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.