Digital Storage Oscilloscope

DIGITAL STORAGE OSCILLOSCOPE

In the digital storage CRO the data can be stored in the memory. The memory can store data as long as required without degradation. In this CRO, the waveform to be stored is digitised and then stored in a digital memory. The conventional cathode ray tube is used in this oscilloscope hence the cost is less.

The power to be applied to memory is small and can be supplied by small battery. Due to this the stored image can be displayed indefinitely as long as power is supplied to memory.

The Block digram of digital CRO is shown in figure 5.59

The input signal is applied to the amplifier and attenuator circuit. The digital CRO uses same type of amplifier and attenuator circuitry as used in the conventional CRO. The attenuated signal is then applied to the vertical amplifier.

The vertical amplifier output is digitised by an analog to digital converter to create a data set that is stored in the memory. The data set is processed by the microprocessor and then sent to the display.

The main requirement of ADC in the digital CRO is its speed, while in digital voltmeters accuracy and resolution were the main requirement.

The digitised output needed only in the binary form and not in BCD. The successive approximation type of A/D converter is most oftenly used in the Digital CRO.

The sampling rate and memory size are selected depending upon the duration and the waveform to be recorded. Once the input signal is sampled, the ADC digitises it. The signal is then captured in the memory. Once it is stored in the memory, many manipulations are possible as memory can be read out without being erased.

Acquisition Methods in Digital CRO

In Digital CRO, it is necessary to capture the digital signal and store it. Depending upon the application, there are three different acquisition methods used in the digital CRO. They are

(i) Real time sampling

(ii) Random repetitive sampling

(iii) Sequential repetitive sampling.

Real time Sampling

In this method, in response to single trigger event, the complete record of 'n_s' samples is simultaneously captured on each and every channel. From these samples recorded in a single acquisition cycle, the waveform is displayed on the screen of digital storage oscilloscope.

The key features of this method are,

(i) Display and analysis of waveform can be carried out at later stage while the signal gets recorded in memory at an earlier stage.

(ii) It is very easy to capture the signals.

(iii) Truly simultaneous capture of multiple signals is automatic.

This method can be used in a continuously repeating mode but each waveform displayed is captured from a single acquistion cycle. The larger memory and fast sampling rate plays an important role in the real time sampling.

The higher sampling rate is required to capture long time interval signal capturing. This is possible due to large memory. The sampling theorem helps to select the proper sampling rate.

Random Repetitive Sampling

The bandwidth limited to ƒs/4 in real time sampling. The main disadvantage of this is increasing bandwidth means increasing sampling rate and fast sample rate digitizers and memory are very expensive.

In this method, repeated real time data acquisition cycles are performed. Each acquistion cycle produces random time interval t_d between tigger point and sample clock it is shown in figure 5.60.

The time between the samples from that capture is 't_s' with an offset of 't_d' from the trigger point.

Each successive acquistion is plotted at its measured random offset. This progressively fills the picture of the waveform.

Sequential Repetitive Sampling

An oscilloscope having bandwidth 20 to 50 GH, need very fast sweep speed settings. In such case, random repetitive method can not work satisfactorily Hence sequential repetitive sampling is used.

In this method, one sample value per tigger event is captured at a cerefully controlled time delay 't_ds' after the triggering pulse as shown in figure 5.61.

This delay is increased by small amount t_se after each point is captured. The single sample acquisition cycle is repeated till the entire waveform has been plotted. In this method the increase in delay which is t_ds is the effective sampling time. This method used only in microwave bandwidth digital oscilloscopes.

Advantages of Digital Storage Oscilloscope (DSO)

(i) The storage time is infinite

(ii) It is easier to operate and has more capability

(iii) It has more flexibility, the number of traces that can be stored and recalled depends on the size of the memory.

(iv) Cursor measurement is possible

(v) The waveform information displayed on the screen such as amplitude, frequency, maximum, minimum, etc.

(vi) Keeping the records is possible by transmitting the data to computer system.Where the further processing is possible.

(vii) Brighter and bigger display with colour to distinguish multiple traces.

(viii) Slow traces like temperature variation can easily stored.

(ix) The built in interfaces such as Rs 232 serial communication, paralled port, IEEE 488 Bus are available.

Application of Digital Storage Oscilloscope (DSO)

(i) Measurement of various parameters of alternating signal such as RMS, average, crest factor, duty cycle etc.

(ii) Measurement of Frequency, time period, phase, phase difference for periodic signal.

(iii) Measurement of transient parameters delay time, rise time, peak overshoot etc.

(iv) Mathematical operations such as addition, subtraction, integration etc of waveform is obtained.

(v) To obtain p - v diagram, B - H curves, Hysteresis loop etc.

(vi) For transmission line analysis to obtain standing waves, modulation characteristics etc.