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

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

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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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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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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:

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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?

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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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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.

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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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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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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?

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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.

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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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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:

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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?

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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.

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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)

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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:

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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?

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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.



modulating signal (Sensitivity). A receiver with good selectivity will


using tuned LC circuits resonating at the desired frequency. LC circuits with a high



hence distortion of the transmitted information.



amplification. There are two types of communications receiver; the Tuned Radio Frequency (TRF)


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Date:

Receiver kit, set of connecting wire, Oscilloscope. Multi meter, RF signal



modulating signal (Sensitivity). A receiver with good selectivity will


using tuned LC circuits resonating at the desired frequency. LC circuits with a high Q value have





ver; the Tuned Radio Frequency (TRF)


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CE LAB MANUAL

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.

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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.

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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:

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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:

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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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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.

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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:

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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

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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.

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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,

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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

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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).


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.

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CE LAB MANUAL

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


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


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

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

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


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


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

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

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


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


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

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:


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


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


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

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


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


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

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

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


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?