Storage cathode ray tube has several limitations. They are as follows,

1. There is a short duration of time, in which it can preserve a stored waveform, so the wave formmay lose.

2. Trace of storage tube is not as fine as that of a normal CRT.

3. Writing rate of the storage tube is less than that of a conventional CRT which in turn limits the speed of the analog storage oscilloscope.

4. It is more expensive than a conventional CRT and requires additional power supplies.

5. Only one image can be stored. For comparing two traces they are to be superimposed on the same and displayed together.

6. Digital storage oscilloscope is used to limit these limitations. In DSO, the waveform to be stored is digitized, stored in a digital memory and retrieved for display on the storage oscilloscope.

Stored waveform is continuously displayed by repeatedly scanning it. Therefore a conventional CRT can also be used for the display. The stored display can be displayed continuously as long as the power is applied to the memory which can be supplied from a small battery. Digitized waveform can be analyzed by oscilloscope or by reading the contents of the memory into the computer. Display of the stored data can be possible both in amplitude versus time or x-y modes. In DSO, fast memory readout is used for CRT display in addition to this a slow readout is also possible

Which is used for development of hard copy externally.

Figure shows the block diagram of DSO which consists of,

1. Data acquisition

2. Storage

3. Data display.

Data acquisition is earned out with the help of both analog to digital and digital to analog converters, which are used for digitizing, storing and displaying analog waveforms. Overall operation is controlled by control circuit which is usually consists of microprocessor.

Data acquisition portion of the system consist of a Sample-and-Hold (S/H) circuit and an analog to digital converter (ADC) which continuously samples and digitizes the input signal at a rate determined by the sample clock and transmit the digitized data to memory for storage. The control circuit determines whether the successive data points are stored in successive memory location or not, which is done by continuously updating the memories. When the memory is full, the next data point from the ADC is stored in the first memory location writing over the old data. The data acquisition and the storage process is continues till the control circuit receive a trigger signal from either the input waveform or an external trigger source. When the triggering occurs, the system stops and enters into the display mode of operation in which all or some part of the memory data is repetitively displayed on the cathode ray tube.

In display operation, two DACs are used which gives horizontal and vertical deflection voltage for the CRT Data from the memory gives the vertical deflection of the electron beam, while the time base counter gives the horizontal deflection in the form of staircase sweep signal. The screen display consist of discrete dots representing the various data points but the number of dot is very large as 1000 or more that they tend to blend together and appear to be a smooth continuous waveform. The display operation ends when the operator presses a front-panel button and commands the digital storage oscilloscope to begin a new data acquisition cycle.

Various mode of operation of DSO

1. Roll mode:

Roll mode is the most useful mode and has been available on digital storage oscilloscopes (DSO) from the earliest stage. It is fundamentally different from a conventional scope display and it leads in nicely to the other operating modes of digital storage oscilloscopes. This mode is used to display very fast varying signals, clearly on the screen. The fast varying signal is displayed as if it is changing slowly, on the screen. In this mode, the input signal is not triggered at all. The stored signal is rolled slowly from right to left across the screen.

1. Refresh mode:

This is the most commonly used mode. In this mode, the input initiates a trigger circuit. This initiates the memory write cycle. The digital data is transferred to the memory. When the memory is full, the write cycle stops. Using digital to analog converter, the memory data is converted to analog and then displayed on the screen. When the next trigger occurs the memory is refreshed.

1. Single Shot mode or Save Mode:

This is also called automatic refresh mode. When new sweep signal is generated by time base generator, the old contents get overwritten by new one. If a particular signal is to be stored then by pressing hold or save button, overwriting can be stopped and previously saved signal is locked.

Single shot events, such as the waveform of an explosion are transient in nature and are lost very quickly. The observer cannot see such events, unless the waveform is photographed or stored. Such events can be stored in memory of digital storage oscilloscope and reading the memory rapidly and repetitively the continuous waveform can be obtained.