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Oscilloscopes, 5th Edition by Ian Hickman

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5
Using
oscilloscopes
It seem superfluous to say that, when using
an
oscilloscope
to
view a waveform, one should choose an instrument appropriate
to the ,job in hand. Yet,
as
explained in the course
of
this chapter,
besides the more obvious requirements ('does it have
a
band-
width wide enough
to
display my signal faithfully?', 'is
it
sensitive
enough to see the very small signal
I
wish to view?') there are
quite a few other considerations that are a little less
obvious.
Some have already been pointed out, notably in Chapter
3,
and
others will become apparent in the course
of
this chapter. We
shall also consider
the case where there
is
no choice and one
is
faced with the task
of
trying
to
obtain some useful information
about
a
waveform with an oscilloscope which
is
hardly adequare
for the purpose.
Use
of
probes
()iiestions
that
people
HC'W
to
using
oscilloscopes oftcn
ask
arc:
'Do
1
always need
a
probe?
If
not, how
do
1
know when
1.0
use
one
and
whrn
not?' The first part
of
Chapter
4
should
havc
provided
a
good
deal
of'
insight into this:
if
you
are
still pu~~1t.d
il
might
be
worth reading again.
But
for
a
short, simple answer, the
author's advice
is
always
to
use a
1O:l
passive divider probe
(correctly
set
up
for
the oscilloscope you are using) as
a
matter
of
habit.
If.
owing
to
the attendant attenuation factor
of
10,
the
signal
you
wish to view gives insufficient vertical deflcction even
with the
Y
input setting
at
its most sensitive position, then it will
be necessary
to
consider whether
it
is possible to depart
from
your standard practice
of
using such a probe.
For example,
if
you are using
a
metre or
so
of
general-purpose,
audio screened
lead
to
connect the signal
to
be viewed
to
the
oscilloscopc,
the
total capacitivc loading on thc circuit
may
wcIl
be
scvcrnl hundrcd picofarads. This will
he
of
no consequence
if
looking
at,
say,
the
secondary voltage
of
a
mains transformer,
and
gerierally
acceptable
for
viewing the output
of
a
hi-fi
amplilier
over the whole
audio
range. However,
200
pF
has a reactance
of

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