How oscilloscopes work (3)" storage c.r.t.s 233
threshold' (WT). Unlike all other collector voltage limits (FP,
UWL, RT), this one is not a limit due to basic constructional
features of the tube; it is dependent on the beam velocity which
we specified.
For such a specified velocity, the writing threshold represents
the lower limit of the collector voltage operating margin to which
we referred earlier. Neither can we operate successfully above the
upper writing limit since trace spreading occurs. This defines the
collector operating range and is shown in Figure 11.9. A writing
speed specification is only realistic if it puts the writing threshold
in approximately the position shown in Figure 11.9, giving a
usefully large operating range.
target voltage.- -
USP- m m
("" ~~ ._
e') L_
...................... ,i ....
_J
lstX --r~ over ARP RP .............
]
distance across screen
--on same scale ..... collector voltage
FP
UWL
OL
WT
RT
floodgun
cathode
USP upper stable point FP fade-positive level
RP rest potential UWL upper writing limit
ARP
average rest potential OL operating
level
LSP
lower stable
point W'I-
writing threshold
RT
retention threshold
Figure 11.9 As Figure 11.6, but showing the writing threshold WT (courtesy
Tektronix UK Ltd)
2
34
Oscilloscopcs
Now to the other effects
of
departing from t.he normal
collector operating
level.
We
said
(hat
as
the
collector voltage
is
raised, the
ARP
goes
up. Thcrcforc the light
level
or
the
unwritten area will increase,
Rut
also,
since the upper stable
point follows the collector voltage
up,
the
brightness
of
the
written trace increases. The converse
is
true when the collector
voltage is decreased. We
must
consider whether, on balance,
these effects produce traces with more
or
less contrast, and
whether,
if
one has the choice, it is more important to get the
maximum possible contrast or the maximum possible absolute
light output. (Contrast, as defined here, means the brightness
ratio
of
written to unwritten areas.) The brightness
of
the
unwritten areas increases more rapidly with increased collector
voltage than the brightness
of
the written trace,
so
the contrast
becomes poorer. On the other hand, with increasing ambient
light, the contrast decreases,
hut
it
decreases least
if
the c,r,t..
light output is high,
because the ambient light cannot then
swamp the tube light
as
easily.
Which
is
prcIchrablc?
To
see the trace
al
all,
we need contrast
-
and the more we have, ihc hcitcr.
But
ir
turns
out rhar. for
rliffrren
I
a
rnbirn
I
I
jgh
t
i
rig
conditions di ffcrcnt collector voltages
will give best contrast,
so
110
hard-ancl-fasr
rule
is
possible.
Phorograpliy,
of
course, takes place
in
total darkness as
the
camera shuts out
all
amhicnt Iight. and would therefore benelit
from
a
low collector voltage.
Changes in collector volrage,
as
we have seen, affect writing
speed, absolute light output and contrast. They
also
affect tube
life. We can summarize by saying that
increased
collector voltage
will
increase
writing speed and absolute light output, and wilI
decrease
contrast and tube life cxpecrancy
-
and vice versa. If you
wish
to
favour one
of
these factors you
can
adjust the collector
accordingly. But remember that whenever you depart from the
normal
OL
voltage
in
either direction
you
are moving away from
the
centre
of
thc operating range which we tried
to
make large to
give
long,
t
roiihlc-frcc periods bctwwn rccalibraiions.
It
has already been
said
thai thy improvernrni
in
wrilirig speed
which can be achieved with higher collector voltage is only
Inarginal. There are
two
other techniques, howcvcr, which
arc
How oscilloscopes work (3): storage c.r.t.s 235
capable of increasing the writing speed by a factor of 10 or more.
These will now be discussed.
To understand how they work, we must first visualize what
happens when the beam moves faster than the maximum writing
speed and fails to store. In such a case, the dwell time-intensity
product is not enough to raise the target voltage above the first
crossover, and as soon as the writing beam is passed, the
floodbeam begins the destructive process of moving the target
back to the rest potential. Nevertheless, the writing beam did
raise the target above its rest potential. The secret of the two
techniques is to make use of this charge pattern before the
floodbeam can destroy it.
The first technique is useful on repetitive sweeps, and is called
the 'integrate' mode. By stopping the floodbeam altogether, the
destructive process can be halted. Any charges laid down by the
writing beam will remain on the target, if not indefinitely, at any
rate for minutes. If the signal is repetitive, successive beam
passages will scan the same target areas and will add to the charge
pattern. This is a cumulative process which must eventually lead
to the point where the written target areas cross the first
crossover. If the floodbeam is then restored it will move these
areas to the written state and the trace will be seen.
But imagine now that we wish to store a single transient, some
unique event, at a speed exceeding the normal writing speed.
Since we cannot repeat the event, the integration technique is
useless. Yet even that one sweep did leave
some charge behind.
The second technique, called 'enhance' mode, again attempts to
salvage the situation. A positive pulse is applied to the collector,
Figure 11.10, of such amplitude that capacitive coupling will lift
the whole target by just the amount needed to bring the written
area above the first crossover. The floodbeam will then imme-
diately set to work separating the written and unwritten potential
further. We maintain the positive pulse long enough to ensure
that at its end the written areas do not drop back below the first
crossover. The curvatures recall the fact that the floodbeam is
most effective at voltages where the secondary emission ratio
departs most from unity, and floodbeam action slows down as a
of 1 is approached.
236 Oscilloscopes
OL
USP
first
crossover
target
ARP-~ !? ~
"'-
T
beam
---~adjustable
.j-"
i " J"J
Time
passage
enhance pulse
...J
collector
/
J
s
.J
s
J
/
written target
, ~2ms '
,-~ ~,v I
unwritten target
/
Figure 11.10 Enhance mode can increase storage writing speed by a factor of
ten (courtesy Tektronix UK Ltd)
Figure 1 1.10 also makes the point that immediately after the
beam passage the floodbeam starts removing the laid-down
charge. The enhance pulse must therefore be applied as soon as
possible- in other words, as soon as the sweep is completed. But
on slow sweep speeds, say 5 i~s/div or slower, even this may be
too late. The enhance pulse will only rescue the later portions of
the trace while those near the beginning of the sweep will already
have been partly or wholly destroyed by the floodbeam.
Nevertheless, if enhancing were that simple one would have to
ask why the technique is not made a permanent feature of the fast-
sweep storage, giving at a stroke a tenfold improvement in writing
speed. But Figure 1 1.10 is oversimplified in an important respect.
The average rest potential is a fictitious level, and the actual target
rests over a broad range of levels. When the writing beams adds a
charge to this, the written areas, too, will end up over a broad
range of levels. There will therefore be no one correct amplitude of
enhance pulse which can raise all the written, and none of the
unwritten, areas above the first crossover.
In fact, the smaller the charge left behind by the writing beam,
the more likely it will be that even with optimum enhance pulse
How oscilloscopes work (3): storage c.r.t.s 237
amplitude some written parts will remain unstored, and some
unwritten parts will become stored. The exact amplitude then
becomes a matter of experimentation until the user subjectively
feels that he or she has achieved the best compromise, making for
clearest visibility.
When we said that the enhance technique allowed a tenfold
increase in writing speed, this was meant as a guideline only. In
any given situation it depends on the kind of compromise the
user still finds acceptable. (Luckily, the interpretative powers of
eye and brain far exceed that of any computer.) By contrast, the
integrate technique really has no upper speed limit; it just
depends on whether you can afford enough time to integrate
long enough to accumulate enough charges to reach the first
crossover. In cases where the signal repetition rate is 1 Hz or so
and the required sweep speed very fast, this can become a
question of operator patience.
The next topic in this section is the erase process used in
phosphor-target tubes. Basically, the erase pulse is a negative
pulse applied to the collector, which capacitively moves the
whole target negative. The aim is to move the written portions
from the upper stable point to below the first crossover, after
which the floodbeam can complete the erasure. But there are two
problems. The first arises from the fact that sooner or later we will
have to return the collector back to its normal operating level,
and if we do this too fast we will capacitively move the target
back up. This is true even if the negative pulse was long enough
to give the floodbeam a chance to stabilize the target at the rest
potential, because the voltage separating rest potential and first
crossover is much smaller than that between first crossover and
operating level through which the collector must move. The
solution is to make the trailing edge of the erase pulse so slow
that any capacitive coupling effects on the target can be
countered by floodbeam action.
The other problem with erasing is that when small written
areas are surrounded by large unwritten areas, and the target is
capacitively lowered, the unwritten areas will move to a potential
which is so greatly negative that the floodbeam is totally repelled
from the target. The small written areas are in effect then
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