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The Cathode Ray Oscilloscope


The following should give the student some familiarisation with the function and uses of the cathode ray oscilloscope (C.R.O.).

Consider a simple sine wave electrical signal from some source as in Fig. 1a. If we can arrange things so that this sinusoidal voltage is applied to two horizontal conducting plates then in the region between these plates, the electric field will be alternating with period T seconds. It will increase in strength to a maximum, decrease to zero, turn over, and increase in the opposite direction to an equal maximum, then decrease to zero again, in each period of time T.

Now, if there is a beam of charged particles (electrons) streaming between these horizontal plates, the oscillating electric field there will bend the beam first up, then down, then back to the undeflected position in each time period T. Further, if the beam strikes a plate of material which fluoresces, one would see a spot of light on this plate (screen) which moves vertically up and down with period T.

 Now consider a set of vertical plates, also straddling the electron beam. An electric field applied to these plates will deflect the beam horizontally by an amount proportional to the voltage applied across the plates. If, connected to these plates we have a circuit which generates a linear ramp voltage, periodically, with the same period T, as in Fig. 1b, then the spot on the screen would be forced to start at the left side and move linearly in time across the screen, reaching its maximum travel to the right at time T. The spot would disappear then, and instantaneously reappear back where it started at the left hand side. This “sweep” would repeat every time T. You may wonder how one can easily produce a sawtooth wave at exactly the frequency of the input signal (or if you don’t, you should!). The answer is simple; you use the input signal to trigger the ramp (ie. to start it at its lowest voltage) every time the input voltage reaches a particular value going in a particular direction (ie. increasing or decreasing). In this way, if the input is periodic, then the sawtooth will have the same period, by definition.

The first set of plates, driven with the sinusoidally varying voltages will produce on the screen a spot travelling up and down in simple harmonic motion. If fast enough, it will appear as a solid vertical line. (Figure 2a) The second set of plates, driven with the ramp signal, if fast enough, produces a horizontal line on the screen, as in Figure 2b.

A combination of both sets of plates, one with a sinusoidal driving voltage of period T, the other with a ramp period of period T, will produce on the screen a picture like Figure 2c, (if the two circuits are synchronised so that they start as drawn on the voltage vs. time graphs).

 If the ramp period is now doubled, so that the spot sweeps across the screen in a time 2/T, on the sweep left to right the sinusoidal voltage will complete two full cycles and the picture on the screen will look like Figure 2d. Hopefully, this introduction will have presented an idea of how the C.R.O. functions in displaying a.c. (ie. time-varying) signals on the screen.

The C.R.O. in Detail 

The main part of the C.R.O. is a highly evacuated glass tube housing parts which generates a beam of electrons, accelerates them, shapes them into a narrow beam, and provides external connections to the sets of plates described above for changing the direction of the beam. The main elements of the C.R.O. tube are shown in Figure 3.

1. K, an indirectly heated cathode which provides a source of electrons for the beam by “boiling” them out of the cathode.
2. P, the anode (or plate) which is circular with a small central hole. The potential of P creates an electric field which accelerates the electrons, some of which emerge from the hole as a fine beam. This beam lies along the central axis of the tube.
3. G, the grid. Controlling the potential of the grid controls the number of electrons for the beam, and hence the intensity of the spot on the screen where the beam hits.
4. F, the focusing cylinder. This aids in concentrating the electron beam into a thin straight line much as a lens operates in optics.
5. X, Y, deflection plate pairs. The X plates are used for deflecting the beam left to right (the x direction) by means of the “ramp” voltage. The Y plates are used for deflection of the beam in the vertical direction. Voltages on the X and Y sets of plates determine where the beam will strike the screen and cause a spot of light.
6. S, the screen. This is coated on the inside with a material which fluoresces with green light (usually) where the electrons are striking.


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