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Basic Oscilloscope Patterns
oscilloscope pattern
Assume that a sinusoidal voltage signal is applied to the horizontal deflection plates without applying any voltage signal to the vertical deflection plates, as shown in figure. At point A in time, the voltage is zero so the spot remains undeflected at centre point of the screen. At point B in time, voltage V_{h} is maximum positive so the spot is at the extreme right end on the screen. At point C in time, once again the voltage is zero so the spot comes back to the central position on the screen. At point D in time, the voltage is maximum negative and so the spot is at the extreme left end on the screen. At point E in time, the voltage is zero so the spot returns to the central position of the screen. This way for next voltage cycle the spot again moves from point A to point B on the screen. So we get a horizontal line on the screen. One thing is to be kept in mind that this horizontal line is in the central position vertically as no voltage has been applied to vertical defection plates.
If sinusoidal voltage signal is applied to the vertical deflection plates without applying any voltage signal to horizontal deflection plates then we get a vertical line on the screen of CRO, as shown in figure. This line would be in the central position on the screen horizontally. By this time we have seen what type of pattern we get when sinusoidal voltage signal is applied to horizontal or vertical deflection plates alone. Now we would discuss what happens when both horizontal and vertical defection plates are supplied with sinusoidal voltage signals simultaneously. Let us consider a case, when two sinusoidal voltage signals equal in magnitude and frequency and in phase are applied to both of the horizontal and vertical deflection plates, as shown in figure. At point A in time, voltages at both of the plates are zero so the spot is in the centre of the screen. At point B in time, voltages applied to both of the plates are maximum positive, so the spot appears at the extreme right end in horizontal direction and extreme upward in the vertical direction. At point C in time, again both voltages are zero so the spot moves back to centre of the screen. At point D in time, voltages applied to both of the plates are maximum negative, so the spot appears at the extreme left end in the horizontal direction and extreme downward in the vertical direction. As both of the voltage signals are in phase and equal in amplitude and frequency, so at any time voltage signals applied to horizontal and vertical deflection plates are equal in magnitude as well as in sign. That is why, at any instant movement of the spot is same in horizontal (Xaxis) as well as in vertical (Yaxis) directions. Thus a straight line inclined at 45° to the positive Xaxis is obtained on the screen, as shown in figure. Here it is very important to note that at any time the movement of the spot on the screen is the vector sum of the horizontal and vertical deflections and the horizontal and vertical deflections are proportional to the voltages applied to the horizontal and vertical deflection plates respectively. So if sinusoidal voltage signals, in phase, and equal in amplitude and frequency are applied to horizontal and vertical deflection plates we get a straight line inclined at 45° to the positive Xaxis, as explained before. If amplitude of sinusoidal voltage signal applied to the vertical deflection plates is less than that of the voltage signal applied to horizontal deflection plates, then the deflection of the spot along Yaxis would be less than that along Xaxis direction. So we get a straight line inclined at an angle, less than 45° to the positive Xaxis. When voltage applied to the vertical deflection plates is more than that applied to the horizontal deflection plates, we will get a straight line inclined at an angle, more than 45° to positive Xaxis. Now let us consider a case when two sinusoidal voltage signals applied to the horizontal and vertical deflection plates are of equal magnitude but opposite in phase, as shown in figure. At point A in time, both voltage signals are zero so the spot is at the central position of the screen. At point B in time, voltage applied to horizontal deflection plates is maximum negative while the voltage applied to the vertical deflection plates is maximum positive, so the spot moves a maximum distance to the left and upward, as shown in figure. Similarly at point C in time the spot comes back to the central position of the screen and at point D in time, it goes to maximum right and downward, as shown in the fig. Thus we get a straight line inclined at 135° to the positive Xaxis, as shown in figure. In third case, if the two sinusoidal voltage signals, 90° out of phase and of equal amplitude and equal frequency, are applied to the horizontal and vertical deflection plates, a circle would appear on the screen, a shown in figure. At point A in time, voltage applied to the horizontal deflection plates is maximum positive and the voltage applied to the vertical deflection plates is zero, so the spot moves extreme right end on the Xaxis without any movement along the Yaxis. At point B in time, the voltage applied to horizontal deflection plates is zero but the voltage applied to the vertical deflection plates is maximum positive, so the spot moves maximum in upward direction without any horizontal movement. Similarly the spot moves, for points C and D in time, on the screen, as shown in figure. Thus during one cycle of sine wave, the spot traces out a circle on the screen. 
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