Draw neat block diagram of CRO and explain its functioning comment on role of delay line in CRO.

Answer: CRO -The cathode ray oscilloscope is an extremely useful and versatile laboratory instrument used for studying wave shapes of alternating currents and voltages as well as for meas­urement of voltage, current, power and frequency.

The block diagram of CRO is shown below :   

 A general purpose CRO  consists of following parts.

  1. Cathode ray tube                                                                         

  2. Vertical amplifier

  3. Delay line

  4. Time base circuit

  5. Horizontal amplifier

  6. Trigger circuit

  7. Power supply.

Cathode Ray Tube - The instrument employs a cathode ray tube (CRT), which is the heart of the oscilloscope. It generates the electron beam, accel­erates the beam to a high velocity, deflects the beam to create the image, and contains a phosphor screen where the electron beam eventually becomes visible.

Vertical Amplifier- The input signal is applied to vertical amplifier. The gain of this amplifier can be controlled by VOLT/DIV knob. Output of this amplifier is applied to the delay line.In Vertical Amplifier,The input signals are amplified by the vertical amplifier. Usually, the vertical amplifier is a wide

band amplifier which passes the entire band of frequencies.

Delay Line- Delay Line as the name suggests that, this circuit is used to, delay the signal for a period of time in the vertical section of CRT. The input signal is not applied directly to the vertical plates because the part of the signal gets lost, when the delay Time not used. Therefore, the input signal is delayed

by a period of time.

Time Base (Sweep) Generator- This produces a saw – tooth waveform that is used as horizontal deflection voltage of CRT. The rate of rise of positive going part of sawtooth waveform is controlled by TIME/DIV knob. The sawtooth voltage is fed to the horizontal amplifier if the switch is in INTERNAL position. If the switch is in EXT. position, an external horizontal input can be applied to the horizontal amplifier.

Horizontal Amplifier - In Horizontal Amplifier,The saw tooth voltage produce by the time base circuit is amplified by the horizontal amplifier before it is applied to horizontal deflection plates.

Trigger Circuit- In theTrigger Circuit The signals which are used to activate the trigger circuit are converted to trigger pulses for the

precision sweep operation whose amplitude is uniform. Hence input signal and the sweep

frequency can be synchronized.

  1. Power supply:

The Power supply voltages require by CRT, horizontal amplifier and vertical amplifier are provided by the power supply block.

 Power supply block of oscilloscope is classified in to two types

(1) Negative high voltage supply

(2) Positive low voltage supply

Low voltage supply is required for the heater of the electron gun for gen­eration of electron beam and high voltage, of the order of few thousand volts, is required for cathode ray tube to accelerate the beam. Normal voltage supply, say a few hundred volts, is required for other control circuits of the oscilloscope.

 Role of delay line - When the delay line is not used, the initial part of the signal is lost and only part of the signal is displayed. To counteract this disadvantage the signal is not applied directly to the vertical plates but is passed through a delay line circuit.This gives time for the sweep to start at the horizontal plates before the signal has reached the vertical plate. The trigger pulse is picked off at a time after the signal has passed through the main amplifier.The sweep generator delivers the sweep to the horizontal amplifier and the sweep starts.Hence the sweep starts well in time, since the signal arrives. The delay line produces a delay of 0.25 microsecond so that the leading edge of the input waveform can be viewed even though it was used to trigger the sweep.

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