Download - communication engineering lab manual
ECE Dept, KEC
KUPPAM ENGINEERING COLLEGE, KUPPAM - 517425
(Approved by AICTE & Affiliated to JNTUA)
LABORATORY MANUAL
For
COMMUNICATION ENGINEERING
(FOR II DECE-REGULATION – C09)
Department Of
ELECTRONICS & COMMUNICATION ENGINEERING
Academic Year: 2013-2014.
ECE Dept, KEC
CE LAB MANUAL Page 2
INDEX
S.NO NAME OF THE EXPERIMENT PAGE NO.
1 Simple method of generation and detection of A.M.
3-11
2 Simple method of generation and detection of F.M.
12-19
3 Study of AM super heterodyne receiver.
20-23
4 Test audio amplifier section of super heterodyne
receiver.
20-23
5 Measurement of sensitivity, selectivity of a radio
receiver using field strength meter.
24-25
6 Series Resonance
26-30
7 Parallel Resonance
31-35
8 Verification of Thevinen’s theorem.
36-39
9 Verification of Super position theorem.
40-43
10 Verification of maximum power transfer theorem.
44-46
11 Differentiator and integrator circuits.
47-49
12 Design common emitter amplifier using electronic work
bench
50-53
ECE Dept, KEC
Experiment No: 1.1 Date:
AM MODULATOR
Aim: To construct AM modulator and to determine modulation index ‘m’.
Apparatus: Resistors (1KΩ) - 3 No’s
Capacitors (0.01µF) - 1 no
Diode (1N4007) - 1 no
Decade Inductance Box - 1 no
Signal generators - 1 no
CRO - 1 no
Tag board - 1 no.
Connecting wires.
Circuit diagram:
Fig (1.1) AM modulator circuit.
Tank circuit design:
Frequency of oscillation, fc= 1/2π√ (L1C1 )
Assume C1= 0.01uF
We know that,
Carrier frequency, fc = 10 KHz
Therefore L1 = _________ m.H.
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CE LAB MANUAL Page 4
Theory:
Amplitude modulation: “Amplitude modulation is a process in which the amplitude of high
frequency carrier is varied in accordance with the amplitude of a low frequency modulating signal.”
In amplitude modulation, the information signal varies the amplitude of the carrier
sine wave. In other words, the instantaneous value of the carrier amplitude changes in accordance
with the amplitude of the modulating signal. Fig (1.1) shows amplitude modulator circuit. It is a
circuit which generates amplitude modulation. Fig (1.2) shows a single frequency sine wave
modulating a high frequency carrier signal.
Note that the carrier frequency remains constant during the modulation process but its
amplitude varies in accordance with the modulating signal. An increase in the modulating signal
amplitude causes the amplitude of the carrier to increase. An increase or decrease in the amplitude of
the modulating signal causes a corresponding increase or decrease in both the positive and negative
peaks of the carrier amplitude.
Modulation index: In order for proper AM to occur, the modulating signal voltage must be
less than carrier voltage. Therefore, the relationship between the amplitudes of the modulating signal
and the carrier is important. This relationship is expressed in terms of a ratio known as the
modulation index, m.
Modulation index is the ratio of the modulating signal voltage to the carrier voltage.
MODULATION INDEX, M=VM/VC
The modulation index should be a number between 0 and 1. If the amplitude of the
modulating voltage is higher than the carrier voltage, m will be greater than 1. This will cause severe
distortion of the modulated wave form. This condition is called over modulation. When m=1 the
condition is called full modulation.
Whenever the modulation index is multiplied by 100, the degree of modulation is
expressed as a percentage. In this case modulation index is called percentage modulation.
%m= (Vm/Vc) x100
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Procedure:
1. Connections are made as shown in the circuit diagram choosing appropriate components.
2. Apply 1V/1KHz modulating signal and 4V/10KHz carrier signal.
3. Connect the CRO across the tank circuit and observe the AM waveform.
4. Measure Vmax and Vmin from the AM waveform and calculate the modulation index m.
5. Repeat the above step for different values of modulating signal and carrier signal voltages.
6. Sketch the modulating signal, Carrier signal and AM wave forms.
Waveforms:
Fig (1.2) Wave forms a) Carrier signal b) Modulating signal c) AM signal
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CE LAB MANUAL Page 6
Tabular column:
Sl No. Vmax, in Volts Vmin, in Volts Modulation index, m
in percentage
1
2
3
Calculations:
Vmax = Vmin =
Modulation index = (Vmax - Vmin) / (Vmax + Vmin)
=
Result:
ECE Dept, KEC
CE LAB MANUAL Page 7
Self Assessment Questions:
1. Define amplitude modulation?
2. What is the frequency of the modulated signal?
3. Identify the modulating signal in AM.
4. Define modulation index?
5. What is the range of modulation index?
6. Draw the frequency spectrum of AM wave.
7. What is the band width of AM wave?
8. Draw the under modulation critical modulation and over modulation wave form.
9. What are the advantages and disadvantages of AM wave?
10. What are the applications of AM wave?
ECE Dept, KEC
CE LAB MANUAL Page 8
Experiment No: 1.2 Date:
AM DETECTOR
Aim: To construct AM envelope detector and to observe the demodulated waveform.
Apparatus: Resistors (1KΩ) - 3 nos
Capacitors (0.01µF & 0.1µF) - 1no
Diodes (1N4007, 0A79) - 1 no (each)
Decade Inductance Box - 1 no
Decade resistance box - 1 no
Signal generators - 1 no
CRO - 1 no
Tag board - 1 no
Connecting wires.
Circuit diagram:
Fig (1.2.1) AM Modulator
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CE LAB MANUAL Page 9
Fig (1.2.2) AM Detector (Envelope detector)
Filter design:
Filter cut off frequency, fm = 1/2πR4C2
Assume C2 = 0.1uF
We know that
fm = 1 KHz
Therefore,
R4 = _________ K Ohms.
Theory: Fig (1.2.1) shows the amplitude modulator circuit and fig (12.2) shows AM demodulator
(detector) circuit. A demodulator is a circuit that accepts a modulated signal and recovers the original
modulating information. A demodulator circuit is the key circuit in the radio receiver.
Diode detector: The simplest and most widely used amplitude demodulator is the diode detector
shown in fig (1.2.2). The AM signal is applied to the rectifier circuit consisting of diode, capacitors
and resistors. The diode conducts when the negative half cycles of the AM signals occur. During the
positive half cycles, the diode is reverse biased and no current flows through it.
To recover the original modulating signal filter is connected after the diode. The filter
is designed such that capacitors have very low impedance at the carrier frequency. At the frequency
of the modulating signal, they have much higher impedance. The result is that capacitors effectively
short or filter out the carrier, thereby leaving the original modulating signal. The fig 1.2.3b shows the
demodulated signal.
CE LAB MANUAL
Waveforms:
Fig (1.2.3) Waveforms
Procedure:
1. Connections are made as shown in the circuit diagram choosing appropriate components.
2. Connect the output of AM modulator to the in
3. Connect the CRO to the output of envelope detector.
4. Observe the demodulated wave form, measure the frequency of this waveform and compare it
with the original modulating signal.
5. Sketch AM wave and demodulated waveform.
Result:
forms a) AM signal b) Demodulated signal
Connections are made as shown in the circuit diagram choosing appropriate components.
of AM modulator to the input of envelope detector.
Connect the CRO to the output of envelope detector.
Observe the demodulated wave form, measure the frequency of this waveform and compare it
with the original modulating signal.
Sketch AM wave and demodulated waveform.
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Page 10
Connections are made as shown in the circuit diagram choosing appropriate components.
Observe the demodulated wave form, measure the frequency of this waveform and compare it
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CE LAB MANUAL Page 11
Self Assessment Questions:
1. What is the main active component in the demodulation circuit of an AM wave?
2. What is the input signal to the detector circuit?
3. What is the output of a detector circuit?
ECE Dept, KEC
CE LAB MANUAL Page 12
Experiment No: 2.1 Date:
FM MODULATOR
Aim: To study FM modulator and to observe the FM wave form.
Apparatus: FM modulator trainer and CRO.
Circuit diagram:
Fig (2.1) Circuit diagram of FM modulator
Theory: Frequency modulation: “Frequency modulation is a process in which the frequency of a
high frequency carrier is varied in accordance with the amplitude of a low frequency modulating
signal.”
In FM, the carrier amplitude remains constant, while the carrier frequency is changed
by the modulating signal. As the amplitude of the information signal varies, the carrier frequency
shifts in proportion. As the modulating signal amplitude increases, the carrier frequency increases. If
the amplitude of the modulating signal decreases, the carrier frequency decreases. The reverse
relationship can also be implemented.
ECE Dept, KEC
CE LAB MANUAL Page 13
As the modulating signal amplitude varies, the carrier frequency varies above and
below its normal center frequency with no modulation. The amount of change in carrier frequency
produced by the modulating signal is known as the frequency deviation. Maximum frequency
deviation occurs at the maximum amplitude of the modulating signal.
The frequency of the modulating signal determines how many times per second
the carrier frequency deviates above and below its nominal center frequency 100 times per second.
This is called the frequency deviation rate.
An FM signal is illustrated in fig (2.1.1c). With no modulating signal applied,
the carrier frequency is a constant amplitude sine wave at its normal constant center frequency. The
modulating information signal [Fig (2.1.1b)] is a low frequency sine wave. As the sine wave goes
positive, the frequency of the carrier increases proportionately. The highest frequency occurs at the
peak amplitude of the modulating signal. As the modulating signal amplitude decreases, the carrier
frequency decreases. When the modulating signal is at zero amplitude, the carrier will be at its center
frequency point.
When the modulating signal goes negative, the carrier frequency will decrease.
The carrier frequency will continue to decrease until the peak of the negative half cycle of the
modulating sine wave is reached. Then, as the modulating signal increases towards zero, the
frequency will again increase.
Waveforms:
Fig (2.1.1) Wave forms
a) Carrier signal b) Modulating signal c) FM signal
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CE LAB MANUAL Page 14
Procedure:
1. Switch on the FM experimental board.
2. Connect Oscilloscope to the FM O/P and observe that carrier frequency at that point without any
A.F. input.
3. Connect around sine wave (A.F. signal) to the input of the frequency modulator (At AF input).
4. Now observe the frequency modulation output on the 1st channel of on CRO and adjust the
amplitude of the AF signal to get clear frequency modulated wave form.
5. Vary the modulating frequency (A.F Signal) and amplitude and observe the effects on the
modulated waveform.
Calculations:
fmax = fmin =
δf = (fmax - fmin) / 2
=
Mf = δf / fm
=
Result:
ECE Dept, KEC
CE LAB MANUAL Page 15
Self Assessment Questions:
1. What is the modulation index in FM?
2. What is the range of modulation index in FM?
3. What is the bandwidth of frequency modulation?
4. What are the disadvantages of FM?
5. What is the value of ‘m’ for narrow band FM?
6. Where is narrow band FM is used?
7. Draw the FM wave?
ECE Dept, KEC
CE LAB MANUAL Page 16
Experiment No: 2.2 Date:
FM DEMODULATOR
Aim: To study FM demodulator and to observe the demodulated wave form.
Apparatus: FM demodulator trainer and CRO.
Circuit diagram:
Fig (2.2.1) Circuit diagram of FM demodulator.
Theory:
Fig (2.2.1) shows the Circuit diagram of FM demodulator. Demodulation is the process of
recovering the low frequency modulating signal. Here in FM demodulator the frequency modulated
signal is inputted. The output of the demodulator is the original low frequency modulating signal. Fig
(2.2.2) shows original modulating signal, FM signal and demodulated signal.
ECE Dept, KEC
CE LAB MANUAL Page 17
There are literally dozens of circuits used to demodulate or detect FM signals. The
well-known Foster-Seeley discriminator and the ratio detector were among the most widely used
frequency demodulators at one time, but today these circuits have been replaced with more
sophisticated IC demodulators. Anyhow, they are still found in older equipment. The most widely
used detectors today include the pulse-averaging discriminator, the quadrature detector and the phase
locked loop (PLL).
The PLL is the best of all frequency demodulators in use. Its ability to provide
frequency selectivity and filtering give it a signal to noise ratio superior to any other type of FM
detector. The linearity of the VCO ensures the highly accurate reproduction of the original
modulating signal. Although PLLs are complex, they are easy to apply because they are readily
available in low cost IC form.
Waveforms:
Fig (2.2.2) Wave forms
a) Modulating signal (CH1 of CRO) b) FM signal c) Demodulated signal (CH2 of CRO)
ECE Dept, KEC
CE LAB MANUAL Page 18
Procedure:
1. Switch on the FM experimental board.
2. Connect Oscilloscope to the FM O/P and observe that carrier frequency at that point without any
A.F. input.
3. Connect around sine wave (A.F. signal) to the input of the frequency modulator (At AF input).
4. Now observe the frequency modulation output on the 1st channel of on CRO and adjust the
amplitude of the AF signal to get clear frequency modulated wave form.
5. Vary the modulating frequency (A.F Signal) and amplitude and observe the effects on the
modulated waveform
6. Connect the FM o/p to the FM i/p of De-modulator.
7. Vary the potentiometer provided in the demodulator section.
8. Observe the output at demodulation o/p on second channel of CRO.
9. Draw the demodulated wave form
Calculations:
Modulating signal, fm=
and
Amplitude =
Result:
ECE Dept, KEC
CE LAB MANUAL Page 19
Self Assessment Questions:
1. What is the basic FM demodulator circuit?
2. In T.V receiver, what is the purpose of FM detector?
3. What is the maximum allowable frequency deviation in FM?
CE LAB MANUAL
Experiment No: 3&4
AM SUPER HETERODYNE RECEIVER
Aim: To study of AM Super Heterodyne Radio Receiver.
Apparatus: Radio Receiver kit, set of connecting wire, Oscilloscope. Multi meter, RF signal
generator.
Circuit diagram:
Fig (3.1): AM Super Heterodyne Radio Receiver.
Theory:
The basic requirement for any communications receiver is to have the ability to select a signal
of desired frequency, while rejecting closely adjacent frequencies (Selectivity) and provide sufficient
amplification to recover the modulating signal (Sensitivity). A receiver with good selectivity will
isolate the desired signal in the RF spectrum and eliminate all other signals. This can be achieved
using tuned LC circuits resonating at the desired frequency. LC circuits with a high
narrower bandwidths and hence have better selectivity. However it must be noted the bandwidth must
be sufficiently large such that it passes the carrier as well as the sidebands to avoid attenuation and
hence distortion of the transmitted info
The sensitivity of a communications receiver is a function of the overall receiver gain. In
general, higher gain means better the sensitivity. This can be achieved by multiple stages of
amplification. There are two types of communications recei
receiver and the super heterodyne receiver. Although the TRF system is a straightforward concept at
AM SUPER HETERODYNE RECEIVER
To study of AM Super Heterodyne Radio Receiver.
Receiver kit, set of connecting wire, Oscilloscope. Multi meter, RF signal
Fig (3.1): AM Super Heterodyne Radio Receiver.
The basic requirement for any communications receiver is to have the ability to select a signal
of desired frequency, while rejecting closely adjacent frequencies (Selectivity) and provide sufficient
modulating signal (Sensitivity). A receiver with good selectivity will
isolate the desired signal in the RF spectrum and eliminate all other signals. This can be achieved
using tuned LC circuits resonating at the desired frequency. LC circuits with a high
narrower bandwidths and hence have better selectivity. However it must be noted the bandwidth must
be sufficiently large such that it passes the carrier as well as the sidebands to avoid attenuation and
hence distortion of the transmitted information.
The sensitivity of a communications receiver is a function of the overall receiver gain. In
general, higher gain means better the sensitivity. This can be achieved by multiple stages of
amplification. There are two types of communications receiver; the Tuned Radio Frequency (TRF)
receiver and the super heterodyne receiver. Although the TRF system is a straightforward concept at
ECE Dept, KEC
Page 20
Date:
Receiver kit, set of connecting wire, Oscilloscope. Multi meter, RF signal
The basic requirement for any communications receiver is to have the ability to select a signal
of desired frequency, while rejecting closely adjacent frequencies (Selectivity) and provide sufficient
modulating signal (Sensitivity). A receiver with good selectivity will
isolate the desired signal in the RF spectrum and eliminate all other signals. This can be achieved
using tuned LC circuits resonating at the desired frequency. LC circuits with a high Q value have
narrower bandwidths and hence have better selectivity. However it must be noted the bandwidth must
be sufficiently large such that it passes the carrier as well as the sidebands to avoid attenuation and
The sensitivity of a communications receiver is a function of the overall receiver gain. In
general, higher gain means better the sensitivity. This can be achieved by multiple stages of
ver; the Tuned Radio Frequency (TRF)
receiver and the super heterodyne receiver. Although the TRF system is a straightforward concept at
ECE Dept, KEC
CE LAB MANUAL Page 21
high frequencies it becomes difficult to build, is less efficient, has small gain and suffers bandwidth
changes. For these reasons among others the super heterodyne receiver has become the model for all
receivers; AM, FM, television, satellite, radar etc.
Following the block diagram Fig. 3.1 above, the incoming signal is picked up on the antenna
and fed to an RF amplifier. The RF amplifier provides some initial gain and selectivity and minimizes
radiation of the Local Oscillator (LO) signal through the receiving antenna by isolating the Mixer
from the antenna. However, the most important function of the RF amplifier is to eliminate what is
known as the image signal. The frequency of this signal is greater than the LO and will mix to give a
mixer output at the IF frequency. This will cause problems as after down conversion to IF it will
appear at the same frequency as the desired signal and cause interference. Therefore, signals at the
image frequency
fimage = fRF + 2 fIF
The output of the RF amplifier is then applied to the input of the Mixer. It also has an input
from the LO. The Mixer (or Frequency Converter) is a non-linear device, which results in the creation
of sum and difference frequencies. The output from the Mixer is a combination of the received signal
and the LO signal as well as their sum and difference frequencies. This process is called
Heterodyning. The non-linearity is necessary to provide the mathematical equivalent of time
multiplication between the LO voltage and the RF signal voltage. A tuned circuit at the Mixer output
selects the Difference frequency (i.e. the IF or Intermediate frequency). The LO frequency is tunable
over a wide range and therefore the Mixer can translate a wide range of input frequencies to the IF.
The LO frequency is higher than incoming RF frequency (High Side Injection) for engineering
reasons.
fLO = fRF + fIF.
Therefore the difference or intermediate frequency (IF) is fIF = fLO - fRF. This frequency is
selected while the other signals are rejected (fLO, fRF, fLO + fRF). The output of the mixer is amplified
by one or more IF amplifier stages. Most of the receiver sensitivity and selectivity is to be found in
these stages. All IF stages are fixed and tuned to fIF only (this standard is fixed at 455kHz). Hence,
high selectivity can be obtained. The highly amplified IF signal is now applied to the detector or
demodulator where the original modulating signal is recovered. The detector output is then amplified
to drive a Loudspeaker.
ECE Dept, KEC
CE LAB MANUAL Page 22
Requirements for Receiver:
1. Selectivity is that property of a receiver which enables it to differentiate between one
broadcast frequency and another.
2. Sensitivity is that property of a receiver which enables it to pick up signals from distant
stations with easy, and with very little signal energy supplied to its antenna.
3. Fidelity refers to the tone quality produced by the receiver .a receiver possessing good fidelity
provides a rich and true reproduction of sound.
These Principles used in the super heterodyne aid in obtaining all three of these desired
qualities from the receiver.
Procedure:
1. Study the circuit diagram on the front panel of the training board.
2. Patch the circuit as shown in wiring diagram.
3. Connect Antenna output to input of mixer.
4. The output of mixer should be connected to input of IF amplifier (i.e. Primary of(IFT-1).As we
vary the tuning condenser the frequency will change.
5. The output of IFT-1 should be connected to base of transistor BF 195D.
6. Connect output of IFT-2 to Base of transistor BF 195C.
7. The output of IFT-3 should be connected to diode for detector stage.
8. The detected output signal should be connected to the input of AF Amp. Through 10K
potentiometer. As you vary the 10K volume control potentiometer the volume can be increased or
decreased.
The output of the AF Amp is connected to power amplifier input. The power amplifier is a push-pull
amplifier which can drive loud speaker.
ECE Dept, KEC
CE LAB MANUAL Page 23
Observations:
Study of sensitivity of Radio Receiver:
S NO. Frequency( in KHz) Output voltage (in V)
Study of selectivity of Radio Receiver:
Output voltage at resonance=3V
Frequency( in KHz) Observed output Ratio= Voltage off Resonance
voltage (V) Voltage at Resonance
Plot Graph between Ratio (dB) Versus Kilocycle of frequency
Ratio in dB Frequency
Off resonance – at resonance
Result:
ECE Dept, KEC
CE LAB MANUAL Page 24
Experiment No: 5 Date:
MEASUREMENT OF SENSITIVITY, SELECTIVITY
Aim: To Study the Receiver Characteristics and Measurement of sensitivity, selectivity of a radio
receiver
RECEIVER CHARACTERISTICS
Sensitivity, noise, selectivity, and fidelity are important receiver characteristics. These
characteristics will be useful to you when performing receiver tests. They can help you to determine
whether a receiver is working or not or in comparing one receiver to another.
Sensitivity
The ability of a receiver to reproduce weak signals is a function of the sensitivity of a receiver.
The weaker a signal that can be applied to a receiver and still produce a certain value of signal output,
the better the sensitivity rating.
Sensitivity of a receiver is measured under standardized conditions. It is expressed in terms of
the signal voltage, usually in the micro volts that must be applied to the antenna input terminals to
give an established level of the output.
The output may be an ac or dc voltage measured at the detector output or a power
measurement (measured in decibels or watts) at the loudspeaker or headphone terminals.
Noise
All receivers generate a certain amount of noise, which you must take into account when
measuring sensitivity. Receiver noise may originate from the atmosphere (lightning) or from internal
components (transistors, tubes).
Noise is the limiting factor of sensitivity. You will find sensitivity is the value of input carrier
voltage (in micro volts) that must be applied from the signal generator to the receiver input to develop
a specified output power.
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Selectivity
Selectivity is the degree of distinction made by the receiver between the desired signal and
unwanted signals. You will find the better the ability of the receiver to reject unwanted signals, the
better its selectivity. The degree of selection is determined by the sharpness of resonance to which the
frequency determining circuits have been engineered and tuned.
You usually measure selectivity by taking a series of sensitivity readings. As you take the
readings, you step the input signal along a band of frequencies above and below the circuit resonance
of the receiver; for example, 100 kilohertz below to 100 kilohertz above the tuned frequency.
As you approach the tuned frequency, the input level required to maintain a given output level
will fall. As you pass the tuned frequency, the required input level will rise. Input voltage levels are
then compared with frequency.
They can be plotted on paper or you might view them on an oscilloscope. They would appear
in the form of a response curve. The steepness of the response curve at the tuned frequency indicates
the selectivity of the receiver.
Fidelity
The fidelity of a receiver is its ability to accurately reproduce, in its output, the signal that
appears at its input. You will usually find the broader the band passed by frequency selection circuits,
the greater your fidelity.
You may measure fidelity by modulating an input frequency with a series of audio
frequencies; you then plot the output measurements at each step against the audio input frequencies.
The resulting curve will show the limits of reproduction. You should remember that good selectivity
requires that a receiver pass a narrow frequency band.
Good fidelity requires that the receiver pass a broader band to amplify the outermost
frequencies of the sidebands. Receivers you find in general use are a compromise between good
selectivity and high fidelity.
Result:
ECE Dept, KEC
CE LAB MANUAL Page 26
Experiment No: 6 Date:
SERIES RESONANCE
Aim: To design the series resonant circuit for the given resonant frequency and to plot the frequency
response characteristic of that series resonant circuit.
Apparatus: Resistor 100 Ohms, Inductor 1m.H. (DIB), Capacitor 1µ .F. (DCB), Signal generator,
Multi meter.
Circuit diagram:
Fig (6.1) Series resonant circuit
Design: Given,
Series resonant frequency= 5 KHz
We know that,
Series resonant frequency, fs= 1/2π√LC
Assume C= 1 µ.F.
Therefore, L= _________ m.H.
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CE LAB MANUAL Page 27
Theory: The resonant circuit is a combination of R, L and C elements. The interesting characteristic
of such circuit is that, they exhibit maximum or minimum current output at a particular frequency.
This property is called as resonance. The frequency at which resonance occurs is called as resonant
frequency. The R, L and C components may be connected in series or in parallel. Accordingly, there
are two types of resonant circuits.
They are
1. Series resonant circuit
2. Parallel resonant circuit (Anti resonance)
Series resonance: Fig (6.1) shows series resonant circuit. Series resonance is a phenomenon in a
series RLC circuit. At series resonance inductive reactance is equal to capacitive reactance. Hence
inductive and capacitive reactance’s cancel out each other. The two reactance’s act as a short circuit
since no voltage drops across them. All the applied voltage drops across resistor R. At resonance the
net impedance of the circuit is purely resistive.
Fig (6.2) shows frequency response characteristic of series resonant circuit.
This characteristic is also called as universal resonance curve. It is the plot of current flowing through
the resonant circuit v/s frequency. This curve is bell shaped curve, with its peak value at f=fs. For all
other values of frequency current flowing through the circuit is less than Io. This is because for a
frequency less than fs, capacitive reactance will be more and for a frequency more than fs inductive
reactance will be more. In either case, the net impedance of the circuit is more than its resistance R.
Hence the current is less.
From the graph it is clear that current reduces on either side of fs gradually.
Mark a point on current axis, where the current is equal to 0.707 X Ip. Extend that point on the plot.
The extended line cuts the plot at two points. Extend both the points on to the frequency axis. We will
get two frequencies f1 and f2. Observe that between f1 and f2 current flowing through the circuit is
more, whereas below f1 and above f2 current is less. We conclude that the circuit is frequency
selective. It allows more current for a range of frequencies and less current for all other frequencies.
The range of frequency in which current is more is called pass band (between f1 & f2) and range of
frequency in which current is less is called stop band. Frequencies f1 and f2 are called cut off
frequencies because they separate pass band and stop band.
ECE Dept, KEC
CE LAB MANUAL Page 28
Nature of graph:
Fig (6.2) Universal resonance curve (Frequency response characteristic)
Band width = f2-f1= _______Hz
Procedure:
1. Design the series resonant circuit for the given resonant frequency (Assume the value of C &
calculate the value of L using appropriate formula).
2. Make the connections as shown in the circuit diagram.
3. Keep the input voltage Vin= 5V by varying the amplitude knob of signal generator and maintain
it constant throughout the experiment.
4. Vary the input frequency in steps and note down the corresponding current.
5. Plot the graph of current vs. frequency.
6. Find out the band width from the graph.
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Tabular column: Vin= 5V
Sl. Frequency in Current I, in m.A.
No. Hz
1 100
2 200
3 300
4 500
5 700
6 900
7 1K
8 2K
9 3K
10 5K
11 7K
12 10K
Result:
ECE Dept, KEC
CE LAB MANUAL Page 30
Self Assessment Questions:
1. Define resonance
2. Define resonant frequency
3. Define bandwidth
4. Define quality factor
5. Differentiate between series resonance & parallel resonance.
S.NO SERIES RESONANCE PARALLEL RESONANCE
CE LAB MANUAL
Experiment No:7
PARALLEL RESONANCE
Aim: To design the parallel resonant circuit for the given resonant frequency and to plot
frequency response characteristic of that parallel resonant circuit.
Apparatus: Resistor 100 Ohms, Inductor 1m.H. (DIB), Capacitor 1µ .F. (DCB), Signal
Multi meter.
Circuit diagram:
Fig (7
Design:
Given,
Parallel resonant frequency, fp= 5 KHz
We know that,
Series resonant frequency, fp= 1/2
Assume C= 1 µ.F.
Therefore,
L= _________ m.H.
PARALLEL RESONANCE
To design the parallel resonant circuit for the given resonant frequency and to plot
frequency response characteristic of that parallel resonant circuit.
Inductor 1m.H. (DIB), Capacitor 1µ .F. (DCB), Signal
Fig (7.1) Parallel resonant circuit
Parallel resonant frequency, fp= 5 KHz
Series resonant frequency, fp= 1/2π√LC
µ.F.
L= _________ m.H.
ECE Dept, KEC
Page 31
Date:
To design the parallel resonant circuit for the given resonant frequency and to plot the
Inductor 1m.H. (DIB), Capacitor 1µ .F. (DCB), Signal generator,
ECE Dept, KEC
CE LAB MANUAL Page 32
Theory:
Fig (7.1) shows parallel resonant circuit. Parallel resonance is a phenomenon in a
parallel LC circuit. At parallel resonance inductive susceptance is equal to capacitive susceptance.
Hence inductive and capacitive susceptances cancel out each other. At parallel resonance net
admittance of the circuit is given by
Y= G + jbc - jbl
Where, ‘Y’ is admittance
‘G’ is conductance
‘bl’ is inductive susceptance.
‘bc’ is capacitive susceptance
At parallel resonance bl = bc. Therefore Y = G. We conclude that at parallel
resonance net admittance of the circuit is equal to conductance only and is minimum. Since
admittance is minimum at resonance impedance is maximum. Hence current at parallel
resonance is minimum.
Fig (7.2) shows frequency response characteristic of parallel resonant circuit. It is
also called as universal anti resonance curve. It is having inverted bell shape with its minimum
current value at f = fp.
At all other frequencies current flowing through the circuit is more. We conclude
that parallel resonant circuit is also frequency selective. It allows less current at frequencies
surrounding fp. i.e. parallel resonant circuit rejects these frequencies. Hence circuit is called
rejecter circuit
ECE Dept, KEC
CE LAB MANUAL Page 33
Nature of graph:
Fig (7.2) Frequency response characteristic of parallel resonance circuit
Procedure:
1. Design the parallel resonant circuit for the given resonant frequency (Assume the value of C
& calculate the value of L using appropriate formula).
2. Make the connections as shown in the circuit diagram.
3. Keep the input voltage Vin= 5V by varying the amplitude knob of signal generator and
maintain it constant throughout the experiment.
4. Vary the input frequency in steps and note down the corresponding current.
5. Plot the graph of current vs. frequency.
ECE Dept, KEC
CE LAB MANUAL Page 34
Tabular column: Vin= 5V
Sl. Frequency in Current I, in m.A.
No. Hz
1 100
2 200
3 300
4 500
5 700
6 900
7 1K
8 2K
9 3K
10 5K
11 7K
12 10K
Result:
ECE Dept, KEC
CE LAB MANUAL Page 35
Self Assessment Questions:
1. Define resonance.
2. Give the relation between fo, BW and ‘Q’ factor.
3. Give the relation for parallel resonant frequency.
4. How many types of resonance.
5. Calculate fo for the tank circuit
6. Define ‘Q’ in terms of energy
ECE Dept, KEC
CE LAB MANUAL Page 36
Experiment No: 8 Date:
VERIFICATION OF THEVININ’S THEOREM
Aim: to verify the thevinin’s theorem of a symmetrical T-network.
Apparatus:
1. Regulated power supply (0-30 v) - 1 No.
2. Resistors (1k ohms) - 4 No’s
3. Multi meter - 1 No
4. Decade resistance box(0-100 k ohms)-1 No
5. Ammeter (0-25 mA) -1 No
6. Voltmeter(0-25 v) -1No
Circuit diagrams:
Fig 4.2:Measurement Of Load Current For The Given Circuit(IL)
Fig
4.3: Measurement Of Thevinin Equivalent Voltage VTH Or Voc
ECE Dept, KEC
CE LAB MANUAL Page 37
Fig 4.4: Measurement Of Thevinin Equivalent Impedence(RTH)
Fig 4.5: Thevinin Equivalent Circuit.
Theory: Any linear electrical network with voltage and current sources and resistances can be
replaced at terminals A-B by an equivalent voltage source Vth in series connection with an equivalent
resistance Rth.
This equivalent voltage Vth is the voltage obtained at terminals A-B of the network with
terminals A-B open circuited.
This equivalent resistance Rth is the resistance obtained at terminals A-B of the network with
all its independent current sources open circuited and all its independent voltage sources short
circuited.
ECE Dept, KEC
CE LAB MANUAL Page 38
Procedure:
1. Make The Circuit Connections As Per Circuit Shown In Fig:8.2 and measure the load current.
2. Make The Circuit Connections As Per Circuit Shown In Fig:8.3 and measure the thevinins
equivalent voltage Vth.
3. Make The Circuit Connections As Per Circuit Shown In Fig:8.4 and measure the thevinin
equivalent resistance by using Multimeter.
4. Make The Circuit Connections As Per Circuit Shown In Fig:8.5 and measure the load current.
5. Compare the load current from thevinins equivalent circuit and the load current from fig:8.2.
Observations:
S NO SORCE
VOLTAGE
(V)
VTH
(V)
RTH
(OHMS)
LOAD
CURRENT(mA)
(direct ammeter
reading)
LOAD
CURRENT(mA)
(thevinins
equivalent)
Specimen calculations:
For Source Voltage VS= 15 V
1. VOC=VAB=VTH = × + volts.(refer fig 8.3)
2.REQ=R2 + × (refer fig. 8.4)
3.I L=
(refer fig.8.5)
Result:
ECE Dept, KEC
CE LAB MANUAL Page 39
Self Assessment Questions:
1. State thevenin’s theorem.
2. If there are any dependent sources in the circuit what should we do?
3. What are the limitations of thevenins theorem?
ECE Dept, KEC
CE LAB MANUAL Page 40
Experiment No: 9 Date:
VERIFICATION OF SUPERPOSITION THEOREM
Aim: To Verify The Super Position Theorem Of Symmetrical T Network.
Apparatus:
1. Regulated Power Supply (0-30 V) - 2 Nos.
2. Ammeter (0-25 mA) -1 No.
3. Resistors (100, 220, 330 Ohms) - 1 No.
Experimental Setup:
Fig: 9.1
Fig: 9.2
ECE Dept, KEC
CE LAB MANUAL Page 41
Fig:9.3
Theory:
The superposition theorem for electrical circuits states that for a linear system the
response (Voltage or Current) in any branch of a bilateral linear circuit having more than one
independent source equals the algebraic sum of the responses caused by each independent source
acting alone, while all other independent sources are replaced by their internal impedances.
To ascertain the contribution of each individual source, all of the other sources first must be
"turned off" (set to zero) by Replacing all other independent voltage sources with a short circuit
(thereby eliminating difference of potential. i.e. V=0, internal impedance of ideal voltage source is
ZERO (short circuit)).
Replacing all other independent current sources with an open circuit (thereby eliminating
current. i.e. I=0, internal impedance of ideal current source is infinite (open circuit).
This procedure is followed for each source in turn, then the resultant responses are added to
determine the true operation of the circuit. The resultant circuit operation is the superposition of the
various voltage and current sources.
The superposition theorem is very important in circuit analysis. It is used in converting any
circuit into its Norton equivalent or Thevenin equivalent.
Applicable to linear networks (time varying or time invariant) consisting of independent sources,
linear dependent sources, linear passive elements Resistors, Inductors, Capacitors and linear
transformers.
Another point that should be considered is that superposition only works for voltage and current but
not power. In other words the sum of the powers is not the real consumed power.
ECE Dept, KEC
CE LAB MANUAL Page 42
Procedure:
1. Make The Circuit Connections As Per Circuit Shown In Fig: 9.1. Note down The Ammeter
Reading I, at Different Values of Source Voltages.
2. Make The Connections As Per The Circuit Shown In The Fig.9.2.(i.e. Short Circuiting The Voltage
Source V2). Note Down The Ammeter Reading I1’.
3. Make the Circuit Connections as per the Circuit Shown in the Fig.9.3 ((i.e. short circuiting The
Voltage Source V1).Note Down the Ammeter Reading I2’.
4. Check I = I1’ + I2
’ .
5. Repeat The Above Procedure For Different Values Of Source Voltages.
Observations:
S No
V1
(Volts)
V2
(Volts)
I
(M A)
V1(V2=0)
(Volts)
I1’
(Ma)
V2(V1=0)
(Volts)
I2’
(Ma)
I= I1’ +I2
’
(Ma)
Result:
ECE Dept, KEC
CE LAB MANUAL Page 43
Self Assessment Questions:
1. State superposition theorem
2. What are the limitations of superposition theorem
3. Can we use super position? If there is any non linear element in the circuit.
4. For what type of networks, superposition theorem is valid?
5. Is superposition theorem true for the mixture of different types of sources such as current and
voltage sources?
6. Is superposition theorem true for dependent sources?
7. What is the practical use of superposition theorem?
8. What could be possible error sources between measured and calculated values?
9. Is superposition theorem true for powers as well? Why?
ECE Dept, KEC
CE LAB MANUAL Page 44
Experiment No: 10 Date:
MAXIMUM POWER TRANSFER THEOREM
Aim: To verify maximum power transfer theorem.
Apparatus: Power supply, multi meters, Resistor 1K ohm, Decade resistance box (DRB).
Circuit diagram:
Fig (10.1) Symmetrical network
Theory: Statement: “In any symmetrical network, the power transferred from the source to the load
will be maximum, when the source impedance is complex conjugate of load impedance.”
Let Source impedance
Zi = Ri ± jXi
Load impedance
ZL = RL ± JxL
For maximum power transfer, conditions are
Ri = RL
Xi = -XL
We conclude that for maximum power transfer, load resistance must be equal to source
resistance and load reactance must be opposite in nature i.e. if one is the positive reactance the other
one should be negative (if one is inductive other one should be capacitive).
For dc circuits there will not be any reactive components. Therefore source impedance Zi= Ri
ECE Dept, KEC
CE LAB MANUAL Page 45
and load impedance ZL= RL. Hence condition for maximum power transfer in dc circuits is Ri= RL.
In the circuit diagram source resistance Ri is 1 K Ohm. Therefore power transferred from the source
to the load is maximum when load resistance RL is equal to 1 K Ohm.
Procedure:
1. Make the connections as shown in the circuit diagram.
2. Keep the input voltage Vin=10V
3. Set the DRB (RL) to 100 ohms.
4. Measure the current (IL) through load resistor and the voltage (VL) across load resistor.
Record these data in the tabular column.
5. Repeat step 4 for different values of RL.
6. For each reading, calculate output power using the formula Pout=VL.IL
Tabular column:
Sl. Load resistance Current through Voltage across Output power
No. RL, in Ohms load resistor, IL load resistor, VL Pout=VL.IL in
in m.A. in volts m.W.
1 100
2 300
3 500
4 700
5 900
6 1k
7 2k
8 3k
9 4k
10 5k
11 7k
12 10k
Result:
ECE Dept, KEC
CE LAB MANUAL Page 46
Self Assessment Questions:
1. State maximum power transfer theorem?
2. Give the model graph for the maximum power transfer theorem RL=RS
3. Where do you get the maximum power transfer from source to load when load and
source are pure resistance?
4. What are the conditions for maximum power transfer when load and source are
impedance?
5. If the load is an inductive what must be the nature of source impedance for
maximum power transfer.
6. If the load is capacitive in nature what is the nature of source impedance?
ECE Dept, KEC
CE LAB MANUAL Page 47
Experiment No: 11 Date:
DIFFERENTIATOR AND INTEGRATOR
Aim: To draw output characteristics of differentiator & integrator for square wave input at different
time constants.
Apparatus:
1. Resistors (10, 1K, 100K ohms) - 3 No’s
2. Capacitors (0.047uf) - 1 No
3. Square wave generator or function generator - 1 No
4.cathode ray oscilloscope -1 No
circuit diagram:
Fig 11.1: RC low pass filter.
Fig 11.2: RC high pass filter.
ECE Dept, KEC
CE LAB MANUAL Page 48
Theory:
a. Differentiator:
A linear R.C. circuit acts as a high pass filter when its output is obtained across
resistor. A high pass filter acts as a differentiator when « T. where is time constant(RC)
and T is time period of the input wave form.
b. Integrator:
A linear R.C. circuit acts as a low pass filter when its output is obtained across
resistor. A low pass filter acts as a integrator when » T. where is time constant(RC) and T
is time period of the input wave form
Both differentiator and integrator are classified under linear wave shaping circuits.
Procedure:
a. Differentiator:
1. Connect the circuit as shown in the fig.11.1.
2. Apply square wave input of amplitude ‘V’ volts at frequency ‘f’
3. Record input and output waveform using CRO.
4. Repeat the step 3 for different values of time constant (RC) (i.e. by varying either
resistance or capacitance values)
b. Integrator:
1. Connect the circuit as shown in the fig.11.2.
2. Carry out steps a(2),a(3),a(4).
Result:
ECE Dept, KEC
CE LAB MANUAL Page 49
Self Assessment Questions:
1. Can we use high pass filter as differentiator at any time constant? If not, what is the condition for
using high pass filter as differentiator?
2. Can we use low pass filter as integrator at any time constant?
3. Under what condition the output of high pass filter resembles input wave form
4. Under what condition the output of low pass filter resembles input wave form
5. In how many time constants the capacitor charges to the peak value of input wave.
6. What is the output of integrator for square wave input?
7. What is the output of differentiator for square wave input?
8. What is the average value of output wave form of a differentiator for any input wave form?
ECE Dept, KEC
CE LAB MANUAL Page 50
Experiment No: 12 Date:
COMMON EMITTER (CE) AMPLIFIER
Aim: To observe input-output waveforms of common emitter (CE) amplifier. To measure gain of
amplifier at different frequencies and plot frequency response
Theory:
Common emitter amplifier is used to amplify weak signal. It utilizes energy from DC power
supply to amplify input AC signal. Biasing of transistor is done to tie Q point at the middle of the
load line. In the circuit shown, voltage divider bias is formed using resistors 10K and 2.2K.
During positive cycle, forward bias of base-emitter junction increases and base current
increases. Q point moves in upward direction on load line and collector current increases β times than
base current. (β is current gain). Collector resistor drop IcRc increases due to increase in collector
current Ic. This will reduce collector voltage. Thus during positive input cycle, we get negative output
cycle.
When input is negative cycle, forward bias of base-emitter junction and base current will
reduce. Collector current reduces (Q point moves downside). Due to decrease in collector current,
collector resistance voltage drop IcRc reduces and collector voltage increases. Change in collector
voltage is much higher than applied base voltage because less base current variation causes large
collector current variation due to current gain B. This large collector current further multiplied by
collector resistance Rc which provides large voltage output.
Thus CE amplifier provides voltage gain and amplifies the input signal. Without emitter
resistance gain of amplifier is highest but it is not stable. Emitter resistance is used to provide
stability. To compensate effect of emitter resistance emitter bypass capacitor is used which provides
AC ground to the emitter. This will increase gain of amplifier.
CE amplifier does not provide constant voltage gain at all frequencies. Due to emitter bypass
and coupling capacitors reduces gain of amplifier at low frequency. Reactance of capacitor is high at
low frequency, hence emitter bypass capacitor does not provide perfect AC ground (Emitter
impedance is high).
ECE Dept, KEC
CE LAB MANUAL Page 51
There is voltage drop across coupling capacitor at low frequency because of high reactance at
low frequencies. Gain of CE amplifier also reduces at very high frequency because of stray
capacitances.
Audio frequency transistors like AC127, AC128 works for audio frequency range. It does not
provide large voltage gain for frequency greater than 20 KHz. Medium frequency transistors are
BC147/BC148/BC547/BC548 provides voltage gain up to 500 KHz. High frequency transistors like
BF194/BF594/BF200 provides gain at radio frequencies in the MHz range.
If we apply large signal at the input of CE amplifier, transistor driven into saturation region
during positive peak and cut-off region during negative peak (Q point reaches to saturation and cut-
off points). Due to this clipping occurs in amplified signal. So we have to apply small signal at the
input and ensure that transistor operates in active region.
Circuit diagram:
Fig: common emitter amplifier
ECE Dept, KEC
CE LAB MANUAL Page 52
Procedure:
1) Connect function generator at the input of the amplifier circuit.
2) Set input voltage 10 mV and frequency 100 Hz.
3) Connect CRO at the output of the amplifier circuit.
4) Observe amplified signal and measure output voltage
5) Increase frequency from the function generator and repeat above step
6) Note down readings of output voltage in the observation table for frequency range from 100
Hz to 10 MHz
7) Calculate voltage gain for different frequencies and gain in dB. Plot frequency response.
Result:
ECE Dept, KEC
CE LAB MANUAL Page 53
Self Assessment Questions:
1. What will be emitter current in the given circuit diagram in absence of input AC signal?
2. What is bandwidth? What is the approximate bandwidth of CE amplifier that you have used
during your practical
3. What is the effect on gain of amplifier if value of Rc increases?
4. What are the different biasing methods?
5. What happens if emitter bypass capacitor is removed from the circuit?