2666 1530 analog communications_lammanual (1)

MANDSAUR INSTITUTE OF TECHNOLOGY

ANALOG COMMUNICATIONS

LAB MANUAL
EC-405

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

Analog Communications, EC-405 Page 1


EC 405 ANALOG COMMUNICATIONS LAB

1. i) To Measure carrier with audio signal and to observe waveform on CRO.
ii) To Measure the modulation index of the amplitude modulated waveform.
2. To recover original baseband signal from amplitude modulated waveform.
3. i) To modulate a sinusoidal carrier with an audio signal and observe Frequency
modulated waveform on CRO.
ii) To measure the modulation index of the Frequency-Modulated waveform.
4. To recover original baseband signal from frequency modulated waveform.
5. To study AM Transmitter.
6. To study AM Receiver.
7. To modulate a sinusoidal carrier with an audio signal & observe phase modulated
waveform on CRO.
8. To recover original baseband signal from phase modulated wave.



Expt. No: 1 AMPLITUDE MODULATION

Aim: - 1) To Measure carrier with audio signal and to observe waveform on CRO.
2) To Measure the modulation index of the amplitude modulated waveform.

Equipment Required: - Audio frequency Generator, Amplitude Modulation and Demodulation
Trainer Kit, CRO, Cords.

Waveforms:

Fig.1 Wave forms
a) Carrier signal b) Modulating signal c) AM signal

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 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.

Let the signal potential designated as
em = Em cos(ωmt)
And let the unmodulated carrier be written as
ec = Ec cos(ωct)
The carrier frequency (ωc) is much larger than the signal frequency (ωm).
The resulting modulated wave is
Eam = (Ec+Emcos(ωmt))cos(ωct)
Eam = Ec(1+ m cos(ωmt))cos(ωct)
The factor m is known as modulation index
m = Em/Ec
For the practical we can use
m = [(Vmax - Vmin)/(Vmax + Vmin)]/100

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= Em / Ec, or

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= (Em/Ec) x100



Procedure: -
1. Switch ON the mains supply.
2. Connect carrier wave to the socket marked “CARRIER IN” and 1KHz audio signal from audio
frequency generator to the socket marked “Audio In”.
3. Slowly increase the level of audio signal input and keep observing the amplitude-modulated
waveform on CRO.
4. For a particular level of audio signal measure Vmax and Vmin.
5. Calculate % of modulation

Ma = [(Vmax - Vmin)/(Vmax + Vmin)]/100

Observation Table: -

Result: - Thus, we have measured the modulation index on CRO and calculated the percentage of
Modulation.



Expt. No: 2 AMPLITUDE DEMODULATION

Aim: - To recover original baseband signal from amplitude modulated waveform.
Equipment Required: - Amplitude Modulation and Demodulation Trainer Kit, Audio frequency
generator CRO & CRO Cords.
Theory: – Fig 2.1 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.

Fig (2.1) AM Detector (Envelope detector)

Diode detector:- The simplest and most widely used amplitude demodulator is the diode detector
shown in fig (2.1). 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 2.2b shows the
demodulated signal.



Fig (2.2) Wave forms
a) AM signal b) Demodulated signal

Fig (2.3) Amplitude Demodulation



Procedure:-

1. Connect the output of AM modulator to the input of envelope detector.
2. Connect the CRO to the output of envelope detector.
3. Observe the demodulated wave form, measure the frequency of this waveform and compare it
with the original modulating signal.
4. Sketch AM wave and demodulated waveform.

Result:-



Expt. No: 3 FREQUENCY MODULATION

Aim: - 1) To modulate a sinusoidal carrier with an audio signal and observe Frequency Modulated
Waveform on CRO.
2) To measure the modulation index of the Frequency-Modulated waveform.
Equipment Required: – Frequency Modulation Kit, Audio generator, Cathode Ray Oscilloscope
CRO Cords, Patch Cords.
Block diagram:

Fig (3.1) Block diagram of FM modulator

Waveforms:

a) Carrier signal b) Modulating signal c) FM signal



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. A decreasing modulating signal will increase the carrier
frequency above its center value; whereas an increase in modulating signal will decrease the carrier
frequency below its center value. 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 (3.2c). 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 (3.2a)] 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.

Modulation Index:- Modulation Index is the ratio of the maximum deviation frequency to the
frequency of modulation. In other words it is the ratio of the spread in frequency spectrum to the
frequency that was used to modulate the carrier.
Modulation index mf for FM is defined as:
mf = max frequency deviation/ Modulating frequency

= δ/ fm



Procedure: -

1. Feed modulating audio signal to the sockets marked „MODULATING SIGNAL IN‟ and
observe output on CRO. Vary the amplitude and frequency of audio signal and see their
effects.
2. Disconnect the audio signal and measure frequency of carrier waveform with frequency
counter connected between sockets marked „FM OUT‟. Let it be fc.
3. Now again feed audio signal to the „MODULATING SIGNAL IN‟ sockets and observe the
FM wave on CRO. Note down the maximum frequency of a part of the FM waveform on
CRO. Let it be fmax.
Calculate frequency deviation δ = (fmax - fc)
4. Now connect frequency counter to the sockets marked „MODULATING SIGNAL OUT‟
and note down the frequency of audio signal. Let it be fm.
5. Calculate the modulation index
mf = (max frequency deviation)(modulating frequency)
= δ fm

Observation Table :-

S. No. Modulating frequency fm Frequency Deviation δ Modulation Index mf

Result: - We have successfully done experiment.



Expt. No: 4 FM DEMODULATOR

Aim: - To recover original baseband signal from frequency modulated waveform.
Equipment Required:- Frequency Modulation and Demodulation Kit, Patch Cords, Cathode Ray
Oscilloscope, CRO Cords.
Block diagram :-

Fig (4.1) Block diagram of FM demodulator

Waveforms:-

a) Modulating signal (CH1 of CRO) b) FM signal c) Demodulated signal (CH2 of CRO)

Theory:- Fig (4.1) shows the block 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 (4.2) shows original modulating signal, FM signal and demodulated signal.

There are three general categories of FM demodulator circuits, which are, “phase-locked
loop demodulator,” “slope detection/FM discriminator” and “quadrature detector.” Although slope
detection is rarely used in practice because of its nonlinearity, it is relatively simple to implement.
Hence, we consider slope detection to illustrate basic concepts in this experiment.

Fig.(4.3) Circuit of slope detector
The basic idea is to set the center frequency of a tuned circuit such that the FM carrier signal
falls on the slope of the resonance curve. This way, any increase in frequency of the FM signal will
result in a larger signal amplitude at the output of the tuned circuit, and any decrease will result in a
smaller amplitude. Consequently, the output of the tuned circuit becomes an amplitude-modulated
signal (in addition to FM modulation). At this point, an envelope detector such as the one used for
AM detection can be employed to detect the information-bearing signal. As can be seen from the
figure above, frequency-to-amplitude conversion will in general be nonlinear. However, frequency
deviation of FM can be kept small enough to approximate linear transfer characteristics. A
conceptual circuit for the slope detector is shown below. We will, however, use an active transistor
circuit for actual implementation.

Procedure: -

1. Connect the trainer kit to the mains supply and switch ON.
2. Observe the modulating signal at „modulator output‟ by varying „frequency control‟ &
„amplitude control‟ knob.
3. Observe the un-modulated carrier signal at „FM out‟.
4. Connect the „modulator O/P‟ to the „modulating I/P‟.
5. Connect „FM out‟ to „FM in‟ of demodulator.
6. Connect CH1 of CRO to the „modulating I/P‟ & CH2 to the „demodulator O/P‟ as shown in the
block diagram. Observe the demodulated wave form and compare it with the modulating signal.

Result: - We have successfully recovered the original baseband signal from FM wave.


Expt. No: 5 AM TRANSMITTERS

Aim: - To study AM Transmitters.

Theory: - A transmitter performs the modulation process and raises the power level of a modulated
signal to the desired extent for effective radiation. The AM transmitters are divided into two
categories:

 Low Power Level Modulation:-
Fig.(5.1) shows low level modulated AM transmitter block diagram. In this block diagram,
observe that a linear class B power amplifier is used after class C modulator amplifier. The
linear class B power amplifier performs the major power amplification and feeds the amplified
AM signal to antenna, In this block diagram, the modulator amplifier performs modulation at
relatively low power levels. Hence this is called low level modulated AM transmitter. The
modulated AM signal is amplified by class B power amplifier to avoid distortion in the output.

Fig.(5.1) Low level modulated AM transmitter block diagram

 High Power Level Modulation:-

Fig.(5.2) High level modulated AM transmitter block diagram



Fig.(5.2) shows the block diagram of AM transmitter. The crystal oscillator generates carrier
frequency. The buffer amplifiers and driver amplifiers amplify the power level of the carrier to
required value. The amplified carrier is given to class C modulator amplifier. The modulating
signal and given to audio amplifier. It is further amplified by audio power amplifier at a level
suitable for modulation. The class C modulator amplifier modulates the carrier input according
to modulating audio signal. The output of the class C modulating amplifier is AM and it is given
to antenna through some antenna matching network. The antenna matching network is generally
tuned LC circuit in collector circuit of modulator amplifier. In this AM transmitter, the
modulator amplifier operates at high power levels and delivers power directly to the antenna.
This is called High level modulated AM transmitter.

Result: – We have successfully studied AM transmitters.



Expt. No: 6 AM RECEIVERS

Aim: - To study AM Receivers.

Theory: - The TRF (Tuned Radio Frequency) Receiver and Superheterodyne Receiver are the two
main configurations of the receivers, they have real practical or commercial significance. Most of
the present day receivers use superheterodyne configuration. But the TRF receivers are simple and
easy to understand.

 Tuned Radio Frequency Receiver:-
It consists of two or three stages of RF amplifiers, detector, audio amplifier and power
amplifier. The RF amplifier stages placed between the antenna and detector are used to increase
the strength of the received signal before it is applied to the detector. These RF amplifiers are
tuned to fix frequency, amplify the desired band of frequencies. Therefore, they provide
amplification for selected band of frequencies and rejection for all others. As selection and
amplification process is carried out in two or three stages and each stage must amplify the same
band of frequencies, the ganged tuning is provided.

Fig.(6.1) Block diagram of TRF receiver

The amplified signal is then demodulated using detector to recover the modulating signal. The
recovered signal is amplified is further by the audio amplifier followed by power amplifier
which provide significant gain to operate a loud speaker.

 Superheterodyne Receiver :-
To solve basic problems of TRF receivers, in this receivers, first all the incoming RF
frequencies are converted to a fixed lower frequency called intermediate frequency (IF). Then
this fixed intermediate frequency is amplified and detected to reproduce the original


information. Since the characteristics of the IF amplifier are independent of the frequency to
which the receiver is tuned, the selectivity and sensitivity of super heterodyne receiver are fairly
uniform throughout its tuning range.

Fig.(6.2) Block diagram of superheterodyne receiver

Mixer circuit is used to produce the frequency translation of the incoming signal down
to the IF. The incoming signals are mixed with the local oscillator frequency signal in such a
way that a constant frequency difference is maintained between the local oscillator and
incoming signal. This is achieved by using ganged tuning capacitors.
The RF amplifier provides some initial gain and selectivity. The output of the RF
amplifies is applied to the input of the mixer. The mixer also receives an input from the local
oscillator.
The output of the mixer circuit is difference frequency (f0-fs) commonly known as IF
(Intermediate frequency). This signal is amplified by one or more IF amplifier stages, and
most of the receiver gain is obtained in this IF stages.

The highly amplified IF signal is applied to the detector circuit to recover the original
modulating information. Finally the output of detector circuit is fed to audio and power
amplifier which provides a sufficient gain to operate a speaker.
Another important circuit in the superheterodyne receiver are AGC and AFC circuit.
AGC is used to maintain a constant output voltage level over a wide range of RF input signal
levels.

Result: – We have successfully studied AM Receivers.



Expt. No: 7 PHASE MODULATION

Aim: - To modulate a sinusoidal carrier with an audio signal & observe phase modulated waveform
on CRO.
Equipment Required: -

1. Phase Modulated Kit
2. Audio generator
3. Cathode Ray Oscilloscope
4. CRO Cords.
5. Patch Cords

Theory: - In this type of angle modulation, The phase angle ψ(t) is varied linearly with a
modulation signal f(t) about an un-modulated phase angle ωct. That is to say, the instantaneous
value of a phase angle ψ(t) is equal to the phase of the phase of an un-modulated carrier (ωct) plus a
time varying component proportional to f(t) mathematically,

ψi(t) = ωct +Kpf(t) ---------------- (1)

Note that θ0 is time independent and hence, has been ignored.

The proportionally constant kp, is known as phase sensitivity of the modulator, expressed in
radians/volts. The carrier wave after phase modulation has the phase angle given by eq. (1) and is
represented as

Φpm = A cos ψi(t)

= A cos[ωct + Kpf(t)]
Modulation Index – In AM the degree of modulation is measured as a percentage from 0% to 100%
or as a modulation factor from 0 to 1. In angle modulation the degree of modulation is measured by
the modulation index. The equation for modulation index is

m = fd/fm

Where,



fd = The frequency deviation

fm = The modulating frequency

Fig. (7.1) PM modulation

Procedure: –

1. Switch ON the instruments using ON/OFF switch.
2. Connect the oscilloscope channel „A‟ across modulated output and observe the carrier wave
as in fig. Ic 2206 used as a carrier wave generator modulator.
3. Connect the circuit as shown in circuit diagram.



4. Connect oscilloscope channel „A‟ across AF signal output & channel „B‟ across modulated
output. Now set the frequency of AF signal at 10 KHz and increase the AF amplitude. Now
observe the phase modulation corresponding to the AF signal.

Result: – We have successfully done the experiment.



Expt. No: 8 PHASE DEMODULATION

Aim: - To recover original baseband signal from phase modulated wave.
Equipment Required: -

1. Phase Modulated Kit
2. Cathode Ray Oscilloscope
3. CRO Cords.
4. Patch Cords

Theory:-
In this type of angle modulation, The phase angle ψ(t) is varied linearly with a modulation
signal f(t) about an un-modulated phase angle ωct. That is to say, the instantaneous value of a phase
angle ψ(t) is equal to the phase of an un-modulated carrier (ωct) plus a time varying component
proportional to f(t) mathematically,

ψi(t) = ωct +Kpf(t) ---------------- (1)

Note that θ0 is time independent and hence, has been ignored.

The proportionally constant kp, is known as phase sensitivity of the modulator, expressed in
radian/volts. The carrier wave after phase modulation has the phase angle given by eq. (1) and is
represented as

Φpm = A cos ψi(t)

= A cos[ωct + Kpf(t)]
Modulation Index – In AM the degree of modulation is measured as a percentage from 0% to
100% or as a modulation factor from 0 to 1. In angle modulation the degree of modulation is
measured by the modulation index. The equation for modulation index is

m=

Where,



fd = The frequency deviation

fm = The modulating frequency

Fig.(8.1) PM Demodulation



Procedure: –

1. Connect the circuit as shown in fig.
2. Connect the oscilloscope channel „A‟ across AF signal output and set the frequency between
1 KHz to 3 KHz. Now connect channel „B‟ across Demodulated output. Observe the wave
shape. If modulated output is not proper, then adjust potentiometer given on the band and get
proper Demodulator output.

Result: – We have successfully recovered the original baseband signal from phase modulation.


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