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Electronic Measurements and Instrumentation 2019 Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad 501510 Page 1 Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010 UNIT - III CATHODE RAY OSCILLOSCOPE Introduction to CRT The cathode-ray oscilloscope (CRO) is a multipurpose display instrument used for the observation, measurement, and analysis of waveforms by plotting amplitude along y-axis and time along x-axis. CRO is generally an x-y plotter; on a single screen it can display different signals applied to different channels. It can measure amplitude, frequencies and phase shift of various signals. Many physical quantities like temperature, pressure and strain can be converted into electrical signals by the use of transducers, and the signals can be displayed on the CRO. A moving luminous spot over the screen displays the signal. CROs are used to study waveforms, and other time-varying phenomena from very low to very high frequencies. The central unit of the oscilloscope is the cathode-ray tube (CRT), and the remaining part of the CRO consists of the circuitry required to operate the cathode-ray tube. Internal structure of CRT The main parts of a CRT are Electron Gun Assembly Deflection plate Assembly Fluorescent Screen Glass Envelope 1. Electron Gun Assembly The electron gun emits electrons & forms them into a beam. This beam is focused on to screen to cause a luminous spot on the screen. It consists of 1. Heater 2. Cathode 3. Grid 4. Pre-accelerating anode 5. Focusing anode 6. Accelerating anode Heater & Cathode: Electrons are emitted from the indirectly heated cathode. The typical value of current & voltage required by an indirectly heated cathode are 600ma at 6.3V. Grid: The control grid in the CRT is cylindrical, with small aperture in line with cathode. The control grid bias knob is labeled as intensity control in CRO. Pre-accelerating & Accelerating anodes: These anodes are cylindrical in form, with small openings located in the centre of each electrode, coaxial with the tube axis. The Pre-accelerating & Accelerating anodes are connected to a common positive high voltage of about 1500V. Focusing anode:

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Page 1: CATHODE RAY OSCILLOSCOPEclreddy.files.wordpress.com/2019/10/unit_iii-notes.pdf · CATHODE RAY OSCILLOSCOPE Introduction to CRT The cathode-ray oscilloscope (CRO) is a multipurpose

Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 1

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

UNIT - III CATHODE RAY OSCILLOSCOPE

Introduction to CRT

The cathode-ray oscilloscope (CRO) is a multipurpose display instrument used for the observation, measurement, and analysis of waveforms by plotting amplitude along y-axis and time along x-axis.

CRO is generally an x-y plotter; on a single screen it can display different signals applied to different channels. It can measure amplitude, frequencies and phase shift of various signals. Many physical quantities like temperature, pressure and strain can be converted into electrical signals by the use of transducers, and the signals can be displayed on the CRO.

A moving luminous spot over the screen displays the signal. CROs are used to study waveforms, and other time-varying phenomena from very low to very high frequencies.

The central unit of the oscilloscope is the cathode-ray tube (CRT), and the remaining part of the CRO consists of the circuitry required to operate the cathode-ray tube.

Internal structure of CRT The main parts of a CRT are Electron Gun Assembly Deflection plate Assembly Fluorescent Screen Glass Envelope

1. Electron Gun Assembly The electron gun emits electrons & forms them into a beam. This beam is focused on to screen to

cause a luminous spot on the screen. It consists of 1. Heater 2. Cathode 3. Grid 4. Pre-accelerating anode 5. Focusing anode 6. Accelerating

anode Heater & Cathode: Electrons are emitted from the indirectly heated cathode. The typical value of current & voltage required by an indirectly heated cathode are 600ma at 6.3V. Grid: The control grid in the CRT is cylindrical, with small aperture in line with cathode. The control grid bias knob is labeled as intensity control in CRO. Pre-accelerating & Accelerating anodes: These anodes are cylindrical in form, with small openings located in the centre of each electrode,

coaxial with the tube axis. The Pre-accelerating & Accelerating anodes are connected to a common positive high voltage of about

1500V. Focusing anode:

Page 2: CATHODE RAY OSCILLOSCOPEclreddy.files.wordpress.com/2019/10/unit_iii-notes.pdf · CATHODE RAY OSCILLOSCOPE Introduction to CRT The cathode-ray oscilloscope (CRO) is a multipurpose

Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 2

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

The focusing anode is connected to a lower adjustable voltage of 500V. There are two methods of focusing an electron beam

a. Electrostatic focusing b. Electromagnetic focusing

2. Deflection plates Assembly To move the beam in the CRT, the deflecting plates are mounted inside the tube & suitable deflecting

voltages are applied to them. These plates are arranged in two pairs H1 & H2 for deflecting beam horizontally & V1&V2 for

deflecting beam vertically. Deflection of beam

3. Fluorescent Screen The front of the CRT is called face plate. The inside surface of the face plate is coated with phosphor. A phosphor convert’s electrical energy to light energy when an electron beam strikes phosphor crystal

it raises the energy level. This is known as cathodo-luminescence. When light is emitted during phosphor excitation, it is called Fluorescence.

Page 3: CATHODE RAY OSCILLOSCOPEclreddy.files.wordpress.com/2019/10/unit_iii-notes.pdf · CATHODE RAY OSCILLOSCOPE Introduction to CRT The cathode-ray oscilloscope (CRO) is a multipurpose

Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 3

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

4. Glass envelope To the glass envelope graticules are attached.

Aqua-dog: The electrons striking the screen, release secondary emission electrons. These secondary electrons are collected by a aqueous solution of graphite called Aquadog. Vertical amplifier

Block diagram of vertical amplifier

The vertical amplifier consists of several stages, with fixed overall sensitivity gain expressed in V/div. The advantage of fixed gain is that the amplifier can be more easily designed to meet the requirements of stability and B.W. The vertical amplifier is kept within its signal handling capability by proper selection the input attenuator switch. The first element of the pre-amplifier is the input stage, often consisting of a FET source follower whose high input impedance isolates the amplifier from the attenuator.

This FET input stage is followed by a BJT emitter follower, to match the medium impedance of FET output with the low impedance input of the phase inverter.

This phase inverter provides two anti phase output signals which are required operate the pushpull output amplifier. The push-pull output stage delivers equal signal voltages of opposite polarity to the vertical plates of the CRT. The advantages of push-pull operation in CRO are similar to those obtained from push-pull operation in other applications; better voltage cancellation ran the source or power supply (i.e. dc), even harmonic suppression, especially large.

Horizontal deflecting system The horizontal deflecting system consists of time base generator and an output amplifier The CRO is used to display a waveform that varies as a function of time. If the wave form is to be

accurately reproduced, the beam should have a constant horizontal velocity.

Page 4: CATHODE RAY OSCILLOSCOPEclreddy.files.wordpress.com/2019/10/unit_iii-notes.pdf · CATHODE RAY OSCILLOSCOPE Introduction to CRT The cathode-ray oscilloscope (CRO) is a multipurpose

Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 4

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

As the beam velocity is a function of the deflecting voltage, the deflecting voltage must increase linearly with time.

A voltage with such characteristics is called a ramp voltage. If the voltage decreases rapidly to zero—with the waveform repeatedly produced, as shown in Fig. 14-6—we observe a pattern which is generally called a saw-tooth waveform.

The time taken to return to its initial value is known as flyback or return time.

Simple saw-tooth generator & associated waveforms: The circuit shown in Fig is a simple sweep circuit, in which the capacitor C charges through the resistor

R. The capacitor discharges periodically through the transistor T1, which causes the waveform shown in

Fig to appear across the capacitor. The signal voltage, Vi which must be applied to the base of the transistor to turn it ON for short time

intervals is also shown in Fig.

Time-base generator using UJT: The continuous sweep CRO uses the UJT as a time-base generator. When power is first applied to the

UJT, it is in the OFF state and CT changes exponentially through RT. The UJT emitter voltage VE raises towards VBB and VE reaches the plate voltage VP. The emitter-to-base diode becomes forward biased and the UJT triggers ON. This provides a low

resistance discharge path and the capacitor discharges rapidly. When the emitter voltage VE reaches the minimum value rapidly, the UJT goes OFF. The capacitor

recharges and the cycles repeat. To improve the sweep linearity, two separate voltage supplies are used; a low voltage supply for the

UJT and a high voltage supply for the RTCT circuit. This circuit is as shown in Fig.

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 5

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

RT is used for continuous control of frequency within a range and CT is varied or changed in steps. They are sometimes known as timing resistor and timing capacitor

The different types of sweep generated are as follows: 1. Recurrent Sweep: When the saw tooth, being an ac voltage alternates rapidly, display occurs repetitively, so that a lasting image is seen by the eye. This -treated operation is recurrent sweep. 2. Single Sweep: The signal under study produces a trigger signal, which in turn produces a single sweep. 3. Driven Sweep: The saw tooth oscillator is a free running generator when p crated independently. There is a chance that the sweep cycle may start after the signal cycle, thereby missing a part of the signal. Driven sweep removes this possibility because it is fixed by the signal itself. The sweep and signal cycles start at the same time. 4. Triggered Sweep: In a recurrent mode, the pattern is repeated again and again. In this mode the voltage rises to a maximum and then suddenly falls to a minimum. Electron beam moves slowly from left to right, retraces rapidly to the left and I, pattern is repeated. The horizontal sweep action takes place whether the input signal is applied to the oscilloscope or not, and a horizontal line is displayed on the scope screen. A triggered sweep, on the other hand, does not start unless initiated by a trigger voltage, generally derived from an incoming signal. In the absence of the input signal the sweep is held off and the CRT screen is blanked.

The continuous or recurrent sweep uses a free running multi vibrator (m/v) which covers a wide frequency range and can be locked into synchronization by input signal. Sync takes place when the sweep frequency and the input signal frequency are the same or when the former is a multiple of the latter. Horizontal amplifier

The horizontal amplifier basically serves two purposes

1. When the oscilloscope being used in the ordinary mode of operation to display a signal applied to the vertical input, the horizonatal amplifier will amplify the sweep generator output

Page 6: CATHODE RAY OSCILLOSCOPEclreddy.files.wordpress.com/2019/10/unit_iii-notes.pdf · CATHODE RAY OSCILLOSCOPE Introduction to CRT The cathode-ray oscilloscope (CRO) is a multipurpose

Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 6

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

2. When the oscilloscope is being used in the X-Y mode the signal applied to the horizontal input terminal will be amplified by the horizontal amplifier.

The purpose of an oscilloscope is to produce a faithful representation of the signals applied to its input terminals. Considerable attention has to be paid to the design of these amplifiers for this purpose. The

oscillographic amplifiers can be classified into two major categories. (i) AC-coupled amplifier (ii) DC-coupled amplifiers

The low-cost oscilloscopes generally use ac-coupled amplifiers. The ac amplifiers, used in oscilloscopes, are required for laboratory purposes. The dc-coupled amplifiers are quite expensive. They offer the advantage of responding to dc voltages, so it is possible to measure dc voltages as pure signals and ac signals superimposed upon the dc signals.

DC-coupled amplifiers have another advantage. They eliminate the problems of low-frequency phase shift and waveform distortion while observing low-frequency pulse train.

Trigger pulse circuit

The trigger circuit is activated by signals of a variety of shape & amplitudes which are converted to

trigger pulses of uniform amplitude for precision sweep operations. The trigger section is a 3 position switch internal, external & line. The trigger input is applied to a

voltage comparator whose reference level is set by the trigger level control on the CRO front panel The pulse generator that follows the comparator produces –ve pulses each time the comparator

output crosses its quiescent level, which in turn triggers the sweep generator to start the next sweep. The timing resistance is used for sweep rate control & timing capacitor is changed in steps for sweep

rate control. Simple CRO The cathode ray oscilloscope is a very useful general purpose electronic instrument for testing & developing electronic circuits, systems & instruments. The various blocks of CRO are

1. Time base/ Sweep generator 2. Trigger circuit 3. Horizontal amplifier 4. Vertical amplifier 5. Delay lines

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 7

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

6. CRT 7. HV & LV supplies

1. Time base/ Sweep generator: Time base generator is used to generate a saw tooth voltage before it is applied to horizontal deflecting plates. Usually a constant current Miller sweep circuit is employed to generate a saw tooth waveform & is used to deflect the electron beam linearly in X-direction. 2. Trigger circuit: To get a true representation of input signal, the time base signal or X signal& Y signal must be iniated at the same time. This is achieved by the trigger circuit. 3. Horizontal Amplifier: The purpose of this amplifier circuit is to amplify an externally applied signal to the horizontal or X-plates. This also helps in adjusting the magnitude of the internal saw tooth waveform being generated. 4. Vertical Amplifier: This is also called Y amplifier. The electron beam deflection in the Y direction is proportional to the signal amplitude given to the Y input or vertical plates. 5. Delay lines: To allow the operator to observe the leading edge of the signal waveform, the signal drive for the vertical CRT plates must be delayed by at least the same amount of that of horizontal plates. This is achieved by the vertical delay lines. 6. Cathode ray tube: Here the electron beam is generated, accelerated, deflected & made to strike fluorescent screen to give the visual display of the electrical signal input given to the vertical plates. To accelerate, deflect & sweep the electron beam large voltages in kilovolts is required for a CRT. This is generated by the HV power supply circuit Vcc & other voltages which are generated in the low voltage power supply circuits.

Page 8: CATHODE RAY OSCILLOSCOPEclreddy.files.wordpress.com/2019/10/unit_iii-notes.pdf · CATHODE RAY OSCILLOSCOPE Introduction to CRT The cathode-ray oscilloscope (CRO) is a multipurpose

Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 8

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 9

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 10

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

Dual beam CRO

A Dual Beam CRO consists of two complete separately electron guns, two sets of VDPs and a single set of HDPs. Since a single set of HDPs is used, only one beam can be synchronized at a time.

The Sweep produced by the Time Base Generator is common for both channels; hence the signals must have the same frequency or must be related harmonically in order to obtain both beams locked on the CRT screen.

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 11

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 12

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 13

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 14

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 15

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 16

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 17

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 18

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

Analog Storage Oscilloscope

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 19

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

Digital Storage Oscilloscope The digital storage oscilloscope replaces the unreliable storage method used in analog storage scopes with the digital storage with help of memory.

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 20

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

In this oscilloscope the waveform to be stored is digitized & then stored in a digital memory.

The vertical input after passing through the vertical amplifier is digitized by an ADC to create a data set that is stored in memory.

Digitizing the analog input signal means taking samples of periodic intervals of the input signal.

The signal is then captured in the memory; many manipulations are possible as memory can be readout without being erased.

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 21

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

MEASUREMENTS USING THE CATHODE-RAY OSCILLOSCOPE: Measurement of Frequency

Measurement of Phase

Measurement of Phase Using Lissajous Figures

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 22

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 23

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010

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Electronic Measurements and Instrumentation 2019

Department of ECE, Sphoorthy Engineering College, Nadergul, Hyderabad – 501510 Page 24

Courtesy: Electronic Measurements and Instrumentation, K. Lal Kishore, Pearson 2010