Introduction To Cathode Ray Oscilloscope (CRO)

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Since its invention in 1897, the cathode ray oscilloscope (CRO) has come a long way in being used as a means to detect the presence of electrons.

It is an integral part of modern-day oscilloscopes that are used for measuring various waveforms of electrical circuits. An X-Y plotter plots an input signal against another signal or against time, enabling the study of waveforms, transients, time-based or frequency-based analysis.

A CRO is a complex device consisting of many parts and components. Therefore, before going in depth into it, let’s first understand the Cathode Ray Tube (CRT).

Cathode Ray Tube

A CRT is a vacuum tube that acts as the main part through which the functionalities of a CRO is carried out.

Cathode Ray Tube Internal PartsCathode Ray Tube (Image credit: www.circuitglobe.com)

It consists of several parts:

  • Heater – This part is responsible for heating the cathode.
  • Cathode – On being sufficiently heated, electrons are emitted from the cathode. In order to achieve this, a layer of barium oxide is coated on one end of the cathode.
  • Grid – It is kept at a negative potential and helps control the intensity of the electron beam moving towards the anode. The entire structure is made from nickel.
  • Pre-accelerating anode – Provides acceleration to the anode before entering the Focusing Anode.
  • Focusing Anode – It helps align the incoming electron beam.
  • Accelerating anode – Its job is to again accelerate the newly aligned electron beam. Note here that both pre-accelerating and accelerating anodes are connected to a common positive potential of 1500 volts.

All these components collectively form the electron gun.

The beam then moves towards a set of horizontal and vertical deflection plates. These provide an electric field that acts on the electron beam, rendering them a vertical and horizontal motion.

After passing through this final stage, the electron beam strikes the fluorescent screen to give a glow.

CRTs generally find applications in forming television screens.

Cathode Ray Oscilloscope (CRO)

Now that we have learnt the basics of how a CRT works, let’s move onto learning about a CRO.

As mentioned, a CRO is a device that is used for measuring and displaying different forms of electrical signals.

CRO System (Image credit: www.tutorialspoint.com)

It consists of:

  • Cathode Ray Tube – Through this mechanism, electrons are emitted and controlled to form the desired signal image on the fluorescent screen.
  • Vertical Amplifier – It amplifies the input signal for display on the CRT screen.
  • Delay Line – It provides a certain signal delay that is applied to vertical deflection plates of CRT.
  • Trigger Circuit − It produces a triggering signal for synchronizing both horizontal and vertical deflections of the electron beam.
  • Time base Generator − It produces a sawtooth signal for horizontal deflection of the electron beam.
  • Horizontal Amplifier − It amplifies the sawtooth signal and then connects it to the horizontal deflection plates of CRT.
  • Power supply − It produces both high and low voltages. The negative high voltage and positive low voltage are applied to CRT and other circuits respectively.

Through this procedure, a CRO displays the applied input signal on the screen of CRT, providing signals in the time domain.

Applications:

  • While numerous, a CRO can be used for the following purposes:
  • To determine the amplitude of a waveform.
  • Comparison between the phases and frequencies of electrical signals.
  • Help measure capacitance and inductance values.
  • In the medical field and medical trials, it can be used for monitoring various body parameters like heartbeat rates and nervous reactions.

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