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Thursday, 18 April 2019

Vertical Deflection System in CRO

Vertical Deflection System with Block diagram

The vertical deflecting system of a cathode ray oscilloscope has critical requirements. The functions and requirements are listed below: The vertical deflecting system has to provide the following facilities : 

1. Amplify and reproduce the input signal. The amplifier must have good fidelity That is it must amplify the input signal within the limits of its bandwidth, without effecting the amplitude, frequency and phase. 

2. It has to isolate the cathode ray tube from the input signal. That is it has to act as a buffer. 

3. It must have provisions for the different modes of operation. 

The functional block diagram of the vertical deflecting system of a general purpose oscilloscope is shown in figure.

Block diagram of vertical deflection system

The vertical deflecting system consists of the following elements: 

1. Probe. 
2. Selector for input signal. 
3. Attenuator to adjust the gain of amplifier. 
4. The vertical amplifier that amplifies the signal.

(a) The Probe : 

it connects the vertical amplifier to the circuit in which the waveform is to be observed on CRO. There are many types of probes. The general purpose probe shown in the block diagram above ;as ire probe. This passive probe has a resistor which acts as the attenuator for the signal. The capacitor is to act as the compensator. These two components are used in the probe Tip of the probe connects the circuit. Ground connection is established through a clip connector. The other end of the probe will be connected to the vertical input terminals of the cathode ray oscilloscope using a BNC connector or the like.

(b) Input Selector : 

The input selector is nothing but a single pole three way switch. It's pole is connected to the input terminals of the vertical amplifier. The two extreme positions namely way one and way three are connected to the attenuator through the capacitor and directly, respectively. The middle position of the 'switch that is the second way is connected to earth. In the first position of the switch the input is directly connected to the input attenuator. This position is marked as DC. The d.c input signals will directly appear at the attenuator. In this position both and d.c and a.c components are available after due attenuation at the input of the amplifier. This mode is convenient for measurement of total instantaneous valued of signal voltages.

In the third position the input signal passes through a capacitor before it reaches the attenuator. Hence any D.C is blocked. Only the alternating component of the signal will appear at the attenuator. In this mode we can measure alternating voltages superimposed over d.c.

While changing the switch from the a.c position to d.c was two connects the attenuator to ground. This discharges any stored charge in the attenuator.

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