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Applications — Type
3A74
For
more
precise
measurements,
you
could
increase
equally the vertical sensitivity of all channels used and the
sweep
rate
established
in steps
6, 7, and
8,
but
do
not
change
the
setting
of the
time-base
Variable
Time/Div.
control.
However,
when
you
increase
the
sweep
rate,
you
must take this into consideration
in your calculations.
For example, if you increase the sweep rate by a factor
of 5, and
then
measure
the distances
between
waveforms,
each
division
will
represent
8°
(40°
+ 5) of a cycle.
By
doing
this, you
can
measure
phase shift up to 80°
more
accurately.
When
preparing
to
make
the
measurement,
horizontally
position
the
waveforms
to
points
where
the
graticule
markings
aid
in determining
the exact
distance.
{To
be
certain
that
the
waveform
is
centered
vertically,
check
the
DC.
BAL.
adjustment
before
vertical
sensitivity
is increased.)
Fig.
3-4,
for example,
shows
how
the
phase
shift of channel
2 waveform
can
be computed
using
this
method. Other phase-shift measurements can be determined
using this same basic procedure.
Time-Delay
Measurements
The
calibrated
sweeps
of Tektronix
oscilloscopes
cause
any horizontal distance on the screen to represent a definite
known
interval
of time.
Using
this feature
in combination
with
the
multi-trace
feature
of the Type
3A74,
you
can
measure the time lapse or delay between events displayed
on the crt. This is done
by using the following
method:
1. Set the AC-GND-DC switch to AC or DC depending on
the type
of coupling
desired.
2. Apply
the
reference
signal
to channel
1; apply
the
signals which have more delay than the reference signal to
the other
channels.
Use
coaxial
cables
or probes
having
equal delay.
3. Pull
the
TRIGGER
switch
to
its outward
position
and
set
the
CHOP.-ALT.
switch
to the
position
desired.
4, Set the MODE
switches to NORM.
5. Set
the
VOLTS/DIV.
switches
and
the
VAR.
GAIN
controls
to
obtain
suitable
vertical
deflections.
Set
the
time-base
sweep
rate and
triggering
controls
to obtain
a
stable display at a sweep rate that will allow you to measure
the
distance
between
waveforms
accurately.
Be sure the
Variable Time/Div. control is set to the Calibrated position.
6. If some
of
the
waveforms
are
negative-going
and
others are positive-going, you may prefer to set the appro-
priate MODE switches to the INV. position to make all wave-
forms go in the same direction on the display.
7. With
the
aid
of
the
graticule
markings,
measure
the horizontal distance between the reference waveform and
each
of the other waveforms.
For most measurements
the
distance is usually measured between 50%
amplitude points
on the rising portion of the waveform.
To make the measure-
ments easier, switch the waveforms not being measured "off
by setting the appropriate MODE
switches to the OFF
posi-
tion.
8. Multiply
the
distance
measured
for
each
channel
by the setting of the time-base Time/Div. switch to obtain
the apparent
time interval.
3-4
9. To obtain the actual time interval, divide the apparent
time
inteval
by 5 if 5X
sweep
magnification
is used,
by
10
if 10X
sweep
magnification
(Type
565
Oscilloscope)
is
used,
and
1
if sweep
magnification
is not used.
(Time/Div. Switch Setting) (Distance in Div.)
Sweep Magnification
Time Delay =
For
example,
assume
the Time/Div.
switch
setting
is 2
uSec, the 5X
magnification
is in use, and
you
measure
a
horizontal distance of 3 divisions between the leading edge
of the reference waveform and the leading edge of the wave-
form displayed on another channel.
In this example, 3 divi-
sions multiplied
by 2 microseconds
per division
gives you
an apparent time delay of 6 microseconds.
The apparent time
delay divided by 5 gives you an actual time delay of 1.2
microseconds.
Four-Trace
X-Y
Displays
It is
possible
to
display
up
to
four
different
sets
of
sampled X-Y parameters on the oscilloscope screen through
the use of two Type 3A74 Plug-In Units.
To display four sets of X-Y parameters at the same time,
proceed
as follows:
1. Insert
the Type
3A74's
into the
vertical
(Y-axis) and
horizontal
(X-axis)
openings
of the
oscilloscope.
2. Set the CHOP.-ALT. switch of the Y-axis unit to CHOP.
and the X-axis unit to ALT.
Best transient blanking response
is obtained by setting these switches to the positions as stated.
3. Connect
@ pair
of (X-Y}
signals
to each
channel.
For
example, connect one signal of a pair to channel 1 of the
Y-axis
plug-in.
Connect
the
other
signal
of the
pair
to
channel
1 of the X-axis plug-in.
Proceed in this manner to
connect each pair of signals to the other channels of both
units.
NOTE
Channel
4
in
both
units
synchronize
together
through
pin 18 of the interconnecting
plugs. There-
fore, it is important that you always
leave channel
4 "on"' in both units by setting their MODE switches
to either
NORM.
or INV.,
as desired.
In addition,
always
use the same
number
of channels
between
units
to
assure
proper
synchronization
between
pairs of signals.
4. Set the VOLTS/DIV.
switches and the POSITION
con-
trols for the desired displays.
If the displays do not properly
synchronize, set the channel 4 MODE switch of the left-hand
or Y-axis plug-in to OFF momentarily and then back to the
"on"
position.
If you
are applying
sine-wave
signals of the same
fre-
quency
to each
channel,
the
resulting
displays
will appear
as
a
straight
line,
ellipse,
a
circle,
or
other
variations
of
the Lissajous pattern for phase differences between 0° and
360°.
Fig. 3-5 shows an example of possible displays that
can
be obtained.
To calculate the sine of the phase dif-
ference between a pair of signals, use this method:
{a) Center
one
of
the
X-Y
displays
horizontally
and
vertically on the crt.
Position the others off the crt.
®
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