Marks: 17(25)
CH-2 ELECTRONIC INSTRUMENTS
Cathode Ray Oscilloscope (CRO)
Principle: CRO is most versatile instrument. It allows the amplitude of electrical signal to be displayed as a function of time. CRO depends on the movement of electron beam which is bombarded on screen coated with fluorescent material to produce visible spot. If electron beam is deflected on both X-axis and Y-axis, a two dimensional display is produced. The beam is deflected at constant rate relative of time along X-axis and is deflected along Y-axis in response to electrical signal.
Block Diagram of CRO:
Working:
1. Vertical Deflection System:
(a) Vertical amplifier: The signal waveform to be viewed on CRT screen is applied to the vertical amplifier input. The gain of the amplifier is set up by a calibrated input attenuator marked as volts/div. This is deflection sensitivity. Vertical amplifier adjusts the level of input signal required for vertical deflection of beam. The input circuit sects input modes AC/DC or GND (ground).
(b) Delay line: The overall operation of CRO causes certain amount of time delay in transmission of signal voltages to the deflection plates It is desired that vertical deflection system should synchronize with horizontal deflection system. It takes some extra time. So vertical signal must be delayed by 80 nanoseconds.
2. Horizontal Deflection System:
(a) Trigger Circuit: It is used to get steady waveform on screen. A sample of input waveform is fed to as trigger circuit. It produces a trigger pulse at some selected point on input waveform. This trigger pulse is used to start a time base generator. So that time base voltage will trace same path again and again.
(b) Time Base Generator: It is also called sweep generator that generates a sawtooth waveform. It is used as horizontal deflection voltage of CRT. It causes the repetitive movement of spot from left to right with particular rate. Its rate of rise is set by front panel control marked Time/Div.
(c) Horizontal Amplifier: The gain of horizontal amplifier is larger than vertical amplifier. It amplifies the time base sawtooth voltage.
3. Cathode Ray Tube: It is an electronic device that produces visual representation of signal with high velocity of electrons. CRT consists of an electron gun, deflection plates and fluorescent screen inside the evacuated glass envelope. Electron fires electrons at a very high speed. These electrons strike the fluorescent screen. Kinetic energy of these electrons gets converted gets converted into light energy and input signal is displayed on screen.
4. Power Supply: Low voltage power supply is required for the electronic circuits. While high voltage power supply is required to accelerate electron beam from electron gun towards fluorescent screen.
Cathode Ray Tube (CRT):
It is heart of CRO. It is an electronic device which gives visual representation of input signal. CRT consists of three main sections an electron gun, deflection plates and fluorescent screen. These parts are placed in high evacuated glass envelope.
Electron gun consists of cathode which emits electrons. Numbers of electrons emitted by cathode are controlled by control grid. This control is marked on front panel control of CRO as INTENSITY. It controls brightness of display.
These electrons coming out of control grid are then accelerated by accelerating anode and then pass through focusing anode. Focusing anode controls sharpness of display. This control is marked as FOCUS on front panel of CRO.
A focused beam then passes through deflection plates assembly. It consists of a two pairs of parallel plates. One plate is vertical deflection plates or Y-deflection plates and other pair is horizontal deflection plates or X-deflection plates. Any voltage applied to these plates move electron beam up and down or left and right.
When sharply focused beam strikes the fluorescent screen, a bright spot is produced. Fluorescent material absorbs kinetic energy of beam and converts it into light energy. Colour of display depends on type of fluorescent material.
Inner part of funnel shaped tube is coated with conducting aqueous solution of graphite. It is called aquadag coating. This coating collects the electrons produced by secondary emission when electron strikes the screen.
Front Panel Controls of CRO:
1. POWER ON: It is toggle switch used for ON/OFF the power to the instrument.
2. LED: It indicates ON and OFF the power.
3. INTENSITY: It controls brightness of trace (display). Clockwise direction of this knob increases its brightness.
4. FOCUS: It controls sharpness of display.
5. Y-POSITION: It controls vertical position of display. Clockwise rotation of knob shifts display upward and anticlockwise rotation of knob shifts display downward.
6. DC-AC-GND: It selects AC or DC coupling of vertical input signal or grounding the vertical amplifier input.
7. INPUT BNC: It is input terminal of Y-input.
8. VOLTS/DIV: It selects vertical deflection sensitivity. It is 12 step attenuator from 5mV/div to 20V/div in 1, 2, 5 sequence.
9. 0.2V: It is 200mV peak to peak calibration signal.
10. TIME/DIV: It selects sweep period in specified range. It is 18 step switch to select calibrated sweep periods from 0.2sec/div to 0.5microsec/div in 1, 2, 5 sequence.
11. VARIBLE: In CAL position the selected sweep holds indicated calibration. Clockwise rotation increases sweep period.
12. CT: It is a component tester, when pressed this button, converts CRO into component tester.
13. X-POSITION: It controls the horizontal position of display.
14. LEVEL: It is used for adjustment of trigger level.
15. TRACE ROTATION: This control is used for aligning the horizontal trace with graticule horizontal line.
Applications of CRO:
1. Voltage Measurement:
(a) DC voltage measurement:
The steps in DC voltage measurement are as follows-
The DC input to be measured is fed to Y-input of CRO.
Set AC-DC-GND knob in DC mode.
Observe the vertical shift of trace.
Keep the Volt/Div scale at a suitable position depending on magnitude of input voltage such that trace lies within screen.
Count the number of pulses by which trace is shifted vertically.
Calculate DC voltage using the following formula
DC voltage = Volt/Div scale x No. of vertical divisions.
e.g. If and Volt/div scale = 5 and number of vertical divisions = 4, then
unknown DC voltage = 5x4 = 20V.
(b) AC voltage measurement:
The steps in the AC voltage measurement are as follows-
The AC input to be measured is fed to Y-input of CRO.
Set AC-DC-GND knob in AC mode.
Observe the waveform on screen.
Adjust Volt/Div and Time/Div knobs so that 1 to 2 complete cycles can be viewed on screen.
Count the number of vertical divisions in peak to peak values of AC.
Calculate peak to peak AC voltage using following formula-
Peak to peak AC voltage = Volt/Div scale x No. of vertical divisions
2. Frequency Measurement:
The steps in the frequency measurement are as follows-
The AC input signal whose frequency is to be measured is fed to Y-input of CRO.
Set AC-DC-GND knob in AC mode.
Observe the waveform on screen.
Adjust Volt/Div and Time/Div knobs so that 1 to 2 complete cycles can be viewed on screen.
Count the number of horizontal divisions lying in one cycle of signal.
Calculate AC frequency using following formula-
Period (T) = Time/Div scale x No. of horizontal divisions in on cycle.
Frequency (F) =
e.g. H – divisions = 4, Time/div = 0.5 ms
T = 4 x 0.5 = 2ms
Frequency = 1/T = 1/2ms
= 1000/2
= 500 Hz
3. Frequency measurement using Lissajous figures:
Lissajous pattern can be used for accurate measurement of frequency. In this method the signal whose frequency is to be measured is applied to Y-input and accurate standard signal is applied to X-input. A pattern of loops called Lissajous figures appeared on screen. The frequency of unknown signal is calculated by measuring number of loops on horizontal line and vertical line.
If fV – the frequency of unknown signal (f – unknown)
fH – frequency of standard signal ( f – std.)
In this case, fV/fH = 2/1
fV = 2 x fH
In this case fV/fH = 1/3
fV = 1/3 x fH
4. Phase measurement:
The phase difference between two sine waves can be calculated by applying one signal to X-plates and other signal to Y-plates.
The display lissajous pattern may be ellipse, circle or sloping line.
The display is centered on screen.
The distances between A and B are measured. Distance A is the vertical measurement between two points where the trace crosses vertical central line. Distance B is maximum vertical height of display.
Sinθ = A/B
θ = Sin-1A/B
If lissajous pattern display is circle, phase difference between two signals is 900.
5. Component Testing:
This facility is available in both single trace and dual trace oscilloscopes. The steps for component testing are as follows-
Press CT switch to bring oscilloscope in component test mode.
Set coupling switch to in ‘GND’
Set volt/Div knob at 5mV/Div scale.
Connect component under test to CT terminals.
Adjust X-pos to bring trace in middle of screen.
We observe different patterns for different components on screen.
Compare the observe patterns with standard test patterns.
Types of Oscilloscopes:
In the analysis of electronic circuits many times it is essential to view two signals simultaneously. This can be achieved by using CROs. But this is an expensive way and this method can not compare same event of two signals simultaneously. For this purpose following types of CRO are used.
1. Dual beam oscilloscope: It is an oscilloscope used to compare one signal with another. There are two beams produced in a special type of CRT. A dual beam CRO produces two separate beams to display two vertical input signals.
2. Dual trace oscilloscope: It has only one electron beam. It displays only one channel at a time, but switch to other channel either on alternate sweep or many times per sweep. There are four modes of operation
(a) Channel A mode- When set to this mode only input to channel A is displayed.
(b) Channel B mode- When set to this mode only input to channel B is displayed.
(c) Alternate mode- In this mode signal applied to channel A is displayed for one sweep and then signal a signal for channel B is displayed for next sweep. The alternate mode is used to compare two higher frequency signals.
(d) Chopped mode: In this mode signal applied to channel A is displayed for a short segment and then signal applied to channel B is displayed for next short segment. This chopping action happens fast enough that both waveforms displayed appear continuous; the segments will not be seen. The chop mode is used to compare two low frequency signals.
3. Analogue storage oscilloscope: Storage feature allows the trace pattern that normally decays in a fraction of a second to remain on the screen for several minutes or longer. It uses principle of secondary emission. There are two electron guns in this CRT.
4. Digital storage oscilloscope: It uses digital memory that can store data as long as required time without degradation. It processes the signal by high speed digital processing circuits. The screen image can be directly recorded on paper by means of an attached printer or plotter.
Digital Multimeter (DMM):
Introduction:
It is a common & important laboratory instrument. It is used to measure AC/DC voltage, AC/DC current and resistance with digital display. It gives digital display, which is very accurate. It has an advantage of very high input resistance. It also provides over ranging indicator i.e. if the unknown electrical quantity increases beyond measuring capacity it shows 1— on the display.
Advantages/features of DVM (Over Analog multimeter):
It eliminates human errors in reading
It eliminates parallax errors.
Easy readability as readout is in numerical format.
It increases speed of reading.
Automatic zero and polarity setting.
As output is in digital form, it may be directly fed to memory.
Readings may be carried out to any number of significant figures.
Power requirement is low.
It is light in weight.
It is less costly.
High accuracy and high input impedance
Principle:
Digital multimeter uses Analog to Digital converter (ADC). In a typical Digital multimeter the input signal i.e ac or dc voltage, current, resistance, temperature or any other parameter is converted to dc voltage with ADC. The analog to digital converter then converts the dc voltage into its equivalent digital form. It is then displayed on the display unit. DMM just converts each and every quantity into DC voltage and uses digital voltmeter circuit. DMM is basically a digital voltmeter.
Block diagram:
Working:
1. Attenuator: The DMM has a rotary switch used selecting proper range. When unknown voltage is connected across its probes, first of all, it is checked for its magnitude within the specified range.
If voltage is high, then it is attenuated proportionally. The attenuator is a ladder of high wattage resistors. It has number of steps for attenuation from several volts to kilovolts. To select a particular range for measuring voltage, first switch to higher range.
2. Resistance to voltage converter: The proportional current flows through the resistor, from constant current source. According to Ohm’s law voltage is produced across it. This voltage is directly proportional to its resistance. This voltage is buffered and fed to A-D converter, to get digital display in Ohms.
3. AC voltage to DC voltage converter: AC voltage is rectified to convert it into proportional DC voltage. It is then fed to A-D converter to get the digital display in Volts.
4. AC /DC current to DC voltage converter: AC current is passed through known resistance. AC voltage obtained across resistance is rectified to get proportionate DC voltage. DC current is passed through known resistance to get DC voltage.
5. Analog to digital converter (ADC): After converting each quantity into DC voltage ADC plays its role of converting DC voltage into digital signal or binary signal. There are different circuits of ADC. Practically dual slope ADC is most widely used in digital multimeter due to its better accuracy. Now a days ICs are available for ADC, which are directly used in DMM.
6. Counter and Display unit: In DMM, after converting input quantity into digital signal or clock signal with help of ADC, it is applied to counter. It consists of decade counters and decoder driver. Numbers of clock pulses are displayed by 7-segment displays.
Front Panel controls of DMM:
Display: Where measurement read out can be viewed.
Buttons: For selecting various functions.
Dial: For selecting primary measurement value (volt, ampere, ohm).
Input jacks: Where test leads are inserted.
Applications of DMM:
Resistance measurement:
(a) Keep the selector switch to resistance part marked on front panel
(b) Connect the unknown resistance between input probes.
(c) Read the display carefully which is indicated in Ω, kΩ or MΩ.
AC/DC voltage measurement
(a) Keep the selector switch to DC volt or AC volt part marked on front panel
(b) Connect the terminals of probes across the component directly.
(c) Observe the reading carefully on the display.
AC/DC current measurement:
(a) )Keep the selector switch to DC current or AC current part marked on front panel.
(b) Beak the circuit, connect DMM probes in series. Take care of polarity while connecting.
(c) Turn power ON of the circuit.
(d) Observe the reading carefully on the display.
Function/frequency generator:
Function generator generates specific types of waveforms such as sine, square, triangular, sawtooth and pulse waveforms. The frequencies of these waveforms may be adjusted from few hertz to several hundred kHz.
Block diagram:
Working:
Fig. shows basic elements of function generator. This instrument delivers sine, triangular and square wave. The function generator is provided with frequency control network. The frequency control voltage regulates two currents, upper current source and lower current source.
The upper source supplies constant current to integrator circuit. The output voltage increases linearly with time. The voltage comparator multivibrator is used to change state at predetermined level. This cuts off the upper current source supply and switches on the lower current supply.
The direction of current from lower current source is opposite to that of upper current source. Thus output voltage decreases linearly with time. When it reaches to predetermined level, voltage comparator multivibrator cuts off lower current source and switches on upper current source.
Hence we get a triangular wave shape. Its frequency is determined by the magnitude of current.
A square waveform is obtained at he output of the multivibrator.
The triangular waveform is converted into a sine wave by resistance diode wavw shaping network.
Output amplifiers are used to amplify the signals.
Applications:
Function generators are used to
Study the frequency response of audio and radio circuits.
To study waveforms.
Square wave as clock input to digital circuits.
Sawtooth may be used to drive horizontal plates of cathode ray tube in CRO.
Questions
Marks wise Questions: 1M(2), 3M(3), 4M(2), 6M(1)
1. Fill in the blanks.
1. Heart of oscilloscope is ----------.
(a) Monitor (b) CRT (c) CRO (d) None
2. Input to CRO is always given in -----------.
(a) Horizontal amplifier (b) Vertical amplifier
(c) CRT (d) None
3. ------------- is used on inner surface of CRT screen.
(a) Lithium (b) Phosphorous (c) Aluminium (d) Graphite
4. Time base generator is nothing but ------------ oscillator.
(a) Sine wave (b) Square wave (c) Sawtooth wave (d) None
5. Lissajous pattern obtained on CRO can be used to determine -------------.
(a) Frequency (b) Voltage (c) Current (d) Voltage & current
6. The purpose of focus control in CRO is to control -------------.
(a) Brightness of display (b) Sharpness of display
(c) Brightness & sharpness of display (d) Trigger of time base
7. CRO always measures ------------- AC voltage.
(a) Peak to peak (b) average (c) rms (d) average & rms
8. If Lissajous pattern seen on CRO is circle, then phase difference between two signals is --------.
(a) 00 (b) 900 (c) 1800
9. Deflection sensitivity of CRT is measured in --------.
(a) ohm/volt (b) volt/ohm (c) mm/volt
10. DMM basically measures ---------.
(a) DC voltage (c) DC current (c) AC voltage
11. DMM measures --------- value of AC voltage.
(a) RMS (b) peak (c) Average
12. .Basic waveform used in function generator is ---------.
(a) Sine wave (b) Square wave (c) Triangular wave
13. The instrument which generates sine, square, triangular and sawtooth wave is called---------.
(a) RF signal generator (b) AF signal generator (c) Function generator
14. Screen of CRO is coated with ----------.
(a) Phosphor (b) Phosphorous (c) Aquadag
15. Magnitude of beam current through electron gun of CRT is adjusted by -------.
(a) Focus control (b) Brightness control (c) Intensity control
16. Property of phosphor material to emit light when stimulated by radiation is --------.
(a) Phosphorence (b) Luminance (b) Florescence
17. DMM has ----------.
(a) Low input impedance (b) Low output impedance (c) High input impedance
18. Deflection sensitivity of CRT is measured in ------.
(a) Ohm/volt (b) Volt/ohm (c) mm/volt
2. Match the following.
A B
(a) Electron gun (1) Lissajous pattern
(b) Time base generator (2) Sharpness
(c) Phase measurement (3) Brightness
(d) Focus (4) Sawtooth oscillator
(e) Intensity (5) CRT
3. State true or false.
1. In CRO, Lissajous pattern is used to measure DC voltage.
2. The signal waveform to be observed is applied to vertical amplifier in CRO.
3. In CRO, aquadag coating collects electrons produced by secondary emission when electrons strike the screen.
4. Focus controls brightness of display.
5. Intensity controls sharpness of display.
6. X - position controls horizontal position of display in CRO.
7. CRO measures peak to peak AC voltage.
8. Frequency control network circuit is used in function generator.
4. Answer the following questions.
1. State and explain principle of CRO.
2. Draw block diagram of CRO.
3. Draw internal sketch of CRT. Explain its construction and working.
4. State and explain front panel of CRO.
5. Explain how DC voltage can be measured with CRO.
6. Explain how AC voltage can be measured with CRO.
7. Explain how frequency can be measured with CRO.
8. Explain frequency measurement using Lissajous figures in CRO.
9. Explain phase measurement between two sine waves using CRO.
10. State and explain types of CROs.
11. Explain working principle of dual trace CRO.
12. Write the advantages of DMM over an analog multimeter.
13. Draw a neat block diagram of DMM and write the function of each block.
14. What is function generator?
15. Draw block diagram of function generator and explain its working.
16. Write the applications of function generator.
17. Write front panel controls of DMM.
18. State and explain applications of DMM.
