Operating Instructions-Type
453/R453
When the signal
is
coupled directly to the input of the
Type 453, the input impedance
is
about one megohm
paralleled by about
20
pF.
When the signal
is
coupled to
the input through a coaxial cable, the effedive input ca-
pacitance depends upon the type and length of cable used.
See the following discussion for information on obtaining
maximum frequeqcy response with coaxial cables.
Coaxial Cable Considerations
The
signal cables used to connect the signal to the Type
453
INPUT
connectors have a large effect
on
the accuracy
of the displayed high-frequency waveform.
To
maintain the
high-frequency characteristics of the applied signal, high-
quality low-loss coaxial cable should be used.
The
cable
should be terminated
at
the Type
453
INPUT
connector
in
its
characteristic impedance.
If
it
is
necessary to
use
cables
with differing characteristic impedances,
use
suitable imped-
ance-matching devices to provide the correct transition,
with
minimum
loss,
from
one impedance to the other.
The
characteristic impedance, velocity of propagation and
nature of signal losses
in
a coaxial cable
are
determined
by the physical and electrical characteristics of the cable.
losses caused by energy dissipation
in
the dielectric
are
proportional to the signal frequency. Therefore,
much
of
the high-frequency information
in
a fast-rise pulse can be
lost
in
only a few feet of interconnecting cable
if
it
is
not
the correct type.
To
be sure of the high-frequency response
of the system when
using
cables longer than about
five
feet,
observe the transient response of the Type
453
and the
interconecting cable with a fast-rise pulse generator (gen-
erator risetime
less
than 0.5 nanoseconds).
Input Coupling
The
Channel 1 and 2 Input Coupling switches allow a
choice of input coupling.
The
type of display desired
will
determine the coupling used.
The
DC
position can be used for most applications.
How-
ever,
if
the
DC
component of the signal
is
much
larger than
the
AC
component, the
AC
position
will
probably provide a
better display.
DC
coupling should
be
used to display
AC
signals below about 16 hertz as they
will
be attenuated
in
the
AC
position.
In
the
AC
position, the
DC
component of the signal
is
blocked by a capacitor
in
the input circuit.
The
low-fre-
quency response
in
the
AC
position
is
about 1.6 hertz
(-3
dB
point). Therefore, some low-frequency distortion can be
expected near
this
frequency
limit.
Distortion
will
also
appear
in
square waves which have low-frequency com-
ponents.
The
GND position provides a ground reference
at
the
input of the Type 453.
The
signal applied to the input con-
nector
is
internally disconnected but not grounded.
The
input circuit
is
held
at
ground potential, eliminating the need
to externally ground the input to establish a
DC
ground
reference.
The
GND position can also be used to pre-charge the
coupling capacitor to the average voltage level of the signal
applied to the
INPUT
connector.
This
allows measurement
of only the
AC
component of signals having both
AC
and
2-12
DC
components.
The
pre-charging network incorporated
in
this
unit
allows the input-coupling capacitor to charge
to
the
DC
source voltage level when the Input Coupling switch
is
set to GND.
The
procedure for
using
this
feature
is
as
follows:
1.
Before connecting the signal containing a
DC
compo-
nent to the Type
453
INPUT
connector, set the Input Coupling
switch to GND.
Then
connect the signal to the
INPUT
connector.
2.
Wait about one second for the coupling capacitor to
charge.
3.
Set the Input Coupling switch to
AC.
The
trace
(dis-
play)
will
remain on the screen and the
AC
component of
the signal can be measured
in
the normal manner.
Deflection Factor
The
amount of vertical deflection produced by a signal
is
determined by the signal amplitude, the attenuation factor
of the probe
(if
used), the setting of the
VOLTS/DIV
switch
and the setting of the
VARIABLE
VOL
TS/DIV
control.
The
calibrated deflection factors indicated by the
VOL
TS/DIV
switches apply only when the
VARIABLE
control
is
set to
the
CAL
position.
The
VARIABLE
VOL
TS/DIV
control provides variable
(uncalibrated) vertical deflection between the calibrated
settings of the
VOLTS/DIV
switch.
The
VARIABLE
control
extends the maximum vertical deflection factor of the Type
453
to
at
least
25
volts/division
(10
volts position).
Dual-Trace Operation
Alternate Mode.
The
Al T position of the
MODE
switch
produces a display which alternates
between Channel 1 and
2 with
e·ach
sweep of the
CRT.
Although the
ALT
mode
can be used
at
all sweep rates, the CHOP mode provides
a more satisfactory display
at
sweep rates below about
50
microseconds/ division.
At
these slower sweep rates, alternate
mode switching becomes visually perceptible.
Proper internal triggering
in
the
ALT
mode can be ob-
tained
in
either the
NORM
or
CH
1
ONLY
positions of the
TRIGGER
switch. When
in
the
NORM
position, the sweep
is
triggered
from
the signal on each channel.
This
provides a
stable display of two unrelated signals, but does not indicate
the time relationship between the signals.
In
the
CH
1
ONLY
position, the two signals are displayed showing true time
relationship.
If
the signals
are
not time related, the Channel
2 waveform
will
be unstable
in
the
CH
1
ONLY
position.
Chopped Mode.
The
CHOP position of the
MODE
switch
produces a display which
is
electronically switched between
channels.
In
general, the CHOP mode provides the best
display
at
sweep rates slower than about
50
microseconds/
division, or whenever dual-trace, single-shot phenomena
are
to be displayed.
At
faster sweep rates the chopped switch-
ing becomes apparent and may interfere with
the·
display.
Proper internal triggering for the CHOP mode
is
provided
with the
TRIGGER
switch set to
CH
1
ONLY.
If
the
NORM
position
is
used, the sweep circuits are triggered
from
the
between-channel switching signal and both waveforms
will
®
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