Storage Oscilloscope Notes

The storage oscilloscope finds many applications in the field of mechanical engineering and bio-medical science. It can represent very slow sweep signals. The persistence of the phosphor in conventional CR tubes ranges from few micro-seconds to a few seconds. In applications where the persistence of the screen is smaller than the rate at which the signal sweeps across the screen, the start of the display will disappear before the end of the display is written. The Lecture Notes of Storage Oscilloscope are explained below.

The storage oscilloscope or variable persistence CRO has persistence times greater than few seconds extended over hours. In these types the persistence of the screen can be adjusted to match the sweep time. This storage oscilloscope has all the electrodes commonly available in a conventional CRO. In addition to the above there are some special electrodes. The constructional details of the storage type CR Tube are shown below in Figure.
Storage Oscilloscope block diagram explanation

The storage mesh or storage target, mounted just behind the phosphor screen is a conductive mesh covered with a highly resistive coating of magnesium fluoride. A narrow focused beam of high energy electron gun called the 'write gun' is used to write the information to be stored. The write gun etches a positively charged pattern on the storage target by knocking of secondary emission electrons. Because of the excellent insulation properties of the magnesium fluoride coating this positively charged pattern remains exactly in the same position out the storage target where it was first deposited. An electron beam deflected in the conventional manner both in the horizontal and vertical directions traces the waveform pattern on the storage target.

The stored pattern may be made available for observation at a later time using two special electron guns, called flood guns. The flood guns are located inside the CRT in a position between the deflecting plates and the storage target. They emit low-velocity electrons over a large area towards the entire screen. When the flood guns are switched on in the viewing mode, low energy electrons are sprayed on the screen. The electron trajectories are adjusted by the -collimation electrodes' which constitute a low voltage electrostatic lens system. Hence the flood electrons cross over the screen area. The collector mesh collects most of the flood electrons. Therefore they never reach the phosphor screen. In the area near the stored positive charge on the storage target, the positive field pulls some of the flood electrons through the storage mesh and these electrons continue to bombard the phosphor. The display of the CRT will be an exact copy of the pattern which was initially stored on the target. The display will be visible as long as the flood guns continue emission of low energy electrons. To erase the pattern which is etched on the storage mesh, a negative voltage is applied to the storage target. This neutralises the stored positive charge.

In order to obtain variable persistence, the erase voltage is applied in the form of train of pulses instead of a steady d.c. voltage. The rate of erasure can be varied by varying the width of the pulses in the train. The variable persistence control is made available on the front panel of the CRO. It is the width control of the erase pulse generator.

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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