Question

Describe with block diagram; the principle of operation of digital storage oscilloscope.

Answer 1

The block diagram of DSO is shown above. Sampling rate may be as high as 1,00,000 samples per sencod. For such high sampling rates, a fast conversion ADC is requried. Flash ADC convertor are used at the cost of decresed resolution with increase in sampling rate. The memory size is related to the amount of horizontal segment of the trace that can be divided into one sweep of the time base.

If the 10 bit ADC is used then the frequency response of 25khz is obtained.The total memory storage capacity is 4096 for single channel, 2048 for two channels and 1024 for 4 channels each. The selection of sampling rate and memory size depends on the type of waveform being recorded. A continuous storage oscilloscope consist of feature called “Pre-Trigger view”. This mode is useful when failure occurred.

The DSO has three modes of Operation: 

i) Roll mode 

ii) Store mode 

iii) Hold or save mode 

i) Roll mode :This mode is used to observe the fast varying signal.In this mode input signal is not trigger at all.The stored signal is rolled across the screen from right to left. 

ii) Store mode:This mode is most commonly used it called as refresh mode.Once trigger pulse is obtained the memory write cycle is initiated ADC converts input signal to digital code and fed to memory.When memory is full the memory write cycle stops.The digitally stored signal is converted back to the analog using DAC.When the next trigger is received , the memory is refreshed with the next signal. 

iii) Hold and save mode:The modern DSO operate on automatic refresh system.When a new sweep signal from the time base circuit is received. Then the new contents are over written on the old contents.If it is required to retain the particular signal in the memory then overwritting should be stopped.This obtained by using Hold and Save button.