Oscilloscope and Cathode Ray Tube Notes

The Oscilloscope

• An oscilloscope is a monitoring device used to analyze and visualize varying voltages.

Cathode Ray Tube (CRT)

• The heart of the oscilloscope is the cathode ray tube (CRT), which is similar to a television screen, used to display the graph of varying voltages versus time.
• Components:
  • Heater
  • Cathode
  • Grid
  • Anodes
  • Deflection plates (X and Y)
  • Electron gun
  • Screen (phosphor-coated)
  • Black graphite coating on the inside wall of the tube
• Terminals:
  • Y-input terminal (high)
  • Common input terminal (sometimes connected to earth)
  • X-input terminal

Using an Oscilloscope

1. Connection: An oscilloscope is connected between the two points across which the voltage is to be studied, similarly to how a voltmeter is connected.
2. Graph Production:
   • A beam of electrons, also known as cathode rays, is produced at the back of the tube using an electron gun.
   • The electron gun consists of a hot current-carrying filament that shoots electrons towards a high positive potential in which a hole is drilled.
   • Electrons are accelerated to the anode, and those passing through the hole continue in a narrow beam to hit a screen coated with a fluorescent material that emits light when struck by electrons.
   • The beam can be deflected by two pairs of charged parallel plates.
     • Vertical plates: responsible for horizontal deflection.
     • Horizontal plates: produce vertical deflection.
   • Once deflected, the cathode rays strike different points on the screen.
   • The vertical plates are supplied with voltages controlled by an internal circuit called the timebase, which controls the horizontal deflection of the beam, causing it to move repeatedly at a constant rate from left to right.
   • As it moves, the beam is also deflected vertically by the horizontal plates across which the voltage we wish to measure is maintained.
   • Thus, the point of contact of the beam with the screen traces out a visible graph of voltage versus time.
   • The pattern of shining fluorescent particles which makes up the graph is called the trace.
   • The oscilloscope responds very quickly to sudden changes in voltage and so is ideal for examining alternating voltages.
   • The traces are called waveforms.
3. Grid:
   • The grid on the outside of the screen is similar to the lines on graph paper and serves the same purpose.
   • The horizontal scale has units of time, and the vertical scale has units of voltage.
4. Adjustments:
   • Vertical Voltage Scale: Can be adjusted by means of a multi-position switch to provide the clearest graph with the best scale, otherwise, the trace might disappear or vary undetectably from the straight line of $V = 0$.
     • One can read from the position of the switch the number of volts that correspond to each large division of the grid. Thus, readings of peak value may be taken.
   • Horizontal Time Scale: Can be adjusted by means of a multi-position switch to provide the clearest graph with the best scale; otherwise, the sinusoidal trace may contain hundreds of cycles crammed into the screen, or a broad trace with no perceptible pattern may appear.
     • One can read from the position of the switch the number of seconds that corresponds to each large division of the grid. Thus, readings of periodic time may be taken.

Demonstration

• Connect a '6 V, 50 Hz' supply and a simple cell to the input terminals of the oscilloscope.
• Operation of a transparent cathode-ray tube to see more clearly the structure of the electron gun.
• An e/m tube, so called because of its use in determining the charge-to-mass ratio of the electron (Thompson's Experiment), is available in the lab. Note the high voltages needed to both accelerate and deflect the electron beam.
• The tube has an oblique fluorescent screen and a flat beam which allows the path of the beam to be clearly seen, but you are not 'seeing' electrons.