The flood guns are located just outside the horizontal deflection plates. A cloud of electrons is emitted by each flood-gun cathode. These clouds are combined, shaped, and accelerated by the two control grids, as well as the collimator. The collimator consists of a coating on the inside of the tube. The positive voltage on the collimator is adjusted so the flood-gun electron cloud only fills the CRT viewing screen. The cloud is further accelerated towards the storage mesh and viewing screen by the collector mesh. After passing through the collector mesh, the flood electrons are further controlled by the potentials of the storage mesh and storage layer. The cathode side of the storage mesh is coated with the nonconductive storage material, which is where the pattern to be displayed is stored. Because of the nonconductive property, only a capacitive coupling exists between the storage layer and the storage mesh. This capacitive coupling is required for the storage and erase functions. The rest potential of the storage mesh is approximately + 1 V with respect to the flood-gun cathodes. In the write and erase routines, the potential of the storage layer varies from O V to negative. This is accomplished through the storage mesh and the capacitive coupling.
Storage oscilloscopes are used in applications where the display time at the screen is too short to examine the signals to be measured. If a single-shot signal is to be measured, only one sweep is generated. During this sweep, the screen is excited by the high-energy electron beam. When the beam is suppressed at the end of the single sweep, a phosphorescence remains for some time. The time that the phosphorescence remains visible is dependent upon the type of phosphor used and is referred to as the persistence of the tube. The persistence is the time that the intensity, after the excitation, takes to decay to a level of I/ e of the level attained during excitation (e = 2.72 = base of natural logarithms).
The delayed timebase may start immediately after the main timebase has reached the level at the DELAY potentiometer. But now the following may happen. Assume that the signal to be tested is a pulse train and that the time between two successive pulses is not constant, but varies a little around the set repetition rate. The result will be a somewhat unstable display; this is known as jitter. The time between the first and the second pulses varies a little, as does the time between the second and the third pulses. The third pulse varies twice as much with respect to the first one as the second pulse does. The fourth pulse varies three times as much, and so on.
VARIABLE HOLD-OFF PERIOD
Suppose a series of double pulses must be displayed. The end of the first time-base ramp is reached after pulse 5 and the end of the hold-off period is reached before pulse 6. The second sweep will then be triggered at pulse 6. This means that the following pulses appear at the screen. By making the hold-off period longer, the second sweep will be triggered by pulse 7. As a result, the waveforms during the first and second sweeps will coincide, and the proper picture will be obtained. The same result could have been obtained by shortening the time base sweep by means of the Vernier control, but then the time scale would no longer be calibrated. For this reason, a variable hold-off control is sometimes built into an oscilloscope. However, relatively few oscilloscopes possess this feature, such as the Philips PM 3260 and PM 3265. The hold-off time must be related to the time-base sweep speed. If not, at high sweep speeds the hold-off time would be too long, and the successive sweeps will appear only after a relatively long time. Consequently, a fast sweep would be displayed at a low repetition rate, which would reduce the light output (brightness) to a large extent. For this reason, the range of the hold-off time is automatically set appropriately with the TIME/mv switch.