24. THE USE OF THE OSCILLOSCOPE

Oscilloscopes range from simple general-purpose instruments to highly sophisticated pieces of equipment.

THE CATHODE RAY TUBE

Basically, however, any oscilloscope consists of a cathode ray tube with associated electronic circuitry which enables us not only to measure signal waveform's, amplitude, frequency etc. but also to see the actual waveform displayed on a screen enabling us to check the signal for distortion - an important factor in circuit repair and adjustment.

figure

Fig. 226 shows a simple schematic diagram of the heart of any oscilloscope: the cathode ray tube, which consists of an electron gun, envelope and screen.

The electron gun contains a cathode which, when heated, emits electrons. These electrons are attracted from the cathode by a cylindrical anode carrying a very high positive voltage. The electrons are then accelerated and move at high speed in direction of the screen.

In fig. 226 you see that, in practice, the cylindrical anode is in two parts, called focussing anode an accelerating anode. By adjusting the potential difference between the two anodes, the electron-flow is focussed to a small spot at the point where it reaches the screen. The screen is coated with fluorescent material. So that a bright spot shows the point of impact.

The brillance of the spot can be adjusted by means of a negatively biased control-grid placed between the cathode and the anode. If the control-grid is made more positive or negative, a correspondingly greater or smaller quantity of the electrons is attracted by the anode, thus varying the intensity of the spot on the screen.

Between the anode and the screen there are two sets of deflection plates set at right angles to each other: two parallel plates used for horizontal deflection of the spot - the so called X-plates, and two parallel plates for the vertical deflection of the spot - the so called Y-plates.

When a voltage is applied between the two X-plates, the electron beam will be deflected in horizontal direction and thus the spot on the screen will move horizontally. The amplitude of this deflection is a function of the amplitude of voltage applied to the plates and the direction of the deflection depends on the polarity. In the same way the Y-plates are used for the vertical deflection of the electron beam.

If a sine-wave voltage is applied to the Y-plates, the electron beam will be alternately attracted and repelled by the plates and the spot will move up and down the screen.

Due to the persistence of the fluorescent material on the screen face, a solid vertical trace line will appear whose length will depend on the magnitude of the sine-wave voltage. Similarly, if the same voltage is applied instead to the X-plates, a horizontal trace will appear.

FUNCTION OF THE “SCOPE”

In practice, the signal waveform to be studied is applied to the Y-plates, whilst the X-plates are employed to provide a variable, linear time-base by means of a sawtooth waveform generator (see fig. 227).

fig. 227

The voltage delivered by this generator increases linearly with time. Hence, the spot on the screen moves with constant speed from left to right and its position is a function of time. At the end of this deflection period, the voltage returns to its original value very quickly.

The frequency of the time-base voltage can be varied by means of a time-base or velocity control. When an ac-voltage is applied to the Y-plates, it then appears in its familiar sine-wave form on the screen of the cathode ray tupe through the combination of the waveforms acting on the electron beam.

Finally, as can be seen in fig. 227. amplifiers are connected to the deflection plates, so that small signals can be seen more clearly and measured more accurately. In most oscilloscopes the amplifiers can switched out for the measurement of large signals, which in themselves will give sufficiently large traces. It is of course important when a periodic signal is displayed on the screen that at the beginning of each period of the time-base the signal always starts at the same place of the screen. This can be done in two ways:

SYNCHRONISATION OR TRIGGERING.

A synchronisation circuit ensures that the frequency of the time-base (or a multiple of it) is exactly the same as the frequency of the signal.

The frequency of the time-base must, therefore, be manually adjusted until the signal on the screen does not move anymore.

A triggered time-base is started by the signal. This means that, after a given time, the time-base waits for the next positive or negative edge of the signal. This system is commonly used in modern oscilloscopes.