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Sunday, 28 April 2019

Working Principle of Sampling Oscilloscope

A sampling oscilloscope is a modified oscilloscope. It is meant for avoiding the difficulties of bandwidth requirements. The problem of reduction in the intensity of the image on the screen of the C.R.O, while working with high frequency signals is also averted in the sampling oscilloscope. It improves the high frequency performance of the oscilloscope.

These are used for continuous display for frequencies above 300 MHz. Sampling techniques are to be used to obtain good display of the signal. The display is made up of dots of as many as 1000 in number. Vertical deflection for each dot is obtained from the later points progressively in each successive cycle of input.

The horizontal deflection of the beam is obtained by supplying the stair case wave to the horizontal deflecting plates.

Principle of Sampling Oscilloscope:

In a sampling oscilloscope as the name itself indicates the input signal is not directly applied to the vertical amplifier. The waveform is reconstructed taking several samples from the recurrent cycles of the input signal. Therefore the beam will not have the writing speed of the input signal. Instead it will be moving step by step, or point by point utilising number of samples that may be around 1000 or more. This not only relaxes the bandwidth requirement of the vertical amplifier but also improves the intensity of image formation.

Explanation of Block Diagram of Sampling Circuitry of a Sampling Oscilloscope:

Description of the block diagram: The sampling gate is the first stage that receives the input signal. It receives the sampling pulse from the voltage comparator. The output of the sampling gate drives the vertical amplifier. The vertical signal out of the vertical amplifier is given to the vertical deflecting plates as usual. The block diagram is shown in Figure.
Sampling Oscilloscope Block Diagram

The trigger input is given to the blocking oscillator. The output of the blocking oscillator is given to the ramp generator. The ramp generators frequency is controlled by the time scale switch. The output of the ramp generator is given to the voltage comparator. The voltage comparator receives the input from the attenuator also which is connected the output of the range selector. The stair case generator receives its input from the comparator. The output of the stair case generator goes to the horizontal deflecting circuit.

Working of Sampling Oscilloscope: 

The input signal is received and is reconstructed in the sampling oscilloscope. The input signals voltage will be measured at very short intervals of time when the sample pulses occur. The sample pulses will turn on the sampling circuit only during the short intervals. The spot on the screen of the cathode ray tube will be positioned vertically to those voltage levels only. As the second and subsequent samples are taken during the next cycles of the input voltage the position of the spot will represent the different amplitudes of signal, over the different sampling periods. At the same time the horizontal plates will be applied with increasing voltage corresponding to the sample instants. This results in the formation of the pattern following point by point movement of the spot on the screen of the C.R tube.

The sampling gate is connected to the input terminals. The input signal forward biases the diodes of the gate. Thus the gate input capacitance is offered to the input terminals. So the gate input capacitor gets charged to the input voltage. This voltage is amplified by the vertical amplifier and is applied to the vertical deflecting plates. This occurs only when the sampling gate is permitted by the sampling pulse. The sampling is done in synchronisation with the input signal. Therefore a delay circuit is provided in the vertical amplifier. The sweep circuit is triggered by the input signal.

On receiving the trigger pulse the blocking oscillator starts a perfectly linear ramp voltage. This goes to the comparator. The comparator compares the output of the staircase generator with the amplitude of the ramp voltage. When the ramp voltage amplitude is equal to the amplitude of the staircase voltage, the staircase voltage generator is permitted to advance one step. At this instant a sampling pulse is applied to the sampling gate. Therefore only at this instant the sampling input is taken by the vertical amplifier. It is amplified and is applied to the vertical deflecting plates.

It is to be noted here that the displacement of the beam horizontally is synchronised with the trigger pulses. As explained earlier the trigger pulses determine the instants of sampling. The final image on the screen of the C.R. tube is determined by the size of the steps of the staircase generator. The larger the step the more will be distance between the consequent horizontal spots that make up the image.

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