Page 55 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Audio Frequency (AF) Generator (Fig 2): Audio frequency generators produce sine wave signals from 20 Hz
to 20 kHZ. Certain type of AF generators produce sine wave upto 100 kHZ. In addition to sine wave there may
be provision to produce square waves too.

Fig 2

These generators contain a variable amplitude control which changes the signal amplitude from 10 mv to 20V.
With the help of this generator the audio amplifier stages in radio, TV recorders and audio amplifier could be
tested.
While the frequency range switch selects the desired frequency range switch selects the desired frequency range,
the frequency dial is used to select the frequency within the desired range.
Cathode ray oscilloscope (CRO)
Introduction: The oscilloscope is an electronic measuring device which provides a visual presentation of any
wave form applied to the input terminals. Cathode ray tube (CRT) like a television tube provides the visual display
of the signal applied as a wave form on the front screen. An electron beam is deflected as it sweeps across the
tube face, leaving a display of the input signal.
An oscilloscope usually consists of:
– Attenuator
– amplifiers
– saw-tooth generator
– gate amplifiers or Z-amplifier
– Trigger
– CRT (cathode ray tube)
– power supply
The block diagram of a simple cathode ray oscilloscope is shown in Fig 3.

Attenuator : The input signal should be attenuated to a suitable magnitude before it is applied to the amplifier.
The attenuators are employed at the input of both vertical and horizontal amplifiers.
Amplifier : The amplifiers of an oscilloscope consist of a vertical amplifier and a horizontal amplifier. The vertical
amplifiers amplify the vertical input signal before it is applied to the Y-plates. The horizontal amplifier amplifies
the signal, before it is connected to the X-plates.

Saw-tooth generator: The measuring signal of any shape is connected to the Y-input(plates) and then it appears
on the screen. The signal on X-plates should be such that the image on the screen is similar to that on the
Y-plates. Hence a saw-tooth signal is required to be connected to the X-plates which makes the image on the
screen like the signal connected at the vertical plate. The saw-tooth signal is called the time base signal, and is
produced by the saw-tooth generator. The shape of the saw-tooth signal is shown in Fig 4. The time-base signal
consists of trace, retrace and hold off period.

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CITS : Power - Electrician & Wireman - Lesson 60-69