Sampling Oscilloscope

Definition: The sampling oscilloscope is a special type of digital sampling oscilloscope which is used to examine a very fast signal. It works on the principle of stroboscopic light in which sample is taken at different portions of the waveform, over successive cycles, and then the total picture is stretched, amplified by relatively low bandwidth amplifiers, and display as the continuous wave on the screen.

The advantage of a sampling oscilloscope is that it can measure very high-speed event and uses instruments which have lower bandwidth. The disadvantage of the sampling oscilloscope is that it can only make measurements on the repetitive waveform signals, a continuous display for frequencies in the 50 – 300MHz range depends on the design of the oscilloscope. Above this range of frequencies, sampling techniques must be used for obtaining the satisfactory display.

The sampling technique immediately transforms the input signal into the lower frequency domain, where the low-frequency circuitry is then capable of producing a highly effective display. But the sampling technique cannot be used for the display of the transient wave.

The sampling oscilloscope can respond and store rapid bits of information and present them in a continuous display. The sampling oscilloscope can be used beyond 50MHz into the UF range around 500 MHz and beyond up to 10GHz.

Block Diagram of Sampling Oscilloscope

The block diagram of the sampling scope is shown in the figure below. The input signal is delayed and then sampled by the diode gate. The sampled signal is saved on a capacitor store, then amplified and fed to the vertical plates. The unity feedback is used from the amplifier to the sampling diode gate. Thus ensure that the voltage on the capacitor store is only increased by the incremental value of the input voltage change between each sample.

block-diagram-of-sampling-oscilloscope-

The staircase waveform is reset after a certain number of steps, typically 100 – 1000, and then it starts again. Therefore up to about 1000 points are used to create the waveform on the screen. The staircase waveform also feeds the horizontal plates of the cathode ray tube, and it is used to move the spot across the screen in the series of rapid movement.

principle-of-sampling-oscilloscopeThe sampling frequency used in sampling oscilloscope can be as low as one hundredth of the signal frequency, so a signal frequency of 1GHz needs an amplifier bandwidth of only 10Mhz.

Delayed Sweep in sampling Oscilloscope

It is a special feature which increases the versatility of the instrument. It magnifies the selected portion of the undelayed sweep, measure waveform jitter or rise time, and check pulse modulation as well as many other applications. It is a technique that added an accurate amount of time between the trigger point and the starting point of the scope sweep

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