Use Of Probes - Tektronix 545A Instruction Manual

Operating
Instructions
—
Type
545A
low-frequency components.
For applications
where you
are
observing
low-level signals, unshielded input leads are
entirely unsatisfactory
due
to
their
tendency
to pick
up
stray signals
which produce erroneous
oscilloscope
dis-
plays.
Shielded cables should be
used
whenever
possible,
with the shield
connected
to the chassis of
both
the
oscillo-
scope
and
the signal source. Regardless of the type of input
lead used, leads should
be
kept as short as
possible.
Distortion of the input
waveform may
result
if
very low-
frequency
input signals
are ac coupled
into
the oscilloscope,
if
high-frequency waveforms are
not properly terminated,
or
if
the input
waveform
contains high-frequency
compon-
ents
which exceed the pass
band
of the oscilloscope
and
plug-in
unit
combination.
You
must
be aware
of the
limita-
tions of the instrument.
In
analyzing the displayed
waveform, you must
consider
the
loading
effect
that the oscilloscope has
on
the
input-
signal source.
In
most
cases
this
loading
effect
is
negligible;
however
in
some
applications, loading caused by the
oscillo-
scope
may
materially alter the
results
obtained.
In
such
cases
you
may
wish
to
reduce
the
amount
of
loading
to
a
negligible
amount
through the use of a probe.
Use
of Probes
Occasionally connecting
the input of
an
oscilloscope to
a signal source loads
the source
sufficiently to
adversely
affect both the operation
of the source
and
the
waveform
displayed on the oscilloscope.
In
such cases
an
attenuator
probe
may
be used
to
decrease both
the capacitive
and
resistive
loading
caused by
the oscilloscope
to a negligible
value.
In
addition
to
providing
isolation
of the oscilloscope
from
the signal source,
an
attenuator
probe
also decreases
the
amplitude
of the displayed
waveform by
the attenuation
factor of the probe. Use
of a
probe allows you
to increase
the vertical-deflection factors of the oscilloscope
to
observe
large-amplitude
signals
which
are
beyond
the
normal
limits
of the oscilloscope
and
plug-in
combination.
Signal ampli-
tudes,
however, must be
limited to the
maximum
allowable
value
of the
probe
used.
Before
using a
probe you must check (and
adjust
if
neces-
sary)
the
compensation
of the
probe
to
prevent
distortion
of the
applied waveform. To
adjust the
probe compensation
control, place the
HORIZONTAL
DISPLAY
switch
to
A, the
Fig.
2-3.
When
compensating
the
probe
it is
adjusted
to
obtain an undistorted presentation of the calibrator
square-waves.
2-3