Horizontal Amplifier and Sweep Generator of CRO

Before discussing the horizontal amplifier we need discuss the basic concept of sweep generator. Because the sweep generator feeds the horizontal amplifier in a CRO. Then the horizontal amplifier feeds the vertical plates for horizontal deflection of the electron beam.

So far we have discussed the vertical amplifier and its function. Actually there are two sets of plates in a CRO. One is the set of vertical plates and another is the set of  horizontal plates. The function of horizontal plates is to provide vertical electric field. The vertical electric field produces the deflection of the electron beam vertically on the screen of the CRO. On the other hand, the vertical plates provide a horizontal electric field. In absence of the horizontal electric field the electron beam only deflects vertically. Therefore, the beam creates only a vertical line on the screen.

A time based ramp signal applied between the vertical plates, provides the solution. The time best ramp signal is a sawtooth wave of the same frequency as that of the input signal. Because of the sawtooth wave, the electron beam deflects horizontally from left to right during each cycle of the waveform. On reaching the right side of the screen the beam flies back instantly to its starting position on the left. As a result the electron beam draws a straight horizontal line on the middle of the screen in absence of any signal on the horizontal plates. But when both the input signal and the ramp signal are present in the CRO, the device is able to display the complete waveform of the input signal.

The instantaneous magnitude of the horizontal deflection of the electron beam depends on the instantaneous voltage applied across the vertical plates. Since the rate of rise of voltage at each cycle of the waveform (ramp) is constant, the speed of the horizontal deflection of the beam is also constant.

horizontal amplifier of cro

Sweep Generator

A very basic form of sweep generator, generates the signal by discharging and charging a capacitor continuously. In this way, it creates a chain of ramp voltages. Therefore, a sawtooth waveform appears. For stationary imaging of the input waveform the frequency of the sawtooth waveform matches the input signal. We can achieve this synchronisation by adjusting carefully the sweep frequency of the sawtooth waveform. But this method has some disadvantages. When the frequency of the input signal varies significantly, it becomes  difficult to adjust the exact frequency of the sweep generator.

The waveform which we observe on the CRO screen may not be always a periodic one. There are many types of signals which are not only nonperiodic  but occurs at very irregular intervals. In such cases sweep generator actuates with the signal under test. Under such circumstances we use triggered sweep generator. It deflects the beam once on the screen. It displays a very transient or a one time signal. Since in that case, the cathode ray oscilloscope can display the signals once at the instant of appearing the signal, the pattern must be photographed. We can obtain the trigger signal for the sweep generator from an external source or from the signal under measurement and analysis.

In the driven trigger, the sweep is recurrent but triggered by a signal under the examination.

For some applications like comparison between two waveforms we use non sawtooth voltage for sweeping the electron beam.

Horizontal Amplifier

The sweep signal is amplified before it gets fed to the the vertical plates for creating horizontal deflection of the beam. The function of a horizontal amplifier is the same. The amplifier maybe a cascaded amplifier.

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