Electronics Unit 5 - B.Tech 2nd Year.pdf

Electronic engineering unit 5 notes

1. SYLLABUS
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Unit 03
Electronic instrumentation and measurements: Digital voltmeter: Introduction,
RAMP techniques digital multimeters: Introduction Oscilloscope: introduction,
basic principle, CRT, block diagram of oscilloscope, simple, measurement of
voltage, current phase and frequency using CRO, introduction of digital storage
oscilloscope and comparison of DSO with analog oscilloscope.
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Digital Voltmeter:-
Similar to an analog voltmeter, the digital voltmeter is used for the measurement of potential
differences between two specific points in an electric circuit.
The voltage that has to be measured can be either alternating or direct current and the
measured value is displayed as discrete numerals.

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Ramp Type Digital Voltmeter

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The working of ramp types DVM is essentially dependent on the measurement of time. The
device consists of a ramp generator that produces a signal that represents a ramp. Because of
the ramp generator used in this circuit, the device is named ramp type DVM.
1.The measurement of voltage in this device is initiated by providing an unknown voltage
signal to the ranging and attenuation section which either amplifies or attenuates the signal
as needed.
2.Also, from the ramp generator, either a positive or negative ramp voltage signal is
considered. Let us consider a negative ramp signal which will be compared with the
unknown signal.
3.The amplified or attenuated signal is given to a comparator that compares both the ramp and
unknown input signals.

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3. During the comparison, when the input voltage matches with ramp voltage, then a pulse is
generated which opens the gate. With respect to time, the ramp voltage tends to decrease until it
reaches ‘0’ voltage. Now, the ground comparator delivers a pulse to close the gate.
4. The time period between the opening and closing of a gate is termed a gating time interval.
During this time interval, the pulses from the clock generator pass through the gate and those will
be counted by the counter and displayed.
5. The oscillator generates clock pulses which are permitted to get through the gate to a counter
where it is a counter for the total number of pulses passed through the gate.
The sample rate multivibrator specifies the rate when the measurement cycles are initiated and is
used to provide the initiating pulse for the ramp generator to initiate the next ramp voltage signal.

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Advantages of Digital Voltmeters:
1.Outputs on the screen are accurate without any errors
2.Readings are taken faster
3.Output can be stored in memory devices
4.Versatile and accurate
5.Power consumption is less
6.Portable instrument
7.Cheap cost and compact

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Applications of DVM:
•Using a digital voltmeter, the actual voltage levels of various components can be known
easily.
•With the known voltage values from DVM, current levels can be found.
•With a digital voltmeter, one can check whether there is power in the circuit or not.

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Digital Multi-meters

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Digital Multimeter:
To measure voltage (AC, DC), current (AC, DC) and resistance, two types of
instruments, analog and digital meters, are utilized. The measurements of these
fundamental electrical quantities are based on either one of the following:
i)Current sensing. The instruments are mostly of the electromagnetic meter
movement type, such as an analog multimeter.
ii)Voltage sensing. The instruments are mostly electronic in nature, using amplifiers
and semiconductor devices, such as a digital multimeter.

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BLOCK DIAGRAM OF DIGITAL MULTIMETER:

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•The main part of most of the digital multimeter (DMMs) is the analog to digital
converter (A/D) which converts an analog input signal to a digital output.
•Since the DMM is a voltage sensing meter; current is converted to volts by passing
it through a precision low resistance shunt while ac is converted to dc at the AC
converter by employing rectifiers and filters.
•For resistance measurement, the meter includes a precision low current source that
is applied across the unknown resistor. Then the dc voltage drop across the resistor,
which is proportional to the value of the unknown resistor, is measured.

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Advantages of Digital Multi-meters:
•High accuracy
•High input impedance so there is no effect for loading
•A clear reading at higher viewing distances can be obtained.
•The electrical output can be used to interface with additional equipment
•These are less costly due to the integrated technology
•Output display can be provided automatically.

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Applications of DMM:
•AC/DC voltage measurement
•AC/DC current measurement
•Resistance & continuity measurement
•To check diode
•Measurement of capacitance
•Measurement of frequency
•To test batteries

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OSCILLOSCOPE

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Introduction:
An oscilloscope is a voltage sensing
electronic instrument that is used to visualize
certain voltage waveforms. An oscilloscope can
display the variation of a voltage waveform in
time on the oscilloscope’s screen. The heart of
the oscilloscope is the CRT.

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Cathode Ray Oscilloscope Principles:

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The above Figure shows the structure, and
the main components of a cathode ray tube
(CRT). In this Electron beam generated by the
electron gun first deflected by the deflection
plates, and then directed onto the fluorescent
coating of the CRO screen, which produces a
visible light spot on the face plane of the
oscilloscope screen.

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Cathode Ray Tube (CRT):
CRT is composed of two main parts,
1)Electron Gun
2)Deflection System

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1) Electron Gun:

Electron gun provides a sharply focused electron beam directed toward the
fluorescent-coated screen. The thermally heated cathode emits electrons in many
directions. The control grid provides an axial direction for the electron beam and
controls the number and speed of electrons in the beam.
2) The Deflection System:
The deflection system consists of two pairs of parallel plates, referred to as
the vertical and horizontal deflection plates. One of the plates in each set is
permanently connected to the ground, whereas the other plate of each set is connected
to input signals of the CRO.

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As shown in above Figure, the electron beam passes through the deflection
plates. In reference to the schematic diagram in above Figure, a positive voltage applied
to the Y input terminal causes the electron beam to deflect vertically upward, while a
negative voltage applied to the Y input terminal causes the electron beam to deflect
vertically downward. Similarly, a positive voltage applied to the X input terminal will
cause the electron beam to deflect horizontally toward the right, while a negative
voltage applied to the X input terminal will cause the electron beam to deflect
horizontally toward the left of the screen.

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The amount of vertical or horizontal deflection is directly
proportional to the corresponding applied voltage. When the electrons hit the
screen, the phosphor emits light and a visible light spot is seen on the screen.
Since the amount of deflection is proportional to the applied voltage,
actually the voltages Vy and Vx determine the coordinates of the bright spot
created by the electron beam.

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Block Diagram of a CRO:

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•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 delay produced is about
0.25msec.

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•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.

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•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.

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•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.

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Applications of CRO:
•The CRO’s are used in huge applications like radio stations for observing the transmitting
& receiving the properties of the signal.
•The CRO is used to measure the voltage, current, frequency, inductance, admittance,
resistance, and power factor.
•This device is also used to check the AM and FM circuits characteristics
•The shape of voltage and current waveform can be observed by CRO which helps to take
the necessary decision in a radio station or communication station.
•Transistor curves can be traced.

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Digital Storage Oscilloscope

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DSO:
Digital storage oscilloscope is define as an electronic device that stores and analyses the signal
in the digital format. When the input signal is given to the DSO, then it is processed, stored in
the memory, and displayed on the screen. It stores the signal in the form of digital data as
either 1 Or 0.

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Block Diagram of DSO

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As seen in the above figure, at first digital storage oscilloscope digitizes the analog
input signal, then the analog input signal is amplified by amplifier if it has any weak
signal. After amplification, the signal is digitized by the digitizer and that digitized
signal stores in memory. The analyzer circuit process the digital signal after that the
waveform is reconstructed (again the digital signal is converted into an analog form)
and then that signal is applied to vertical plates of the cathode ray tube (CRT).

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The cathode ray tube has two inputs they are vertical input and horizontal input. The
vertical input signal is the ‘Y’ axis and the horizontal input signal is the ‘X’ axis. The
time base circuit is triggered by the trigger and clock input signal, so it is going to
generate the time base signal which is a ramp signal. Then the ramp signal is
amplified by the horizontal amplifier, and this horizontal amplifier will provide input
to the horizontal plate. On the CRT screen, we will get the waveform of the input
signal versus time.

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