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1
Introduction
The cathode ray oscilloscope is an instrument designed to display
the voltage variations, periodic or otherwise, that are met with in
electronic circuits and elsewhere.
The word is an etymological hybrid. The first part derives
from the Latin, to swing backwards and forwards; this in turn
is from
oscillum,
a little mask of Bacchus hung from the trees,
especially in vineyards, and thus easily moved by the wind. The
second part comes from the Classical Greek
skopein,
to observe,
aim at, examine, from which developed the Latin ending-
scopium,
which has been used to form names for instruments
that enable the eye or ear to make observations. For some
reason the subject of the design and use of oscilloscopes is
generally not called oscilloscopy but oscillography, from oscillo-
and
graphein,
to write.
There are other types of oscilloscope besides those using
cathode ray tubes. For example, pen recorders, ultra-violet chart
recorders and XY plotters are all oscilloscopes or oscillographs of
a sort, as indeed is 'Fletcher's Trolley' of school physics fame.
However, this book is concerned mainly with cathode ray
oscilloscopes, together with the increasing number of similar
instruments using LCD (liquid crystal display) technology.
Representing a varying voltage
The basic principle of oscillography is the representation, by
graphical means, of a voltage that is varying. The voltage is
plotted or traced out in two-dimensional Cartesian coordinates,
named after Descartes, the famous French seventeenth-century
philosopher and mathematician.
Figure 1.1 shows the general scheme for the representation of
any two related variables. Both positive and negative values of
each variable can be represented. The vertical axis is called the Y
axis, and the horizontal the X axis. The point where the axes
cross, where both X = 0 and Y = 0, is called the 'origin'.
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2
Oscilloscopes
-,
-3
-2
-1
-
0
1
2
3
X
-1
I
-2
Figure
1.1
may
he
two
different scales, even diilcrrnt units,
for
graphical purposes
Cartesian
or
graphical cnordinarcs.
The
horizontal and vertical axes
Any point is defined by its
X
and
Y
coordinates.
Thus
the
point
P
is
the point
(3,
2),
because
its
distance
to
the right (called
its
'abscissa'
or
X
coordinate)
is
3
units and its distance
up
(called
its
'ordinate' or
Y
coordinate) is
2
units.
Figtirt.
1.2
is
an
rxan~plt~
of
a
graph
ploltcd
on
Carlesian
coordinates and
shows
an
imaginary
plot
of
t.hc tcmprrat.urr
W
I
1Jan
I
(midnight)
midday
-5
Figure
1.2
Fictional
plot
of
teniperatiire in tirst
wcck
ot
January.
An
exarnple
of
a
graph
where the horironral and vertiral axcs
arc
to
different scales and
in
different units

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