How oscilloscopes work (3): storage c.r.t.s 223
In this way the limits of definition are dictated by the fineness of
the writing beam only.
You will remember that bistable target operation depends on a
nearby collector to collect secondary electrons. The discontinuous
nature of the phosphor deposition allows the use of the
conducting foil on which the target is deposited ('storage target
backplate' in Figure 11.4) as a collector. This foil is so extremely
thin as to be transparent, so that light emitted from the phosphor
can be seen through it by the observer. Secondary electrons
knocked off the target will therefore be attracted through the
gaps in the phosphor to the higher potential collector.
Perhaps we should consider briefly why the primary stream of
flood electrons does not itself go directly through these gaps to the
collector, thus defeating the whole purpose of the arrangement.
scattered
ill I!/
jphosphor
Ill ~ particles
storage-
I~ ~
!il
target
....
. oo^ll l
faceplate
storage-
Ill !/I target
target dots Ill III backplate
ceplate
(b)
....
Figure 11.4 Types of phosphor target (courtesy Tektronix UK Ltd)
224 Oscilloscopes
The reason is that the flood electrons arrive with a fair amount of
kinetic energy and are not easily diverted at the last moment to the
minute gaps between phosphor particles. By contrast, secondary-
emission electrons have much lower energy and therefore move at
much slower speed, which makes them more manoeuvrable. It is
incidentally the difference between the high energy of the landing
electrons and the lower energy of the secondaries which is
converted into heat and light emission from the phosphor.
Let's pause at this point to summarize briefly what we have
seen of phosphor-target storage tube construction and operation.
These tubes have a target composed of phosphor which can be
written- that is, lifted by a writing beam to a higher potential-
and will then attract electrons from a floodbeam whose landing
energy is partly converted to light and partly used to dislodge
secondary electrons. The secondary electrons find their way
through gaps in the target to the storage-target backplate which
acts as collector. The floodbeam is therefore used in the first place
to make the written areas visible, but it also has the effect of
shifting the target from whatever voltage it may have been left at
by the writing beam or erase pulse to the upper or lower stable
point, making this storage tube a bistable one. The floodbeam
originates from a floodgun and is deliberately dispersed to cover
the whole target area. The writing beam comes from the writing
gun which is so negative with respect to the target that when the
writing beam lands it causes much secondary emission, thus
lifting the target voltage. The writing beam is intensity controlled,
focused and deflected in the usual way.
With this basic picture in mind we must now go a little more
deeply into the problems of target construction, since this will
considerably increase our understanding of storage tube behav-
iour. These are problems which are of concern at the design stage,
but also have important effects on operating characteristics.
A suitable target material must be chosen. Then it must be
decided whether to deposit particles according to Figure 1 1.4(a)
or (b). Nowadays the semicontinuous method shown in Figure
1 1.4(a) is so predominant that we will base our further discussion
on this, although very similar problems would be encountered
with the other method. Having made both these decisions we can
How oscilloscopes work (3)" storage c.r.t.s 225
then also vary the thickness of this target layer, and this has a
surprising number of repercussions which are presented at length
in the remainder of this chapter. If your main interest is
transmission tubes and you are reading this chapter merely to
understand bistable principles I suggest you now move on to the
section on transmission tubes.
As target thickness increases a number of factors are affected in
a beneficial way. Luminance increases fairly linearly, since the
presence of additional material (and the higher operating voltage
that this permits) generates additional light. Resolution increases
rapidly at first: when the target is only molecules thick, wide
spaces exist between particles and these fill in as thickness
increases. Predictably, once a certain thickness has been reached,
the increase in resolution levels off. But perhaps the most
significant improvement resulting from greater target thickness is
the increase in contrast, as shown in Figure 11.5.
Against this catalogue of benefits resulting from increased
target thickness we must set one factor which, after reaching a
peak, decreases again. This is the stable range of collector
operating voltages, and to understand what it is, and why it is so
important to us that we sacrifice a great deal of contrast to it, we
must consider one aspect of this type of storage tube which has
hitherto been ignored: the possibility of leakage from the target
surface to the backplate on which it is deposited.
preferred
design ~ .~ I
oov ,
/ --I 3:1
"i
target thickness ~__
Figure 11.5 Effect of target thickness (courtesy Tektronix UK Ltd)
226 Oscilloscopes
In unwritten screen areas there is in fact some leakage through
the target, to the collector, which sits at a high positive voltage,
lifting the phosphor surface above the lower stable point (LSP)
and causing a slight amount of light emission because of the
increased landing energy of the floodbeam. It might seem to
contradict basic theory that the target can rest at a point above
LSP, since the secondary emission ratio is then less than unity and
it ought to gain electrons, but this effect is balanced by the
migration of electrons through the target to the collector.
The amount of leakage will vary from point to point across the
screen, since the phosphor layer is randomly semicontinuous, but
some leakage will be observed almost everywhere and we can
surmise that the rest potential might look something like the solid
line RP in Figure 1 1.6. This will cause light emission varying
target voltage ..... on same scale ....
USP
)ver
RP
ARP
collector voltage
9
UWL
t-
..... OL
ls, Xover
LSP J !
''
I .... j,
w VV"
distance across screen
.... L
y
RT
floodgun
cathode
USP upper stable point FP fade positive level
RP rest potential UWL upper writing limit
ARP average rest potentialOL operating level
LSP lower stable point RT retention threshold
Figure 11.6 Random points whose rest potential exceeds the first crossover will
'fade positive' to the upper stable point (courtesy Tektronix UK Ltd)
How oscilloscopes work (3): storage c.r.t.s 227
across the screen in a correlated manner. From normal viewing
distances these variations average out and we simply observe an
average background light level, corresponding to the average rest
potential (ARP).
The solid line RP is no more than an artist's impression, but
given such wide variations across the target, some points will
inevitably exceed the first crossover level, and these points will
therefore move to the upper stable point (USP), a process which
is often called 'fading positive'. Being individual, randomly
distributed bright dots on a microscopic scale we can again see
only their contribution to the average background light level.
Although on theoretical grounds one might wish to exclude
these written dots from the calculation of the average
rest
potential, in practice this is not possible. The ARP is a purely
theoretical value which cannot be measured directly since the
target is floating. We assess the average rest potential on the basis
of average light emission, and when making such light measure-
ments we are bound to include the written dots as well as those
in various unwritten states.
The full picture, then, is that dot by dot across the screen the
rest potential varies in a random manner, causing a correspond-
ing slight light output, with the exception that all those dots
which happen to exceed the first crossover level will fade
positive and emit the written light level. Only the average of all
these light contributions can be perceived on a macroscopic
scale, and from this average light level we can deduce the
average rest potential.
The situation is illustrated in Figure 11.6, in a purely qualitative
way, for the condition where the collector voltage is set to a
typical operating level, OL. Naturally, as the collector voltage is
varied up and down, the amount of leakage also varies and the
RP curve will shift up and down to some extent.
If we set the collector to increasingly positive levels, a point will
be reached where spreading of the written trace occurs because
areas adjacent to it are so near the crossover that capacitive
effects or local dielectric breakdown are sufficient to make them
fade positive. This collector voltage level is known as the upper
writing limit, UWL. At some still higher level, so much of the RP
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