Mumbai University > Electronics Engineering > Sem3 > Electronic Instruments and Measurements

Marks: 10M

Year: Jun15, Dec14, Jun14, Dec13

Traditional storage CROs have several limitations like short storage duration, low data writing rate, expensive than a conventional CRT, and the inability to store multiple images. Digital Storage Oscilloscope (DSO) 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.

Below drawn is the block diagram of a DSO. It consists of data acquisition, storage, and data display blocks.

Working –

Digital storage oscilloscopes work in various modes which are discussed below.

1. Roll Mode – It is the most basic mode of operation which is similar to that of a general purpose CRO. When an input is applied, its trace is displayed on the screen. A user can use this mode to keep an eye on the waveform and its various characteristics.

2. Refresh Mode – It is used when the sample rate of a waveform becomes too high, and when the waveform of interest is repetitive or nearly so. The DSO produces a stale, triggered display with a higher sweep time.

3. Single Shot Mode – The DSO is in an inactive state in this mode, displaying the last trace captured, until a given sequence of events take place.

4. Equivalent Time Mode – It basically arranges the ADC to work on limited sweeps, so as to allow particular signals to sync properly and give out a clearer trace.

Additionally, there are three acquisition modes in a DSO, which are explained briefly below - Sample - In this acquisition mode, the oscilloscope samples the signal in evenly spaced intervals to construct the waveform. This mode accurately represents signals most of the time. (can result in aliasing)

Peak Detect - In this acquisition mode, the oscilloscope finds the highest and lowest values of the input signal over each sample interval and uses these values to display the waveform. (to display narrow pulses, while noise can be higher in this mode)

Average - In this acquisition mode, the oscilloscope acquires several waveforms, averages them, and displays the resulting waveform. (Used to reduce random noise).

Working of a DSO is similar to that of a CRO, save for the involvement of memory, as is described below.

• 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 continue 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 consists of discrete dots representing the various data points, but the number of dots is so large 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.

Applications of DSO –

1. Take cursor and pulse width readings
2. Measuring rise time and propagation delay
3. Implementing math functions like subtraction and addition
4. Acting as a simple signal tracer, a DSO enables technicians to probe electronic device’s individual connections and components, to determine the malfunctioning part.
5. In measuring the functions of the individual component of the device, the DSO locates where an expected signal is incorrect or absent.
6. The DSO can also measure components’ minor variations in operations and alert the technician of the need for fine-tuning or replacement.
7. To prevent erroneous replacement of parts, the DSO also helps technicians identify the parts that are still working.