Block Diagram of a Cathode Ray Oscilloscope

Vertical Deflection System

When we use electrostatic vertical deflection we need to feed the input signal to the horizontal plates. The input signal passes through an attenuator and a number of amplifier stages. The vertical deflection system requires a vertical amplifier to amplify a very small input signal. It is for proper observation and measurement the waveform on the screen.

The amplifier must be capable of passing the signal of frequencies within entire bandwidth of the measurement.
On the other hand, an attenuator reduces the amplitude of the high voltage input signal before the signal enters in the vertical amplifier. This is because the output waveform must not go beyond the edge of the active screen. Means the entire waveform must be within the active portion of the screen.

We need to trigger the horizontal amplifier system with the help of the the vertical input signal. Hence, the vertical amplifier also feeds the synchronising amplifier through a synchronizer selector switch.

Horizontal Deflection System

The horizontal deflection system provides the deflection of the electron beam in horizontal direction. So, it essentially has a voltage input signal. The plates for the horizontal deflection system are the vertical plates.  An amplifier feeds the vertical plates. This amplifier is the horizontal amplifier. Although if the voltage is sufficiently high that can be directly fed to the vertical plates without any amplification stage. But we supply small external signals to the vertical plates through the the horizontal amplifier.

Block Diagram of a Cathode Ray Oscilloscope
Block Diagram of a Cathode Ray Oscilloscope

Synchronizing Selector Switch

Before the signal reaches to the the horizontal amplifier it passes through a synchronizing selector switch. When the selector switch is on the internal position, the horizontal amplifier receives the input from the sweep generator. The synchronising amplifier triggers the sweep generator .

In order to obtain a static pattern on the screen screen, synchronising has to be done. The horizontal system should be operated with the frequency which matches the frequency of the input signal. The synchronising selector switch  has three prepositions.

Internal Position

At that condition the input signal initiates the triggering of the sweet generator.

External Position

In that case,  an external signal triggers or initiates the sweep generator. The same external signal also concurrently actuates the input signal.

Line Position

In this method the power supply of the instrument triggers the sweep generator.

Line Blanking Circuit

When we apply a sawtooth voltage wave across the vertical plates the electron beam spot on the screen moves horizontally from left to right. The spot moves only during the rise time of each cycle (trace) of the sawtooth wave. Hence, the speed at which the electron beam spot moves horizontally depends on the the rising slope (trace) of the ramp.

If the speed of the moment of the spot is very slow we will observe only one glowing spot moving on the screen from left to right. To form a glowing straight line the movement of the electron beam must exists during the threshold of persistence of human vision. Again if the slope of the ramp is very high the speed of the spot will be also very fast. In that case the thickness of the horizontal line becomes so thin that it may become invisible to the human eyes.

On the other hand, ideally the signal does not require any time to reach its minimum value position from its peak. Although practically it is not possible. So, a minimum time delay is there. During this time the spot moves in opposite direction. This may lead a confusion. So the negative sloped portion of each cycle (retrace) must be blanked out. We do this by supplying a high negative voltage during the retrace time. We call this  negative voltage as blanking voltage. And the time based sweep generator develops this voltage itself.

Position Controlling

In a CRO we apply a DC potential to the vertical plates in addition to the sweep voltage. Similarly, we apply a DC potential to the horizontal plates in addition to the input voltage. A general purpose cathode ray oscilloscope consists of two rotary knobs. One regulates the dc potential to the vertical plates and another regulates the dc potential to the horizontal plates. By adjusting  the DC potential to the vertical and horizontal plates one can manually shift the spot on the screen horizontally and vertically, respectively.

Intensity Modulation

It is done by inserting a signal between the ground and the cathode during the visible portion of the trace. It improves the brightness of the display.

Intensity Control

The intensity of the displayed image of the waveform on the screen can be controlled by controlling the number of electrons in the electron beam. The grid potential determines the amount of electron leaving the cathode. Hence, by controlling the grid voltage with respect to the cathode we can control the intensity of the display. And we do that by using a potentiometer type device.

The excessive intensity may cause a burning spot on the screen. So, a stationary spot should have very low intensity. If the intensity is increased the spot must keep moving on the screen.

Focus Control

The middle anode in the election gun system has a a lower potential than the other anodes. It acts as a electronic lens. The focusing of an electron beam is done by adjusting the potential of the middle anode with the help of a potentiometer. By doing this the electron beam gets narrowed and may make a pinpoint spot on the screen.

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