Circuitry of General Purpose CRO and Description of its Different Parts

This article is about the working of basic circuitry of oscilloscope using its complete block diagram and different sections and their functioning involving deflection plates, sweep generators, focusing control and synchronization

Introduction to Cathode ray oscilloscope

An oscilloscope is a display device that is used for the purpose of analysis and study of the wave form of an electrical signal generated in many transducer and communication processes. Here we will be studying about its various parts that involves in its circuitry that make it possible to use the system for a given stuff of data. These circuit's components include:
1. Free running recurrent sweeps
2. Vertical deflection system
3. Horizontal deflection system
4. Triggering sweep
5. Driven sweep system
6. Non saw tooth sweep generator
7. Synchronization system
8. Blanking circuit
9. Intensity Modulation also called Z-Axis modulation
10. Position Control system
11. Focus control system
12. Intensity control system
13. Calibration circuits
14. Astigmatism

Now before we proceed onwards in our session I would like to make you aware with the basic circuit diagram of the whole oscilloscope circuit assembly comprising of all the components in it and showing their inter connection with each other for the purpose of coordination in their functioning.
Cathode ray oscilloscope circuit diagram
Cathode ray oscilloscope block diagram

All the components shown above have not to be constructed over the same circuit board but may have been constructed upon separate circuit boards. Then all these section are connected with each other through connecting wires.
Now let us start to study all the sections on individual basis by answering the following questions that are listed below in a sequence:

What is Vertical Deflection system?

It involves a pair of deflection plates and these are supplied with the input signal that is intended to be measures. But before feeding it to the vertical deflection plates it requires to make the signal to pass through a number of amplification and attenuation stages. Vertical amplifier becomes necessary when input signal in not of required enough strength so to make it strong enough to produce a measurable deflection upon the screen it must be amplifier before giving to the display unit. While on the other hand taking the case when signal is having very high value of voltage and current that it is again not favorable to feed it directly to the display unit because in that case it can damage them, so in that case its strength must be, first, reduces to bring to a satisfactory and favorable level of voltage and power. The bandwidth of the amplifier used should be wide enough to make satisfactory amplification of entire frequency range applied to it from input port. Synchronizer amplifier is also fed with the output obtained from vertical amplifier through synchronization selector switch. This switch gets the input when it is in internal position. The signal being measured triggers the sweep generator by using this method.

What is Horizontal Deflection system?

The sweeps signals are applied to the horizontal plates this provides a time base for the study of the signal under reference frame of time. An amplifier stage is also established for amplification of the signal being supplied to the horizontal deflection plates but it not always necessary to feed the horizontal plates through the amplifier because when the signal is having enough strength it can also be fed to it directly without having requited any amplifier. When it is required to feed the external signal to horizontal deflection system it can also be done by feeding the signal through horizontal amplifiers which is done by switching the sweep generator switch to the internal position.
When it is made to be in internal position then saw tooth sweep generator supplies the input to horizontal amplifier and this is made to be triggered by synchronizing amplifier.
Here in order to understand synchronizing amplifier it is necessary to have understanding about synchronization that can be answered from following question:

What is synchronization of input signal?

In any type of sweep used in the system it is necessary for it to be synchronized with the input signal. To obtain a stationary pattern it is required to perform synchronization. For this to be done the time base should be operated at a submultiples frequency of signal. The pattern becomes unstable in absence of synchronization and results in drifting of the displaying pattern on the screen. There can be three types of synchronization depending upon its source:
1. Internal
2. External
3. Line

What is internal synchronization?

The signal under investigation is used for triggering the synchronization then it is referred to as internal synchronization.

What is external synchronization

Here some external source is used for the purpose of triggering the investigation of input signal.

What is line synchronization?
Here the power supply, itself, provides the triggering.

Description of different types of sweep generator?

Sweeps are of total four types that also have been named above and now will discuss about them one by one:

Meaning of free running sweep

This is also called as recurrent sweep. A saw tooth wave is used in type of sweep for repetition. First after termination of one sweep the next sweep is generated and in this case the circuit is not started by any external signal under measurement.

Meaning of triggered sweep?

The wave form that is going to be displayed upon the screen may or may not be periodic in nature i.e. it also can be irregular in nature. In such case is may be required that the sweep circuit should remain un-operational and may required the triggering by the waveform under investigation itself. There may be some cases that some of the portion of the wave form is periodic but the portion of interest may be for just a small duration of time. In those cases this type of sweep is used preferably. In these sweeps after getting triggered by the sweep signal the spot sweeps across the screen once in a while. This type of sweep is used for investigation of transient or one time signal and the wave for the signal is recorded in form of photograph for the purpose of its study in onward sessions of investigation.

Meaning of driven sweep

It is used when the signal is repetitive in nature and the sweep is also triggered by the signal under analysis.

Application of non saw tooth sweep

There is such application of the oscilloscope like
• Comparison of frequencies of waves forms
• For determination of phase shift

For these applications this type of sweep generator is used. With the type of oscilloscope used the variation in sweep frequency also takes place. The sweep frequency reaches to several Hz in case of a laboratory oscilloscope. The upper limit of an oscilloscope used for audio work is in the range of 100 KHz. Even, in many television services 1MHz of sweep frequency may be required.

What is blanking circuit?

The beam in made to move across the screen upon application of saw tooth sweep voltage to the X-deflection plates. This movement is from left to right in a straight line and that too within the sweep time called as trace time and denoted by Ts. Due to this a slow movement of the spot will be visible as if it is a straight line but assuming that the rate of movement should be greater than the threshold of the persistence of vision of the human eye. Otherwise if the rate is below this limit is will not be perceived as a straight line rather a moving spot only. Now the other part of the story is that if this movement is very fast this straight line will appear very thin and at certain higher speed it may also be invisible. So in the case when this fly back time is very small then the spot will no longer be visible. Taking the ideal case in which the fly back time is taken as zero resulting in state when spot becomes invisible. But in contrary to the ideal case this fly back time does not come to ne zero. So this means that during retrace journey the spot will be visible and this visibility of the spot in its retrace period may lead to a great confusion and rather a distortion in the wave form of the signal under investigation. To remove the possibility of such a case the retrace must be blanked out. This blanking of the trace in oscilloscope is done by application of high negative signal to the grid during the fly back time. This blacking voltage is normally generated by sweep generator.

What is Intensity modulation?

By insertion of the signal in between the ground and cathode terminals of oscilloscope leads to intensity modulation of the input signal. This type of modulation is also known as Z-axis modulation because intensity of the signal may be considered to be its third dimension of its existence. This modulation is used for controlling the brightness of the displayed waveform. During the sweep period of the signal periodic positive pulses are applied to the grid so that it may be brightened.

What is positioning control?

It is often required to control the position of the waveform generated on the screen. This control over the position can be obtained by application of a DC voltage to the deflection plates. This positioning can be controlled with help of a potentiometer arrangement where the constant DC voltage applied to the deflection plates can be changes on the account of the position of the pointer on the potentiometer.
Oscilloscope control panel
Oscilloscope control panel

What is Focus control?

The focusing of the signal is done with the help of electron lens whose focal length can be changed by application variable voltage to it.

What is intensity control system?

The potentiometer used for controlling the intensity of the electron beam is utilized for purpose of having control over the intensity of the display being shown on the screen. Here the potentiometer arrangement is used or feeing the grid with an adjustable voltage because it is nothing but the grid voltage that decides how many electrons are going to be emitted from by cathode. In this way indirectly it is the potential of the grid that controls the intensity.

What is calibration Circuit and how is calibration circuit used in oscilloscope?

The voltages of known amplitude (value) are used in oscilloscope which a regenerated in the oscilloscope itself by its internal circuitry. These voltages are utilized for the purpose of calibration. In normal cases the wave form of calibration voltage is in square shape.

What is astigmatism and its role in today's oscilloscopes?

The astigmatism is an additional control that is generally introduced in the latest oscilloscope as an addition control system that can also be seen in the figure shown above in control panel view. This process is analogous with the astigmatism that occurs in lenses. For getting the spot focused in a correct way is become very crucial to make it stable at the centre of the screen. This is done when the time base is switched off and X and Y positions are adjusted correctly.

You may prefer to read the construction of cathode ray tube and its working

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