ) .
AVT
40.0mV
AT =
125.3us
Glitch
Catching
Puises that are
present for a
very
short time
duration
during the
viewing of
longer
puise
duration
signals ,
such
as a
logic
pulse
train ,
may
not be
visible at
the sweep
speed in
use
(
see Figure
6-25A
) .
In
digital
logic
circuitry , a
small
switching
transient (
glitch )
may
cross
the
logic
threshold
level
and cause an
error .
Setting up
the
instru
ment to
trigger on
the
error event
should
position the
storage
window to
acquire the
pulse
train
that
contains
the
glitch
.
1
100
|
90
MISSING
PULSE
10
07
20.my
To
catch a
glitch
,
first
set
the trigger
position to
pre
trigger
.
This
will
acquire 7/8
of a
waveform
occurring
before the
trigger .
Select
ACCPEAK
mode
using
the
ACQUISITION
MODE switch . This
will
acquire
the
waveform
maximum and
minimum
points
over a
selected
number
of
sweeps
.
The
location of the
glitch will
be
displayed in
the
accumulative
envelope
display (
see
Figure
6-25B ) .
O
.
50us
4998-29
Figure 6-26 . Missing pulse , ACCPEAK STORE mode .
П
6-23