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Oscilloscopes, 5th Edition by Ian Hickman

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9
How
oscilloscopes
work
(1):
the
carat.
Many
logic analysers and some
DSOs
use magnetically dellecled
c.r.t.s either monochrome
or
coiour. This is the type
of
display
technology
used
in
TV
scts. The operation
of
TV
type
rubes
is well
covrred
in
ol.her
volurnes
in t.he Newnes
series,
10
which
the
reader is rufcrrud
for
further information. In
c.r.t.
storagr
oscilloscopes,
he
cathode
ray
tube is basically similar
to
the
electrostatically deflected type
of
tube described in
this
chapter,
but with
a
special
screen
or the addition
of
one or
more
storage
mcshcs: storage tubes
arc
dcscribcd in Chapter
11.
This chapter deals solely with the high-performance
c.r.t.s
using
electrostatic deflection, used in non-storage oscilloscopes. Such an
oscilloscope may also include
a
digital storage capability,
as
in
Figure
1.5,
and
the same c.r.t. is then used for
both
the
conventional reaI-time display and for the storage
mode
display.
The cathode ray tube is the main component of an oscilloscope.
A
cathode
ray
tube consists basically
of
an electrode assembly
mounted in an evacuated
glass
vessel (Figure
9.1
).
The electrodes
perform
t
tic
following
functions:
0
A
triode assembly geiieratcs the electron beam, originalIy
callccl
thrb
'cathode,
ray'.
Jt
consists
of
a
cathodc
I<
heated
by
a
filament
F,
a
control grid
G
and
ilic first beam-accclcrarion
elcc.1
rod(:
(
1
).
0
An
clcctrodc
(2)
focuscs
the
beam.
0
The
kairi
is
IIirii
furilltar
ac.c.clcraIrd
before
rv~ching
Lhr
deflection plates.
0
'l'hc~ vertical deflection plates change the direction of the
beam
in proportion
10
the potential difference between them.
When
this is
zero.
i.c.
the
two
p1atc.s
arc at
the
same potential, the
beam
passes
through undeflectcd. The vertical deflection
plates are
so
called because they can deflect the
beam
in the
vertical direction,
so
that
it
hits the screen at
a
higher or a
How oscilloscopes work (1)" the c.r.t. 177
gun
/---------~ ~ detlect
ion
screen
1E) ~ _~ . N
"~-
! .o t~ i~ , ~- o
I~ I ~ la i
>~ '-
,,
I I '
! ,
impact of
FK 1 12 I
I I
i I
, ,
i i
I
coating
photons
glass
Figure 9.1 Basic oscilloscope (electrostatic) cathode ray tube (courtesy Enertec
Instrumentation Ltd)
lower point; they are actually mounted horizontally above and
below the beam, as shown in Figure 9.1. Similarly the
horizontal deflection plates permit the beam to be deflected to
left or to right.
9 The deflected beam then hits the fluorescent coating on the
inner surface of the glass screen of the c.r.t. The coating
consists of a thin layer of 'phosphor', a preparation of very fine
crystals of metallic salts deposited on the glass. Further details
of phosphors are given in Appendix 1. The 'spot' or point of
impact of the beam glows, emitting light in all directions
including forwards. Modern c.r.t.s are aluminized, i.e. a thin
layer of aluminium is evaporated on to the rear of the coated
screen. The electrons pass through this with little retardation,
causing the phosphor to glow as before, but now the light
emitted rearwards is reflected forwards, almost doubling the
useful light output.
The potential at the focus electrode is adjusted to obtain a very
small round spot on the end of the tube. Unfortunately, if no other
control were provided, it would often be found that the focus
control setting for minimum spot width was different from that for
minimum spot height. This is avoided by providing an astigmatism
control. In the case of a simple cathode ray tube this consists of a

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