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Deflection Sensitivity of CRT

  • The deflection sensitivity of a cathode ray tube is the deflection on the screen given by the beam in meters per volt of the applied voltage.

    This is different from the deflection sensitivity of CRO. The CRO's deflection sensitivity includes the gain of the amplifiers between the input terminals and the deflection plates.

    The deflection sensitivity of the cathode ray tube of any CRO can be obtained by applying known voltage to the deflecting plates directly. Connection to the deflection plates can be made from terminals provided for this purpose on the frame of the CRO and are accessible from the protecting covers, through a window like opening.

    The following block diagram shows the arrangement for the determination of the deflection sensitivity of the CR Tube.

    (a) Vertical deflection sensitivity :

    To obtain vertical deflection sensitivity, the internal sweep can be used to produce the horizontal line on the CRO. The CRO is to be set up for operation and the horizontal line is to be adjusted in line with one of the horizontal lines on the graticule. A d.c power supply is taken and its voltage is applied between the vertical deflecting plates. A multimeter with good sensitivity or a digital voltmeter is connected across the power supply to indicate the voltage applied to the vertical deflecting plates. On application of the voltage, the horizontal line shifts either up or down depending on the polarity of the applied voltage on the deflecting plates. The voltage is adjusted to get a convenient displacement of the horizontal line. Now the displacement in meters can be noted from the marking on the graticule. From this the deflection sensitivity can be calculated and expressed as meters/volt.

    (b) Horizontal deflection sensitivity :

    In this case the internal sweep is not necessary. The sweep mode switch can be put to external mode. The vertical deflecting plates will be supplied with a low voltage alternating voltage. The screen will display a vertical line. This line is aligned with one vertical line on the graticule. A d.c voltage from a power supply is applied to the horizontal deflecting plates. The voltage applied is measured with a multimeter or the like. Application of the voltage to the horizontal deflecting plates will shift the vertical line either to left or right. This depends on the polarity on the deflecting plates of the applied voltage. The voltage will be adjusted to get convenient deflection over the scale. The voltage required for that displacement will be noted. From that the deflection sensitivity can be calculated and expressed in m/V.

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