Cathode Ray Oscilloscope

Cathode Ray Oscilloscope (C.R.O)

The Cathode Ray Oscilloscope is a common electronic device used to study waveforms, measure voltage and short intervals of time.
Structure of C.R.O
Cathode Ray Oscilloscope (C.R.O)
Courtesy of SPMPhysics
Component Function
Heater supply
(filament)		 Heat the cathode to release electrons
Cathode
(negatively charged)		 Produce electrons on heating
Grid Control the amount of electrons reaching the screen
=> brightness control
Anode
(positively charged)		 Accelerate electrons to high velocity
Y-plates Deflect electron beam vertically (up or down)
X-plates Deflect electron beam horizontally (left or right)
Fluorescent screen Glows when struck by electrons to produce bright spot on screen

Y-gain & Time Base

(1) Y-gain or Voltage gain

This amplifies the deflection of electron beam. The amount of deflection depends on the input voltage at Y-plates.

Y-gain also determines the sensitivity of the oscilloscope.

Unit: Volts per division, V/div

A Y-gain of 1.0 V/div means that for an input of 2.5 V, the deflection would be 2.5 divisions (= 2.5 / 1.0 = 2.5).

(2) Time base

When time base is on, a voltage is applied to the X-plate.

Time base controls the speed at which the electron beam sweep across the screen from left to right horizontally, then jump back to left plate and the sweeping repeats. This is done by altering the frequency of the time base.

Effect of different time base setting
Unit: millisecond/division (ms/div) or Hz

Conversion between ms/div and Hz

Time base frequency is 25 Hz means the electron beam sweeps across the screen 25 times in one second.

Note that each screen of C.R.O has a fixed number of divisions, i.e. 8. This means each time the electron beam sweeps across the screen, it moves 8 divisions.

Therefore, in one second, it sweeps across 25 * 8 = 200 divisions.
So time base = 1 / 200 = 0.005 s/div = 5 ms/div.

Note that Y-gain and Time base are similar to the scale of a graph. Y-gain is for y-axis while Time base is for x-axis.

Application of C.R.O

(1) Measuring Voltage

When time base is off, CRO can actually be used as a voltmeter.

The voltage to be measured is applied to the Y-plates via the Y-input terminals. By doing so, an electric field is set up between the Y-plates, and the electron beam would sweep towards the positively charged Y-plate. The deflection of electron beam by electric field is proportional to the applied voltage.

By adjusting Y-gain, the height of trace can be magnified so that its magnitude is easily to be read off.
(2) Displaying Voltage Waveform

When time base is on with a suitable frequency, a steady waveform of any input voltage can be displayed on the screen. The period of each wave and number of complete wave displayed can be varied, and it is determined by the formula:
(3) Measuring Short Intervals of Time

When time base is on, C.R.O can be used to measure a short interval of time. One of the examples is to measure the time taken for sound to travel a distance and hence find the speed of sound.
Procedure:
  • Setup the apparatus as shown.
  • The microphone first produces a pulse of sound (first pulse on C.R.O), and this pulse is reflected by the wall and is again received by the microphone (second pulse on C.R.O).
  • The distance t between two pulses on C.R.O is actually the time taken for the sound to travel from the microphone to the wall and back.
  • Thus, speed of sound = 2d / t.  
Note that the height of second pulse is smaller than that of first pulse because some energy is lost to the medium and the reflecting wall. 
Screen Displays
When time base is on and input is an a.c supply.
In summary, 
  • Increase the Y-gain / time base / frequency of a.c supply would reduce the size of trace or waveform.
  • Summary on screen display:
Example

(a) Explain how the electron beam can be deflected to any spot on the fluorescent screen.
(b) A waveform is displayed on the C.R.O screen as shown.
The Y-gain control and time base are set to 4 V/div and 1.5 ms/div.
(i) State its peak voltage. 
(ii) Calculate its frequency.
(iii) Below Figure shows a new display of the same waveform when the C.R.O is adjusted.
Describe in detail what changes were made on the controls on the C.R.O to produce the new displayed waveform.
Answer:
(a) Potential differences are applied to X and Y plates respectively to deflect the electron beam horizontally or vertically.
(b)(i) Vp = 4.0 * 4 = 16 V
(ii) T = 1.5 * 10^(-3) * 4 = 6.0 * 10^(-3) s
f = 1 / T = 167 Hz
(iii) Changes observed:
  • Peak Voltage is halved => Y-gain is doubled from 4.0 V/div to 8.0 V/div
  • Trace is moved up by 2 divisions => Y-shift is adjusted to shift the trace upwards by 2 divisions.






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