Working Principle of Sampling Oscilloscope:
A sampling oscilloscope is used to examine very fast signals. It is similar in principle to the use of stroboscopic light to look at fast mechanical motion. Samples are taken at different portions of the waveform, over successive cycles, and then the total picture is stretched, amplified by relatively low bandwidth amplifiers, and displayed as a continuous wave on the screen.
The advantage of a sampling oscilloscope is that it can measure very high speed events, which require sweep speeds of the order of 10 ps per division, and amplifier bandwidths of 15 GHz, using instruments having bandwidth several orders lower.
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The disadvantage of a sampling oscilloscope is that it can only make measurements on repetitive waveform signals, a continuous display for frequencies in the 50-300 MHz range depending upon the design of the oscilloscope. Above this range of frequencies, Sampling Techniques must be employed to obtain satisfactory display.
The display may be made up from as many 1000 dots of luminescence. The vertical deflection for each dot is obtained from progressively later points in each successive cycle of the input waveform as shown in below figure principle of sampling oscilloscope.The horizontal deflection of the electron beam is obtained by the application of a staircase waveform to X deflection plates.
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| Working principle of sampling oscilloscope |
The sampling oscilloscope is able to respond and store rapid bits of information and present them in a continuous display.It is this ability that enables the sampling oscilloscope to side step the usual limitations in conventional high frequency oscilloscopes which have limited sensitivity and bandwidth and small display size.The sampling techniques immediately transform the input signal into a lower frequency domain, where conventional low frequency circuitry is then capable of producing a highly effective display.
The sampling oscilloscopes can be used beyond 50 MHz into the UHF range around 500 MHz and beyond up to 10 GHz. However, it should be understood that sampling techniques cannot be used for the display of transient waveforms.
The below figure shows the block diagram of sampling oscilloscope. The input signal is delayed and then sampled by a diode gate. The sampled signal is saved on a capacitor store, then amplified and fed to the vertical plates.Unity feedback is used from the amplifier to the sampling diode gate. This ensures hit the voltage on the capacitor store is only increased by the incremental value of the input voltage change, between each sample.
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| Sampling oscilloscope block diagram |
The staircase is reset after a certain number of steps, typically 100 to 1000, and it then 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 CRT, and it is used to move the spot across the screen in a series of rapid movements.
Figure working principle of sampling oscilloscope shows the screen display.The sample frequency used in sampling oscilloscopes can be as low as one hundredth of the signal frequency, so a signal frequency of 1 GHz needs an amplifier bandwidth of only 10 MHz.
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Delayed sweep in sampling oscilloscopes:
Many laboratory quality oscilloscopes include a delayed-sweep feature. This feature increases the versatility of the instrument by making it possible to magnify a selected portion of the undelayed sweep, measure waveform jitter or rise time, and check pulse time modulation, as well as many other applications.Delayed sweep is a technique that adds a precise amount of time between the trigger point and the beginning of the scope sweep.
When the scope is being used in the delayed-sweep mode, the start of the delayed sweep can range from a few microseconds to perhaps 10 seconds or more. The delayed sweep operation allows the instrument user to view a small segment of a waveform, for example, an oscillation or ringing that occurs during a small portion of a lower-frequency waveform.
Sometimes the delayed sweep feature is used for convenience to allow triggering at some other point than at the leading or trailing edge. However, in some situations, a measurement is possible only if the delayed-sweep feature is used.
For example, suppose the part of a waveform that is to be measured is too far from the only available trigger point to permit a stable display on CRT screen.The problem can be solved by using delayed sweep to trigger at the only available trigger point and then starting the sweep at the point of interest.
Although there are a few exceptions, delayed sweep is normally a feature of dual time base oscilloscopes which have two completely separate sweep generators.One sweep functions as the main sweep and the other serve as the delayed sweep.The main sweep is initiated by a trigger pulse at the leading edge of pulse 1 shown in the figure below.
Suppose we wish to observe in detail a portion of the waveform near its trailing edge by expanding the waveform and using a higher sweep speed, for example, a sweep speed of 0.1μs/cm. As can be seen in the figure sampling oscilloscope block diagram, the portion of the waveform of interest to us is completely off the screen to the right when the sweep speed is set to 0.1μs/cm. To observe the portion of the waveform that is of interest at the higher sweep speed, we must use the delayed-sweep feature and reset the sweep speed to 5μs/cm.
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| CRT display with sweep speed equal to 5μs/cm |
Basically, the delayed sweep works as follows.The main sweep is initiated by a trigger pulse at time t0 , shown in the below waveforms figure.This time corresponds to the leading edge of pulse 1 in the above figure.The main sweep generator ramp, which is applied to a comparator along with a voltage from a delay control circuit, increases linearly until it trips the comparator at time t1.
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| CRT display with sweep speed equal to 0.1μs/cm |
When the comparator changes states, the delayed sweep generator is triggered, which intensifies a portion of the original display.Adjusting the delay time should intensify the portion of the waveform that is of interest. The exact front panel adjustments that are necessary to display the expanded portion of the waveform on the CRT screen depend on the oscilloscope being used.
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| Delayed sweep triggering waveforms |
Some oscilloscopes have an alternate sweep separation control so that two waveforms are displayed: the original waveform, with the portion of the waveform of interest intensified, and a second expanded waveform, as shown in the below figure.By adjusting the horizontal mode to channel B, we can now increase the sweep time so that we can view the portion of the waveform that is of interest.
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| Delayed sweep measurement |
Conclusion:
Now here we have discussed working principle & block diagram of sampling oscilloscope. You can download this notes to your device in pdf, ppt format.
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