Digital Storage oscilloscope

                                          Digital Storage oscilloscope:   

Storage cathode ray tube has several limitations. 
as follows,

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

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 is possible in both amplitude versus time and 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 as 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 is 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.

Digital Storage Oscilloscope:- (DSO )

• DSO use the digital memory. It can store data as required with out degradation.

• DSO also uses for complex processing of the signal with high speed with the hepl of digital signal processing circuits .

• in this A/D converter use to create the data that is stored in microprocessors memory, and data sent to display on screen

•  DSO convert analog  in to digital form using to A/D convertor ,it stores digital data in memory.

• then processes the signals and to be display on screen.

• the wave form is stored in digital ,advantage of using DSO ,that stored data can be used to visualize the signal at any time.

 

 Advantages :- 

1. Allows for automation.
2. In this,slow traces like the temperature variation across a day can be recorded 
3. With colour Bigger and brighter display, to distinguish multiple traces
4. peak detection

 Disadvantage:-

Digital Oscilloscope is the limited refresh rate of the screen.

                                    SAMPLING OSCILLOSCOPE:

An ordinary oscilloscope has a B.W. of 10 MHz the HF performance can be improved by means of sampling the input waveform and reconstructing its shape from the sample, i.e. the signal to be observed is sampled and after a few cycles sampling point is advanced and another sample is taken.

 The shape of the wave form is reconstructed by joining the sample levels together. The sampling frequency may be as low as 1/10th of the input signal frequency (if the input signal frequency is 100 MHz, the bandwidth of the CRO vertical amplifier can be as low as 10 MHz). 

As many as 1000 samples are used to reconstruct the original waveform.

Fig  : block diagram of a sampling oscilloscope.


 The input is applied to the sampling gate. The input waveform is sampled whenever a sampling pulse opens the sampling gate. The sampling must

be synchronized with the input signal frequency. The signal is delayed in the vertical amplifier, allowing the horizontal sweep to be initiated by the input signal. The waveforms are shown in fig

At the beginning of each sampling cycle, the trigger pulse activates an oscillator and
 a linear ramp voltage is generated. This ramp voltage is applied to a voltage comparator which compares the ramp voltage to a staircase generate-When the two voltages are equal in amplitude, the staircase advances one step and a sampling pulse is generated, 
 which opens the sampling gate for a sample of input voltage.

The resolution of the final image depends upon the size of the steps of the staircase generator. The smaller the size of the steps the larger the number of samples and higher the resolution of the image.

Difference between digital storage oscilloscope and conventional storage oscilloscope:

Digital Storage

Oscilloscope (DSO):

1. It can store the given signal indefinitely as long as the small amount of power is supplied to the memory.

2. It always collects the data and stops when triggered.

3. It employs normal CRT, hence the cost of the tube is much cheaper than the storage tube used in ASO.

4. It can produce bright image even for high frequency signals.

5. In this oscilloscope, time base is generated, by a crystal clock.

6. It has higher resolution than ASO.

7. It has less operating speed than ASO.

8. Because of aliasing effect the useful storage ' bandwidth is limited.

Conventional Storage Oscilloscope

(Analog Storage Oscilloscope (ASO)):

1. In this oscilloscope heavy amount-of power is to be supplied to the storage CRT.

2. It collects the data only after triggering.

3. The cost of the tube is costlier than the storage tube used in DSO.

4. It cannot produce bright image for high frequency signals.

5. In this oscilloscope, time base is generated by a ramp circuit.

6. It has lower resolution than DSO.

7. It has high operating speed than DSO.

8. It doesn't have aliasing effect