Introduction to CRO – Cathode Ray OscilloscopeThe cathode ray oscilloscope is an extremely useful and versatile laboratory instrument used for studying wave shapes of alternating currents and voltages as well as for measurement of voltage, current, power and frequency, in fact, almost any quantity that involves amplitude and waveform. It allows the user to see the amplitude of electrical signals as a function of time on the screen. It is widely used for trouble shooting radio and TV receivers as well as laboratory work involving research and” design. It can also be employed for studying the wave shape of a signal with respect to amplitude distortion and deviation from the normal. In true sense the cathode ray oscilloscope has been one of the most important tools in the design and development of modern electronic circuits.Block Diagram of a CRO

CRO Block DiagramThe instrument employs a cathode ray tube (CRT), which is the heart of the oscilloscope. It generates the electron beam, accelerates the beam to a high velocity, deflects the beam to create the image, and contains a phosphor screen where the electron beam eventually becomes visible. For accomplishing these tasks various electrical           signals and voltages are required, which are provided by the power supply circuit of the oscilloscope. Low voltage supply is required for the heater of the electron gun for gen-eration of electron beam and high voltage, of the order of few thousand volts, is required for cathode ray tube to accelerate the beam. Normal voltage supply, say a few hundred volts, is required for other control circuits of the oscilloscope.Horizontal and vertical deflection plates are fitted between electron gun and screen to deflect the beam according to input signal. Electron beam strikes the screen and creates a visible spot. This spot is deflected on the screen in horizontal direction (X-axis) with constant time dependent rate. This is accomplished by a time base circuit provided in the oscilloscope. The signal to be viewed is supplied to the vertical deflection plates through the vertical

amplifier, which raises the potential of the input signal to a level that will provide usable deflection of the electron beam. Now electron beam deflects in two directions, horizontal on X-axis and vertical on Y-axis. A triggering circuit is provided for synchronizing two types of deflections so that horizontal deflection starts at the same point of the input vertical signal each time it sweeps. A basic block diagram of a general purpose oscilloscope is shown in figure.Time Base GeneratorThe TBG or time base generator in CRO is the heart of its display mechanism. It controls the movement of electron beam, in horizontal direction i.e. along x-axis of the screen. The fundamental approach of TBG is given in the following sawtooth oscillator circuit.It consists of a UJT i.e. a Uni Junction Transistor1. The circuit produces saw tooth wave, which are used to sweep the electron beam on screen. The circuit works as follows. When CRO is switched on, the circuit gets the DC voltage from LV power supply of CRO. But the circuit remains inactive, unless it receives trigger pulse from trigger circuit.

When a signal is connected at y-input of CRO, the circuit is triggered through the trigger circuit. Now the capacitor (C) starts charging through variable resistor (R). So the emitter voltage (Ve) of UJT rises towards the supply voltage. When Ve = Vp, the capacitor quickly discharges through R2. The value of R2 >> R. So the charging time TS of the capacitor is more than the discharging time TB. Thus, we get saw tooth wave across the capacitor. When it charges, rising edge of saw tooth wave is obtained and when it discharges, the falling edge is obtained, as shown in the waveform.The value of Vp for the UJT is given by –

The frequency of saw tooth wave can be adjusted by changing the resistance of variable resistor R and the value of capacitor C marked as Time/div knob.These edges of sawtooth wave are synchronised with the horizontal movement of e-beam. As you must know, there are four deflection plates

inside the CRT: two of them are H-plate and the other two are V-plates. The H-plates are connected with this sawtooth voltage.Now imagine that you are standing in-front of the screen of CRO. Then the two H-plates will be like this: |   |. During starting of rising edge, the left plate starts becoming negative and the right plate becomes more and more positive. So the electron beam is slowly attracted towards the right plate. When rising edge reaches its peak value the e-beam has already reached the right end point of the screen. Now at this very moment, the blanking circuit dumps the cathode emission by applying a strong negative voltage on control grid. So e-beam is cut off momentarily.Now the falling edge starts. It has a short duration, and during this span, the left plate becomes positive and right plate becomes negative. At this instant, the blanking circuit takes off its negative voltage on control grid. So the e-beam rushes out, but now strikes at left end of the screen. Then again and again the same process repeats as explained above and thus the horizontal motion of e-beam is controlled.

The cathode ray oscilloscope is a versatile laboratory instrument. If a laboratory has only CRO in it, other measuring instruments may not be required. This is the importance of CRO in scientific laboratories. With it we can measure, AC/DC voltage, AC/DC current, resistance, phase and phase difference between two or more waveforms, relative frequency of a waveform, observe the amount of noise present on a signal, etc.In addition, CRO is also useful to observe the shape of waveform or signal and observe its real time progression on time axis. The waveform displayed on it, is observed with respect to x-y axes or co-ordinate system. The screen of CRO is plotted in terms of a measuring scale, known as graticule. Using this scale, the amplitude and wavelength of waveform can be accurately measured in centimetres and then converted into required unit. Learn some important terms before going into the details of functional block diagram of CRO.Y-input: It is the main input of CRO, to which the input signal is connected. The waveform of this input signal is displayed on the screen of CRT.Vertical attenuator1: It consists of RC voltage divider, which is marked on the CRO front panel as Volt/div control knob. Thus the ‘gain’ of CRO can be controlled with Volt/div knob.Vertical amplifier: It is a set of preamplifier and main vertical amplifier. The input attenuator sets up the gain of vertical amplifier.

Delay line: The delay line delays the striking of electron beam on the screen. It synchronizes the arrival of the beam on screen when time base generator signal starts sweeping the beam horizontally. The propagation delay2 produced is about 0.25msec.Trigger circuit: It takes the sample of input voltage connected at y-input of CRO and feeds it to the input of time base generator. So the TBG starts only when input signal is present at y-input.Time base generator: It produces a saw tooth wave. The waveform is used to sweep (move) the electron beam horizontally on the screen. The rate of rise of positive going edge of saw tooth waveform is controlled by Time/div control knob. Thus, the saw tooth wave controls the horizontal deflection of electron beam along x-axis.A switch known as INT/EXT is also connected after the output of TBG. When the switch is in INT position, the output of TBG is connected to H-plates through horizontal amplifier. When it is in EXT position, internal saw tooth is cut-off and some external signal can be connected to horizontal plates.Horizontal amplifier: It amplifies the saw tooth waveform coming from TBG. It contains phase inverter circuit also. Due to this circuit, two outputs are produced. One output produces positive going saw tooth and other output produces negative going saw tooth. The first output is connected to right side H-plate and the second output is connected to left side H-plate. So the electron beam moves properly from left to right of the screen.Blanking circuit: It is necessary to eliminate the retrace, which would produce when the spot on screen moves from right to left. This retrace can produce confusion with the original wave. So when the electron beam reaches right end of screen, the negative blanking voltage is produced by TBG. It is fed to control grid of CRT, to stop the electron beam completely.

HV/LV power supply: The high voltage section is used to power the electrodes of CRT and the low voltage section is used to power the electronic circuits of the CRO.

Basic Controls of a CRO

CRO-Block Diagram

Number of controls are required to be provided on a panel of CRO to facilitate its proper functioning. Intensity control is provided for adjustment of brightness of the spot on the screen. It is accomplished by varying the voltage between the first and second anodes. The horizontal and vertical position controls are provided for moving the beam on any part of the screen. It is accomplished by applying a dc voltage to horizontal or vertical deflection plates. Similarly there are other numerous controls in a CRO, which will be discussed, in detail, here.

Vertical Deflection System.

The function of vertical deflection system is to provide an amplified signal of the proper level to drive the vertical deflection plates without introducing any appreciable distortion into the system.

The input sensitivity of Many CROs is of the order of a few milli-volts per division and the voltage required for deflecting the electron beam varies from approximately 100 V (peak to peak) to 500 V depending on the accelerating voltage and the construction of the i tube. Thus the vertical amplifier is required to provide this desired gain from milli-volt input to several hundred volt (peak to peak) output. Also the vertical amplifier should not distort the input waveform and should have good response for entire band of frequencies to be measured.The deflection plates of CRO act as plates of a capacitor and when the input signal frequency exceeds over 1 MHz, the current required for charging and discharging of the capacitor formed by the deflection plates increases. So the vertical amplifier should be capable of supplying current enough to charge and discharge the deflection plate capacitor.

As we know that electrical signal is delayed by a certain amount of time when transmitted through an electronic circuitry. In CRO, output signal voltage of the vertical amplifier is fed to the vertical plates of CRT and some of its portion is used for triggering the time base generator circuit, whose output is supplied to the horizontal deflection plates through horizontal amplifier. The whole process, which includes generating and shaping of a trigger pulse and starting of a time-base generator and then its amplification, takes time of the order of 100 ns or so. So the input signal of the vertical deflection plates of a CRT is to be delayed by at least the same or little more amount of time to allow the operator to see the leading edge of the signal waveform under study on the screen. For this purpose, delay line circuit is introduced between vertical amplifier and the plates of CRT, as shown in figure.

Horizontal Deflection System.

External signal is applied to horizontal deflection plates through the horizontal amplifier at the sweep selector switch in EXT position, as shown in figure. The horizontal amplifier, similar to the vertical amplifier, increases the amplitude of the input signal to the level required by the horizontal deflection plates of CRT. When the function of time is required to be displayed on the screen of CRT, INT position of sweep selector switch is used. Before going further we should make ourselves clear first about the linear time base pattern. Assume that we supply an ideal saw-tooth signal voltage to the horizontal deflection plates, keeping vertical deflection plates at zero potential, as shown in figure.

At the starting point A in time, signal voltage is maximum but negative so the spot on the screen of CRO is at the extreme left position. Further at point B in time, signal voltage applied to the horizontal plates is zero so the spot is in the centre position on the screen. Now when voltage increases in + ve direction and becomes maximum just before the point C, the spot on the screen is at the extreme right side of the screen. Just after the point C, next cycle of saw-tooth voltage signal starts and again voltage becomes maximum negative so the spot goes back to the extreme left position of the screen from right position in no time.

From the above discussions we may conclude that

(i) The spot moves from left to right over the same path again for every cycle of sawtooth voltage applied to the horizontal deflection plates, so a horizontal line appears on the screen of CRO. (it) The spot moves from left to right on the screen with the uniform speed. Thus it produces a linear time base to display function of time on the screen of CRO.

For making idea of time base more clear let us discuss an application. Suppose a sine-wave voltage signal y of time period T is applied to the vertical deflection plates and a saw-tooth voltage signal vh of time period T is applied to horizontal deflection plates, as shown in figure.

At zero time, the spot is at extreme left vertically central position on the screen. Because of zero value of VV and maximum negative values of Vh. At time T/4, the spot is at one-fourth way on the screen in horizontal direction and at maximum positive deflection above the centre line in vertical direction because of maximum positive value of VV. At time T/2, values of

both VV and Vh are zero, so the spot is at the central position of the screen. At time 3T/4, the spot is the three-fourth way on the screen in horizontal direction and at the maximum negative deflection in vertical direction. Finally, at the end of time T, the spot is at extreme right vertically central position of the screen and then it moves back to begin a new trace. In this way, sine-wave voltage applied to the vertical deflection system appears on the screen. If the period of sine-wave is reduced to half then two sine-wave cycle appears on the screen.

From the above discussion, it is obvious that the following conditions are to be satisfied in order to have a waveform of the input signal applied to vertical deflection system as a stationary pattern on the screen of CRO.

(i) Both horizontal and vertical signals must start at the same instant.

(ii) Ratio of frequency of horizontal and vertical signals should be a rational or fractional number.

For satisfying the above conditions, sawtooth-wave is generated and synchronised with the vertical input signal by the trigger circuit and time base generator, as shown in figure and explained above.

In the INT position of sweep selector switch, horizontal amplifier receives an input from the time base generator,which provides a time base, and controls the rate at which the beam is scanned across the face of the CRT.  Time base generator is triggered or initiated by a trigger circuit which ensures that the horizontal sweep starts at the same point of the vertical input signal. As explained earlier, it is necessary to synchronize the sweep with the signal under measurement in order to obtain stationary pattern. Ratio of frequency of the time base and of the signal under measurement should be a rational number, otherwise pattern on the screen will not be stationary. A synchronous selector switch is used, as shown in figure, to select the type of synchronization. In the internal mode of switch the trigger is obtained from the vertical amplifier, input of which is signal under measurement.

In the external position of switch, the trigger is obtained from the external source. In the third position of switch i.e. line, trigger is obtained from the power supply i.e. 230 V and 50 Hz.

Two types of sweep generators are usually used. In the first one sawtooth signal of constant frequency is generated whether there is any input signal for vertical signal or not. That is why it is called free running type. In this it is essential to adjust the frequency of the sawtooth to get stationary pattern. In the second type of sweep generator, sweep is triggered by the signal under measurement so there is no need for any adjustment for synchronization.

Signal output on CRO

Sometimes, non-sawtooth sweep is also used in CRO for some special applications.

Position Controls.

There are two knobs — one for controlling the horizontal position and another for controlling the vertical position. The spot can be moved to left or right i.e. horizontally with the help of a knob, which regulates the dc potential applied to the horizontal deflection plates, in addition to the usual sawtooth-wave. Similarly the spot can be moved up and down i.e. vertically with the help of another knob, which regulates the dc potential applied to the vertical deflection plates in addition to the signal.

Intensity Control.

The potential of the control grid with respect to cathode is controlled with the help of potentiometer in order to control the intensity of brightness of the spot.

Focus Control.

In the electron gun of a CRT, middle anode is kept at lower potential with respect to other two anodes and it acts like an electrostatic lens and focal length of this lens can be varied by varying the potential of the middle anode with respect to other two anodes. So focusing of an electron beam is done by varying the potential of middle anode with the help of a potentiometer, as shown in figure. By increasing the positive potential applied to the focusing anode the electron beam can be narrowed and the spot on the screen can be made a pin point.

Astigmatism.

This is an additional focusing control and is analogous to astigmatism in optical lenses. A beam that is focused at the centre of the screen would be defocused at the edges of the screen because the lengths of the electron paths are different for the centre and the edges. Adjustment of this control gives a sharp focus over the entire screen. This control is affected by varying the potential of deflection plates and accelerating anodes.

Blanking Circuit.

Sawtooth sweep voltage is applied to horizontal deflection plates of CRT which moves the spot on the screen following a straight horizontal line from left to right during the sweep period. When the spot moves slowly so that its rate of movement exceeds the threshold of persistence vision, the spot appears as a solid line. Below this threshold limit, only spot or some portion of line after the spot appears. If the movement of the spot is fast, it appears as thin and dim horizontal line or may be invisible.

Calibration Circuit

Normally an oscillator, which generates a known and fixed voltage in square waveform is fitted in the CRO for calibration purpose.

Q. How do you differentiate between dual beam and dual trace oscilloscope?Sol. There are two separate vertical input channels A, B and these use separate attenuator and preamplifier stages. Therefore the amplitude of each input, as viewed on the oscilloscope, can be individually controlled. After pre-amplification the two channels meet at an electronic switch, this has the ability to pass one channel at a time into the vertical amplifier, via the delay line.

       There are two common operating modes for the electronic switch, called alternate and chop and these are selected from the instrument's front panel. The alternate mode is illustrated. In this, the electronic switch alternates between channels A and B, letting each through for one cycle of the horizontal sweep. The display is blanked during the fly back and hold-off periods, as in conventional oscilloscope provided the sweep speed is much greater than the decay time of the CRT phosphor; the screen will show a stable display of both the waveform at channels A and B. The alternate mode cannot be used for displaying very low frequency signals. In this mode, the electronic switch free runs at a high frequency of the order of 100 kHz to 500 kHz. The result is that small segments from channels A and B are connected alternately to the vertical amplifier and displayed on the screen. Provided the chopping rate is much faster than the horizontal sweep rate, the display will show a continuous line for each channel. If the sweep rate approaches the chopping rate then the individual segments will be visible and the alternate mode should now be used.    Switch allows the circuit to be trigged on either the A or B channel waveforms, or on line frequency or on an external signal. The horizontal amplifier can be fed from the sweep generator or the B channel via switch S. This is the X-Y mode and the oscilloscope operates from channel A as the vertical signal and channel B as the horizontal signal, giving very accurate X-Y measurements. Several operating modes can be selected from the front panel for display, such as channel A only, channel B  only, channels A and B as two traces and signals A+B, A-B, B-A or (A+B) as a single trace.   Dual Beam CRO: The dual trace oscilloscope can not capture two fast transient events, as it cannot switch quickly enough between traces. The dual beam oscilloscope has two separate electron beams, therefore two completely separate vertical channels, as in fig.The two channels may have a common time base system, as shown. Or they may have independent time base circuits. An independent time base allows different sweep rates for the two channels but increases the size and weight of the oscilloscope.Two methods are used for generating the two electron beams within the CRT; the first method uses a double gun tube. This allows the brightness and focus of each beam to be controlled separately but is bulkier than a split beam tube. In the second method known as split beam, a single electron gun is used. A horizontal splitter plate is placed between the last anode and the Y deflection plates. The plate is held at the same potential as the anode and it goes along

the length of the tube between the two vertical deflection plates. It therefore isolates the two channels. The split beam arrangement has half the brightness of a single beam, which has disadvantages at high frequency operation. An alternative method of splitting the beam, which improves its brightness, is to have two apertures in the last anode instead of one, so that two beams emerge from it.The disadvantage of the split beam construction is that the two displays may have noticeably different brightness, if operated at widely spaced sweep speeds. The brightness and focus controls also affect the two traces as the same time.