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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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