Cathode Ray Oscilloscope Introduction

Cathode Ray Oscilloscope Introduction

Useful and versatile laboratory instrument

Allows the user to see the amplitude of electrical signals as a function of time on the screen.

Studying wave shapes of alternating currents and voltages

Measurement of voltage, current, power and frequency

Cathode Ray Oscilloscope Introduction

Very fast X-Y plotter displaying an i/p signal v/s time or v/s another i/p signal

Stylus of the plotter being a luminous spot (produced by a beam of electrons striking a fluorescent screen) which moves over a screen in response to an i/p voltage.

Projectile analogy horizontal (time) and vertical (i/p signal) deflection

Block Diagram

Basic Principle Cathode ray tube (CRT), which is the heart of the oscilloscope, 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 .

The power supply circuit provides various electrical signals and voltages for accomplishing these tasks Low voltage supply is required for the heater of the electron gun for generation 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.

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.

Time base generates a saw tooth, variable in frequency and amplitude. As the saw tooth voltage rises, it causes the spot to sweep from left to right, across the face of the CRT. When the saw tooth suddenly falls to zero, the spot flies back to the left of the screen ready for another sweep. The beam is cut off during the flyback time.

The combination of forces produces a display which represents the input signal.

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. The input signal is sampled by the trigger circuit and used to ensure that the time base runs at exactly the same frequency as the input signal

The sweep trigger generates a pulse coincident with a selected point in the cycle of the trigger signal.

This pulse turns on the sweep generator initiating the sawtooth wave form.

CRT Heart of CRO

Electron gun for producing a stream of electrons

Focusing and accelerating anodes for producing a narrow and sharply focused electron beam

Horizontal and vertical deflection plates for controlling the beam path

An evacuated glass envelope with phosphorescent screen giving bright spot when struck by a high velocity electron beam

CRT Internal structure

CRT - Electron Gun Assembly

Consists of an indirectly heated cathode, a control grid surrounding the cathode, a focusing anode and an accelerating anode

Function of the electron gun assembly is to provide a focused electron beam which is accelerated towards the phosphor screen.

cathode is a nickel cylinder coated with an oxide coating and emits plenty of electrons, when heated.

CRT Control Grid

Control grid is a metal cylinder covered at one end, with a small hole in the cover, kept at negative potential (variable) w.r.t cathode and its function is to vary the electron emission and so the brilliancy of the spot on the phosphor screen.

Hole in the grid is provided to allow passage for electrons through it and concentrate the beam of electrons along the axis of tube

CRT Anodes

Pre-accelerating anode, a hollow cylinder, is at a potential of few hundred volts more positive than the cathode so as to accelerate the electron beam.

This accelerated beam would be scattered now because of variations in energy and would produce a broad ill-defined spot on the screen.

This electron beam is focused on the screen by an electrostatic lens consisting of two more cylindrical anodes called the focusing anode and accelerating anode .

The function of these anodes is to concentrate and focus the beam on the screen and also to accelerate the speed of electrons.

Electrostatic Focusing System

Electrostatic lens consists of three anodes, with the middle anode at a lower potential than the other two electrodes.

Force on the electron is exerted in the direction normal to the equipotential surface.

Equipotential surface acts as a concavelens in geometrical optics. That is why, this focusing system is named as an electrostaticlens.

Deflection Plate Assembly

One pair of deflection plates is mounted vertically and deflects the beam in horizontal or X-direction and so called the horizontal plates and the other pair is mounted horizontally and deflects the beam in vertical or Y-direction and called the vertical plates.

These plates are to deflect the beam according to the voltage applied across them. For example if a constant pd is applied to the set of Y-plates, the electron beam will be deflected upward if the upper plate is positive. In case the lower plate is positive then the beam will be deflected downward.

Screen For CRT Some crystalline materials, such a phosphor,have property of emitting light when exposed to radiation. (fluorescence). These fluorescent materials continue to emit light evenafter radiation exposure is cut off. (phosphorescence). The length of time during which phosphorescence occurs is called the persistence of the phosphor.

The end wall of the CRT, called the screen, is coated with phosphor.

When electron beam strikes the CRT screen, a spot of light is produced on the screen. Phosphor absorbs kinetic energy of the bombarding electrons and emits, energy at a lower frequency in a visual spectrum.

Glass Body And Base The whole assembly - protected in a conical highlyevacuated glass housing through suitable supports.

The inner walls of CRT between neckand screen are usually coated with a conducting material known as aquadag and thiscoating is electrically connected to the accelerating anode. The coating is provided inorder to accelerate the electron beam after passing between the deflecting plates and tocollect the electrons produced by secondary emission when electron beam strikes thescreen. Thus the coating prevents the formation of ve charge on the screen and stateof equilibrium of screen is maintained.

Horizontal and vertical marks are marked on the screen of the CRT to provide user a correct measurement. These marks, usually in rectangular form, are called graticule.

CRO Controls

Number of controls - facilitate proper functioning.

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 vertical amplifier is required to provide this desired gain from milli-volt input to several hundred volt (peak to peak) output.

Delay Circuit

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 platesof a CRT is to be delayed by at least the same or little more amount of time. For this purpose, delay line circuit is introduced between vertical amplifier and the plates of CRT.

Horizontal Deflection System

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.

External signal is applied to horizontal deflection plates through the horizontal amplifier at the sweep selector switch in EXT position

When the function of time is required to be displayed on the screen of CRT, INT position of sweep selector switch is used

Horizontal Deflection System Contd

Assume that we supply an ideal saw-tooth signal voltage to the horizontal deflection plates, keeping vertical deflection plates at zero potential,

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.

Sine Wave

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

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.

Synchronization

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,

Synchronous selector

A synchronous selector switch is used, as shown infigure, 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.

Sweep generators

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

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.

CRO Tube Controls

POWER on / off

Scale

Illumination

Focus. Create spot on screen

Intensity. Brightness (Dont burn a spot on your screen)

Vertical Amp

Position on display

Sensitivity of vertical amp Calibrated. Cal fully clockwise.

Variable sensitivity. Continuous range between calibrated steps.

AC - DC - Gnd.

Selects desired coupling for incoming signal, or grounds amp input. DC couples signal directly to amp. AC connects via a capacitor. (Blocks DC)

Gnd = no signal. Gnd connects Y input to 0 volts. Checks position of 0v on screen.

Horizontal Sweep

Sweep time / Div (or CM) Select desired sweep rate, or admits external sig to horiz amp.

Sweep time / Cm Variable Continuously variable sweep rates. Cal is fully clockwise.

Position Controls horizontal position of trace.

Horizontal variable controls attenuation of signal applied to Horz amp through Ext Horiz connector.

Trigger Set to Auto or normal

Trigger selects timing of the beginning of the Horizontal sweep.

Slope selects trigger at +ve increasing or -ve decreasing portion of signal.

Coupling Selects whether trigger is at a specific DC or AC level.

Source: Int from Vertical Amp

Ext from Ext Trig Input.

Line AC line 50 (60) HZ Volts /Div switch

Volts / Div Variable Fine adjustment

these controls can have a Pull out switch position. May be 5 times mag.

Vertical mode

The operation of vertical deflection plates

Chan 1 and Chan 2 can each operate separately.

Dual. Ch1 and Ch2 are swept alternatively.

Why Dual? Used to measure input and Output signals of a device under test.

Ch1 and Ch2 can be added

Time base

Main, Max, Min, delay.

Selects the sweep for the main mix or delay mode and also X-Y switch

Time/Div provides selection of sweep rates. Range of 0.1 Second, 50 to .1 mS, 50 to 0.1uS per div. Note 5,2,1, sequence.

To determine a frequency use reciprocal.

Frequency = 1/time period (50Hz = 1/20mS)

Time period = 1/Frequency (number of div * ?ms/div. Eg 4div*5ms/div = 20 ms)

Other

Comp Test. Allows individual components to be tested. Connect via banana jacks to test resistors, capacitors, diodes, transistors, etc

Cal delivers calibrated voltage e.g. 2v p-p 1KHz square wave for setting scale.

GND. Earth terminal of scope

Connections

Vertical Input Horizontal Input

External Trigger

Cal. Out

Bandwidth

A 10MHz CRO does not mean it will correctly measure signals at 10MHz.

Vertical Amps are not so wide-band as to amplify all signals. 10MHz is the 3dB point. A 10MHz signal of 1v will measure 0.707v on the screen.

Clipping introduces odd order harmonics. A CRO operating near the max freq. will not show the harmonics and you think you are reading a clean signal.

Square waves begin to look like sine waves.

A rule of thumb is 5 times. To measure 2MHZ use a 10MHz CRO. 3 times is suitable for most Amateur work.

For 7MHz. Times 3 = 21. Use a 20 MHz CRO.

Every CRO will be different

Many instruments made for specific work.

Beam Finder push button

Trace rotation

Chan 1 Vertical input. During X-Y operation this is X axis (abscissa)

Chan 2 Vertical input Chan 2. During X-Y this becomes ordinate input.

Measuring

Voltage RMS is 0.707 * Vp for Sine and Cosine waveforms.

Hint: Try using a multimeter in parallel until you are happy with the measured CRO readings.