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Thursday, 18 April 2019

Beam Transit Time and Deflection Plates in CRT

Beam Transit Time and its Effect on the Frequency Limits of Use:

The deflecting voltage Ed has been assumed as constant in the equation. 

D = L tan θ = L eEd ld / mdvox2 (m) (source: Electrostatic Deflection in CRT)

As the deflection voltage applied to the vertical deflecting plates changes during the transit time of the electrons through the horizontal plates, the deflection sensitivity decreases.

Transit time t1 = l/ vox.

The upper limiting frequency is defined as that frequency at which the transit time is equal to one quarter of the period of the voltage applied to vertical plates.
Therefore the upper frequency limit

fc = l/ 4t1 = vox / 4l

The frequency range of the oscilloscopes can be increased by making division of the deflecting plates in to several sections in the path of the electron beam. The voltage to be analysed will be applied to the vertical plates through an iterative net work. The propagation time of this iterative net work corresponds to the velocity of electrons there by the voltage applied to the vertical plates synchronises with the velocity of the beam. The frequency response of oscilloscope can be extended up to 500 MHz and above using this method.

The Deflection Plates in CRT: 

Two sets of deflecting plates are arranged. One pair is mounted in the vertical plane. This pair produces deflection in the horizontal direction. The other pair mounted in the horizontal plane produces vertical deflection. The deflecting plates are flared to allow the beam to pass the deflecting plates without striking them.

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