TEXT 11
OSCILLOGRAPH
1. A cathode-ray tube is an essential part of all oscillographs and TV receivers. Consider now the way a cathode-ray tube is employed in the oscillograph. The primary purpose of the oscillograph is to convert electric signals into visual images. A simplified diagram of the oscillograph is shown in Fig. 7.
2. The oscillo graph comprises, besides the cathode- nay tube, the scan ning, deflection and synchronizing cir cuits. Like any electronic device,it is supplied with a dc voltage.
3. The voltage of the signal inves tigated enters the vertical deflection input of the ampli
fier On leaving the amplifier, the amplified signal reaches the vertical deflecting plates, creating a potential difference between them. Passing between the plates, the electron beam is deflected from its initial direction, the amount of deflection depending on the voltage of the output signal. The higher is the instant magnitude of the voltage investigated, the more is the spot deflected from the centre of the tube screen.
4. The scanning voltage supplied by the scanning circuit is fed to the horizontal deflecting plates. Under the action of this voltage, the electron beam is made to sweep along a horizontal line, moving from left to right, it is then quickly returned to the left to scan along the same horizontal line again. During this horizontal retrace period the cathode- ray tube is blocked out to make the retrace lines invisible. This retrace must be very rapid in order not to lose the in formation contained in the input signal.
5. Scanning is accomplished by deflecting electron beams. To do this a deflection voltage must pull the electron beam horizontally across the tube. The amplitude of this voltage must rise linearly to provide equal increments in amplitude for equal intervals of time. At the end of each scanning line the deflection voltage must fall back to zero in minimum time; that is, the retrace must be as short as possible. A waveform that provides this relatively slow li near rise in amplitude and rapid retrace to zero has the appear ance of the teeth of a saw and hence, is called a saw tooth waveform The saw-tooth scanning waveform for hizontal scanning called sweep is shown in Fig. 8.
6. When there is no input signal the electron beam traces a horizontal line on the face of the electron
cathode-ray tube. When it appears, both the vertical deflecting voltage and the scanning voltage make the electron ray trace the image of the input signal on the fluorescent screen, thereby showing how the input signal voltage changes with time.
7. To obtain a steady image on the oscillograph
screen it is necessary for each trace produced in each period of scanning to overlay most accurately the previously obtained trace. Such superposition can be obtained only, provided that each sweep coincides with the beginning of each successive period of the input signal. This is ensured by the synchronizing circuit which sends a pulse to start the scanning at the moment the input signal appears.
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