TE 0224 ANALOG COMMUNICATION LAB

Laboratory Manual

DEPARTMENT OF TELECOMMUNICATION ENGINEERING SRM UNIVERSITY S.R.M. NAGAR, KATTANKULATHUR – 603 203.

FOR PRIVATE CIRCULATION ONLY ALL RIGHTS RESERVED

DEPARTMENT OF TELECOMMUNICATION ENGINEERING

TE0224 ANALOG COMMUNICATION LAB (2011-2012)

Revision No: 2 PREPARED BY, Date: Mrs.Kavitha Narayanan

Mrs.S.Murugaveni

HOD/TCE

TEO224 ANALOG COMMUNICATION LAB

List of Experiments

1. AM Modulation and Demodulation(Envelope Detector)

2. FM Modulation using PLL

3. Pulse Amplitude Modulation and Demodulation

4. Pre-emphasis and De-emphasis

5. Analog Multiplexing.

6. Study of FM detection

7. Amplitude Modulation using Pspice

8. AM Modulation using Matlab

9. FM Modulation using Matlab

1. AM MODULATOR & ENVELOPE DETECTOR AIM

To study the amplitude modulation and demodulation and to calculate the

modulation index values for various modulating voltages

APPARATUS REQUIRED 1. Transistor BC108

2. Resistors

3. Capacitors

4. AFO

5. CRO

6. Diode OA79

7. Millimeter

8. Regulated power supply

9. Breadboard and connecting wires

THEORY Modulation is defined as the process by which some characteristics of a carrier

signal is varied in accordance with a modulating signal. The base band signal is referred

to as the modulating signal and the output of the modulation process is called as the

modulation signal.

Amplitude modulation is defined as the process in which is the amplitude of the

carrier wave is varied about a means values linearly with the base band signal. The

envelope of the modulating wave has the same shape as the base band signal provided the

following two requirements are satisfied

(1). the carrier frequency fc must be much greater then the highest frequency components

fm of the message signal m (t)

i.e. fc >> fm

(II) The modulation index must be less than unity. if the modulation index is

greater than unity, the carrier wave becomes over modulated.

PROCEDURE

1. The circuit connection is made as shown in the circuit.

2. The power supply is connected to the collector of the transistor

3. Modulated Output is taken from the collector of the Transistor

4. Calculate Vmax and Vmin from the Output

Vc = 63mv fc = 500Khz fm = 1Khz

Modulating signal Vm(v)

Email (V) Emax(V) Modulation index M = Emax-Emin x 100 Emax + Emin

2

4

6

8

10

15

20

1.05

1

0.9

0.8

0.7

0.4

0

1.2

1.35

1.4

1.5

1.6

1.8

2

6.66

14.89

21.73

30.43

39.13

63.64

100

AM DETECTION THEORY: The process of detection provides a means of recovering the modulating Signal

from modulating signal. Demodulation is the reverse process of modulation. The detector

circuit is employed to separate the carrier wave and eliminate the side bands. Since the

envelope of an AM wave has the same shape as the message, independent of the carrier

frequency and phase, demodulation can be accomplished by extracting envelope.

An increased time constant RC results in a marginal output follows the

modulation envelope. A further increase in time constant the discharge curve become

horizontal if the rate of modulation envelope during negative half cycle of the modulation

voltage is faster than the rate of voltage RC combination ,the output fails to follow the

modulation resulting distorted output is called as “ diagonal clipping : this will occur

even high modulation index.

The depth of modulation at the detector output greater than unity and circuit

impedance is less than circuit load (Rl > Zm) results in clipping of negative peaks of

modulating signal. It is called “negative clipping “

PROCEDURE

1. T he circuit connection are made as shown in the circuit diagram.

2. The amplitude modulated signal from AM generator is give as input to the

circuit.

3. The demodulated output is observed on the CRO

4. The various values modulating voltage signal frequency corresponding

demodulated voltage and frequency are noted and the readings are

tabulated.

SAMPLE READING Fc = 500 KHz fm = 1 KHz

Emin (v) Emax (V) Em(p-p) Modulation index M = Emax-Emin x 100 Emax + Emin

1.05

1

0.9

0.8

0.7

0.4

0

1.2

1.35

1.4

1.5

1.6

1.8

2

0.16

0.24

0.32

0.44

0.56

0.8

1.04

6.66

14.89

21.73

30.43

39.13

63.64

100

RESULT:

Thus the amplitude modulation and demodulation circuit were designed and the

modulation index for various modulating voltage were calculated.

REVIEW QUESTIONS 1. Define Modulation. 2. What is modulation index? 3. Differentiate under modulation & over modulation. 4. List the advantages of AM modulation. 5. What are the different AM modulations Techniques? 6. What is detector? 7. When Diagonal clipping and Negative clipping occur in demodulation and how it

is overcome?

2. FREQUENCY MODULATION AIM: To generate a frequency modulated wave-using IC 566 APPARATUS REQUIRED

1. AFO 2. IC NE566

3. Resistors

4. Capacitor

5. CRO

6. Bread board and connection

7. RPS

THEORY: Frequency modulation is a process of changing the frequency of a carrier wave in

accordance with the slowly varying base band signal. The main advantage of this

modulation is that it can provide better discrimination against noise.

FREQUENCY MODULATION USING IC 566

A VCO is a circuit that provides an oscillating signal whose frequency can be

adjusted over a control by Dc voltage. VCO can generate both square and triangular

Wave signal whose frequency is set by an external capacitor and resistor and then varied

by an applied DC voltage. IC 566 contains a current source to charge and discharge an

external capacitor C1 at a rate set by an external resistor. R1 and a modulating DC output

voltage.

The Schmitt trigger circuit present in the IC is used to switch the current source

between charge and discharge capacitor and triangular voltage developed across the

capacitor and the square wave from the Schmitt trigger are provide as the output of the

buffer amplifier.

The R2 and R3 combination is a voltage divider, the voltage VC must be in the range ¾

VCC < VC < VCC. The modulating voltage must be less than ¾ VCC the frequency Fc

can be calculated using the formula

Fo = 2 (Vcc-Vc) R1 C1 Vcc For a fixed value of Vc and a constant C1 the frequency can be varied at 10:1

similarly for a constant R! C1 product value the frequency modulation can be done at

10:1 ratio

FORMULA: Modulation index: β = ∆ f/fm where ∆ f = Fmax – Fmin /2 PROCEDURE:

1. The circuit connection is made as shown in the circuit diagram. 2. The modulating signal FM is given from an AFO (1KHZ) 3. For various values of modulating voltage Vm the values of Fmax and Fmin are

noted 4. The values of the modulation index are calculated.

SAMPLE READING

Vm Tmin Tmax Fmax Fmin Modulation Index Β = /∆f / fm

1.15 2.1 5.3

0.03 0.02 0.01

0.07 0.1

0,125

33.33 50 100

14.28 10 8

9.5 50 46

RESULTS:

The FM circuit using IC566 was designed and the modulation index for practical and Theoretical.

REVIEW QUESTIONS 1. Define Frequency Modulation. 2. What is Frequency deviation? 3. Differentiate under modulation & over modulation. 4. List the advantages of FM modulation over AM modulation. 5. What are the different AM modulations Techniques? 6. What is detector? 7. When Diagonal clipping and Negative clipping occur in demodulation and how it

is overcome?

3. PULSE AMPLITUDE MODULATION & DEMODULATION

AIM:

To study and obtain pulse amplitude modulation and demodulation

APPARATUS REQUIRED 1. Transistor

2. AFO 3. IC NE566 4. Resistors 5. Capacitor 5. CRO 6. RPS

THEORY: Pulse amplitude modulation is a scheme, which alters the amplitude of

regularly spaced rectangular pulses in accordance with the instantaneous values of a

continuous message signal.

Then amplitude of the modulated pulses represents the amplitude of the

intelligence.

A train of very short pulses of constant amplitude and fast repetition rate is

chosen the amplitude of these pulse is made to vary in accordance with that of a slower

modulating signal the result is that of multiplying the train by the modulating signal the

envelope of the pulse height corresponds to the modulating wave .the Pam wave contain

upper and lower side band frequencies .besides the modulating and pulse signals.

The demodulated PAM waves, the signal is passed through a low pass filter

having a cut –off frequencies equal to the highest frequency in the modulating signal. At

the output of the filter is available the modulating signal along with the DC component

PAM has the same signal to noise ratio as AM and so it is not employed in

practical circuits

PROCEDURE: MODULATION

1. Make the circuit as shown in circuit diagram fig A

2. Set the pulse generated’s output to be 41vpp at 100HZ

3. Set AFO’s output at 2 vpp 100HZ

4. Observe the output wave form on a CRO

5. Tabulate the reading.

DEMODULATION:

1. Connect the circuit as shown in fig (b)

2. Given the modulated output with AFO used to the input of the circuit.

3. Vary the potentiometer so that modulating signal is obtained.

4. Measure the amplitude of the signal and verify with that of the input.

SAMPLE READING: PAM

Modulating voltage Vm (V) Pulse Amplitude modulation (V)

SAMPLE READING: DEMODULATION

Modulating voltage Vm (V) Pulse Amplitude demodulation (V)

RESULT: The pulse amplitude modulation circuit is circuit assembled and studied and

demodulated wave also done.

4. PRE –EMPHASIS AND DE-EMPHASIS CIRCUITS AIM: To study and test the chateristics of pre-emphasis and de-emphasis circuits APPARATUS REQUIRED 1. Transistor

2. AFO

3. IC NE566

4. Resistors

5. Capacitor

5. CRO

6. RPS

THEORY: PRE-EMPHASIS CIRCUITS

The circuits are the transmitting side of the frequency modulator. It is used to

increase the gain of the higher frequency component as the input signal frequency

increased, the impendence of the collector voltage increase. If the signal frequency is

lesser then the impendence decrease which increase the collector current and hence

decrease the voltage.

DE-EMPHASIS CIRCUITS: The circuit is placed at the receiving side. It acts as allow pass filter. The boosting

gain for higher frequency signal in the transmitting side is done by the pre-emphasis

circuit is filtered to the same value by the low pass filter. The cut off frequency is given

by the formula

Fc = 1/2π RC

Where R = 2 π fc L DESIGN FORMULA: Fc = 1/2 π R C (assume =R = 10 KΩ, C = 0.01μf)

R = 2 π fc L; L = R/2 π fc

PROCEDURE:

1. The circuit connection are made as shown in the circuit diagram for the pre-

emphasis and de-emphasis circuits

2. A power supply of 10V is given to the circuit

3. For a constant value of input voltage the values of the frequency is varied and the

output is noted on the CRO

4. A graph is plotted between gain and frequency

5. The cut frequencies are practical values of the values of cut off frequency \are

found, compared and verified.

SAMPLE READING

Frequency

Pre emphasis De emphasis

Vo (v) Gain (db) Vo (v) Gain (db)

RESULTS: The characteristics of pre –emphasis and de emphasis circuits were studied and a

graphs was drawn between gain (in db) and frequency and fc was found.

5. ANALOG MULTIPLEXING

TIME – DIVISION MULTIPLEXING

AIM:

To perform time division multiplexing and de- multiplexing using PAM signals.

APPARATUS REQUIRED:

1. TDM Trainer Kit – ST 2102

2. CRO

3. Patch Chords

4. Probes

THEORY:

An important feature of pulse-amplitude modulation is a conservation of time.

That is, for a given message signal, transmission of the associated PAM wave engages

the communication channel for only a fraction of the sampling interval on a periodic

basis. Hence, some of the time interval between adjacent pulses of the PAM wave is

cleared fro use by the other independent message signals on a time –shared basis. By so

doing, we obtain a time –division multiplex system (TDM), which enables the joint

utilization of a common channel by a plurality of independent message signals without

mutual interference.

Each input message signal is first restricted in bandwidth by a low- pass pre-alias

filter to remove the frequencies that are nonessential to an adequate signal representation.

The pre-alias filter outputs are then applied to commutator, which is usually implemented

using electronic switching circuitry. The function of the commutator is two-fold: (1) to

take a narrow sample of each of the N input messages at a rate f that is slightly higher

than 2 wave, where W are the cutoff frequencies of the pre-alias filter, and (2) to

sequentially interleave these N samples inside a sampling interval Ts = 1/fs. Indeed, this

latter function is the essence of the time –division multiplexing operation .Following the

commutation process, the multiplexed signal is applied to a pulse –amplitude modulator,

The purpose of which is to transform the multiplexed signal into a form suitable for

transmission over the communication channel.

At the receiving end of the system, the received signal is applied to a pulse

amplitude demodulator, which performs the reverse operation of the pulse amplitude

modulator. The short pulses produced at the pulse demodulator output are distributed to

the appropriate low-pass reconstruction filters by means of a decommutator, which

operates in synchronism with the commutator in the transmitter. This synchronization is

essential for satisfactory operation of the TDM system, and provisions have to be made

for it

PROCEDURE:

1. Take the inputs from the function generator and give it to the channel in the

transmitter (Ch0..Ch3)using a patch chords

2. Note down the amplitude and time period of each signal that is available in

(Ch0...Ch3).

3. Measure the voltage and time period at the transmitter output.

4. Using a patch chord, connect transmitter output to receiver input.

5. For synchronization purpose, connect the transmitter clock and receiver clock

and also Txch0 and Rx0.

6. See the output before the filter and the filter for all the channels connected.

RESULTS:

Time division multiplexing and de-multiplexing using PAM signals was

performed and respect ion forms were plotted.

6. STUDY OF FM DEMODULATION

In fm demodulators, the intelligence to be recovered is not in amplitude

variations; it is in the variation of the instantaneous frequency of the carrier, either above

or below the center frequency. The detecting device must be constructed so that its output

amplitude will vary linearly according to the instantaneous frequency of the incoming

signal.

Several types of fm detectors have been developed and are in use, but in this

section we will study two of the most common: (1) the slope detector, (2) Foster-seeley

discriminator.

SLOPE DETECTION

To be able to understand the principles of operation for fm detectors, you need to

first study the simplest form of frequency-modulation detector, the SLOPE DETECTOR.

The slope detector is essentially a tank circuit which is tuned to a frequency either

slightly above or below the fm carrier frequency. View (a) of figure (a)is a plot of voltage

versus frequency for a tank circuit. The resonant frequency of the tank is the frequency at

point 4. Components are selected so that the resonant frequency is higher than the

frequency of the fm carrier signal at point 2. The entire frequency deviation for the fm

signal falls on the lower slope of the band pass curve between points 1 and 3. As the fm

signal is applied to the tank circuit in view (b), the output amplitude of the signal varies

as its frequency swings closer to, or further from, the resonant frequency of the tank.

Frequency variations will still be present in this waveform, but it will also develop

amplitude variations, as shown in view (b). This is because of the response of the tank

circuit as it varies with the input frequency. This signal is then applied to the diode

detector in view (c) and the detected waveform is the output. This circuit has the major

disadvantage that any amplitude variations in the RF waveform will pass through the tank

circuit and be detected. This disadvantage can be eliminated by placing a limiter circuit

before the tank input. This circuit is basically the same as an AM detector with the tank

tuned to a higher or lower frequency than the received carrier.

Figure (a) - Slope detector. VOLTAGE VERSUS FREQUENCY PLOT

Figure (b) - Slope detector. TANK CIRCUIT

Figure (c) - Slope detector. DIODE DETECTOR

FOSTER-SEELEY DISCRIMINATOR

The FOSTER-SEELEY DISCRIMINATOR is also known as the PHASE-SHIFT

DISCRIMINATOR. It uses a double-tuned RF transformer to convert frequency

variations in the received fm signal to amplitude variations. These amplitude variations

are then rectified and filtered to provide a dc output voltage. This voltage varies in both

amplitude and polarity as the input signal varies in frequency. A typical discriminator

response curve is shown in figure (d). The output voltage is 0 when the input frequency is

equal to the carrier frequency (fr). When the input frequency rises above the center

frequency, the output increases in the positive direction. When the input frequency drops

below the center frequency, the output increases in the negative direction.

Figure (d) - Discriminator response curve.

The output of the Foster-Seeley discriminator is affected not only by the input

frequency, but also to a certain extent by the input amplitude. Therefore, using limiter

stages before the detector is necessary.

CIRCUIT OPERATION OF A FOSTER-SEELEY DISCRIMINATOR

Figure (e) shows a typical Foster-Seeley discriminator. The collector circuit of the

preceding limiter/amplifier circuit (Q1) is shown. The limiter/amplifier circuit is a special

amplifier circuit which limits the amplitude of the signal. This limiting keeps interfering

noise low by removing excessive amplitude variations from signals. The collector circuit

tank consists of C1 and L1. C2 and L2 form the secondary tank circuit. Both tank circuits

are tuned to the center frequency of the incoming fm signal. Choke L3 is the dc return

path for diode rectifiers CR1 and CR2. R1 and R2 are not always necessary but are

usually used when the back (reverse bias) resistance of the two diodes is different.

Resistors R3 and R4 are the load resistors and are bypassed by C3 and C4 to remove RF.

C5 is the output coupling capacitor.

Figure (e) - Foster-Seeley discriminator.

CIRCUIT OPERATION AT RESONANCE

The operation of the Foster-Seeley discriminator can best be explained using

vector diagrams figure (e), view (B) that show phase relationships between the voltages

and currents in the circuit. Let's look at the phase relationships when the input frequency

is equal to the center frequency of the resonant tank circuit.

The input signal applied to the primary tank circuit is shown as vector ep. Since

coupling capacitor C8 has negligible reactance at the input frequency, RF choke L3 is

effectively in parallel with the primary tank circuit. Also, because L3 is effectively in

parallel with the primary tank circuit, input voltage ep also appears across L3. With

voltage ep applied to the primary of T1, a voltage is induced in the secondary which

causes current to flow in the secondary tank circuit. When the input frequency is equal to

the center frequency, the tank is at resonance and acts resistive. Current and voltage are

in phase in a resistance circuit, as shown by is and ep. The current flowing in the tank

causes voltage drops across each half of the balanced secondary winding of transformer

T1. These voltage drops are of equal amplitude and opposite polarity with respect to the

center tap of the winding. Because the winding is inductive, the voltage across it is 90

degrees out of phase with the current through it. Because of the center-tap arrangement,

the voltages at each end of the secondary winding of T1 are 180 degrees out of phase and

are shown as e1 and e2 on the vector diagram.

The voltage applied to the anode of CR1 is the vector sum of voltages ep and e1,

shown as e3 on the diagram. Likewise, the voltage applied to the anode of CR2 is the

vector sum of voltages ep and e2, shown as e4 on the diagram. At resonance e3 and e4 are

equal, as shown by vectors of the same length. Equal anode voltages on diodes CR1 and

CR2 produce equal currents and, with equal load resistors, equal and opposite voltages

will be developed across R3 and R4. The output is taken across R3 and R4 and will be 0

at resonance since these voltages are equal and of appositive polarity.

The diodes conduct on opposite half cycles of the input waveform and produce a

series of dc pulses at the RF rate. This RF ripple is filtered out by capacitors C3 and C4.

OPERATION ABOVE RESONANCE

A phase shift occurs when an input frequency higher than the center frequency is

applied to the discriminator circuit and the current and voltage phase relationships

change. When a series-tuned circuit operates at a frequency above resonance, the

inductive reactance of the coil increases and the capacitive reactance of the capacitor

decreases. Above resonance the tank circuit acts like an inductor. Secondary current lags

the primary tank voltage, ep. Notice that secondary voltages e1 and e2 are still 180 degrees

out of phase with the current (iS) that produces them. The change to a lagging secondary

current rotates the vectors in a clockwise direction. This causes el to become more in

phase with ep while e2 is shifted further out of phase with ep. The vector sum of ep and e2

is less than that of ep and e1. Above the center frequency, diode CR1 conducts more than

diode CR2. Because of this heavier conduction, the voltage developed across R3 is

greater than the voltage developed across R4; the output voltage is positive.

OPERATION BELOW RESONANCE

When the input frequency is lower than the center frequency, the current and

voltage phase relationships change. When the tuned circuit is operated at a frequency

lower than resonance, the capacitive reactance increases and the inductive reactance

decreases. Below resonance the tank acts like a capacitor and the secondary current leads

primary tank voltage ep. This change to a leading secondary current rotates the vectors in

a counterclockwise direction. From the vector diagram you should see that e2 is

brought nearer in phase with ep, while el is shifted further out of phase with ep. The

vector sum of ep and e2 is larger than that of e p and e1. Diode CR2 conducts more than

diode CR1 below the center frequency. The voltage drop across R4 is larger than that

across R3 and the output across both is negative.

DISADVANTAGES

These voltage outputs can be plotted to show the response curve of the

discriminator discussed earlier (figure (d)). When weak AM signals (too small in

amplitude to reach the circuit limiting level) pass through the limiter stages, they can

appear in the output. These unwanted amplitude variations will cause primary voltage ep

[view (A) of figure (e)] to fluctuate with the modulation and to induce a similar voltage in

the secondary of T1. Since the diodes are connected as half-wave rectifiers, these small

AM signals will be detected as they would be in a diode detector and will appear in the

output. This unwanted AM interference is cancelled out in the ratio detector and is the

main disadvantage of the Foster-Seeley circuit.

7. AM MODULATOR USING PSPICE DESCRIPTION:

This circuit uses two signal generators to simulate an Amplitude Modulated RF carrier wave. The output can be used to simulate the response of LC and tank circuits.

Two signal generators are used in this circuit, one representing a high frequency

(200 kHz) RF carrier, VG2; the other signal generator is used to inject a 1 KHz audio

signal. The two signals are mixed and amplified by the transistor and an amplitude

modulated signal appears at the collector of the T1 (2N 2222). The DC component is

removed by C2 and R3 and the RF output now appears across the load resistor R3.

SPICE NETLIST:

The spice net list is shown below. Copy all lines between *AM and .END and paste into

a new text file called Vmod.circuit or similar.

Vcc 1 0 30

VG2 2 0 DC 0 AC 1 0 SIN (0 10M 200K 0 0 -90)

VG1 4 0 DC 0 AC 1 0 SIN ( 0 5 1K 0 0 -90 )

C3 5 0 100N

C2 6 3 470P

C1 2 7 100N

R5 0 7 15K

R4 7 1 56K

R3 0 3 1K

R2 4 5 4.7K

R1 1 6 10K

QT1 6 7 5 Q2N2222.

LIB EVAL.LIB.

.TRAN 2us 10 ms.

.PROBE

.END

To produce an output in Spice Opus start the program and load the new Vmod.cir the

modulated signal appears across R3 which is now node 3 and earth. After loading the

circuit the command "listing" will display the net list. The command "run" will then

simulate the circuit; "display" will print a list of all variables in the circuit. The command

plot v(3) will display the AM wave between node 3 and 0 i.e. the load resistor R3.

Note to speed up simulation, the RF carrier has been limited to 200KHz only, and the

output waveform just shows two complete cycles of the audio wave, i.e. 2ms as the

modulating frequency is 1k.

RESULT: Thus Amplitude Modulation using Pspice was generated.

8. AMPLITUDE MODULATION

AIM:

To generate the waveform for Amplitude modulation using Mat lab Simulation.

THEORY:

In Amplitude Modulation or AM, the carrier signal

Has its amplitude

Modulated in proportion to the message bearing (lower frequency) signal

To give

The magnitude of

Is chosen to be less than or equal to 1, from reasons having to do with demodulation, i.e. recovery of the signal

From the received signal. The modulation index is then defined to be

Figures 1 and 2 are some mat lab plots of what the modulated signal looks like for

. The frequency of the modulating signal is chosen to be much smaller than that of the carrier signal. Try to think of what would happen if the modulating index were bigger than 1.

Figure 1: AM modulation with modulation index .2

Note that the AM signal is of the form

This has frequency components at frequencies

.

Figure 2: AM modulation with modulation index .4 Matlab code for

PROGRAM:

t=0:0.001:1;

vd=8*cos(2*pi*5*t);

vc=0.1*cos(2*pi*15*t);

ft=vc.*vd;

am=ft+vc;

figure(1)

plot(t,vd);

figure(2)

plot(t,vc);

figure(3)

plot(t,am);

Carrier signal

Original singal(informationsignal)

Amplitude Modulated signal

The AM waveform in time and frequency domain.

fm=20HZ,fc=500HZ,Vm=1V,Vc=1V,t=0:0.00001:0.09999

fm=20;

fc=500;

vm=1;

vc=1;

interval=0.001;

% x-axis:Time(second)

t=0:0.00001:0.09999;

f=0:1:9999;

% y-axis:Voltage(volt)

wc=2*pi*fc;

wm=2*pi*fm;

V1=vc+vm*sin(wm*t);

V2=-(vc+vm*sin(wm*t));

Vm=vm*sin(wm*t);

Vc=vc*sin(wc*t);

Vam=(1+sin(wm*t)).*(sin(wc*t));

Vf=abs(fft(Vam,10000))/10000;

% Plot figure in time domain

figure;

plot(t,Vam);

hold on;

plot(t,V1,'r'),plot(t,V2,'r');

title('AM waveform time-domain');

xlabel('time'), ylabel('amplitude');

grid on;

% Plot figure in frequency domain

figure;

plot(f*10,Vf);

axis([(fc-2*fm) (fc+2*fm) 0 0.6]);

title('AM waveform frequency-domain');

xlabel('frequency'), ylabel('amplitude');

grid on;

%Plot modulating signal

figure;

plot(t,Vm);

title('AM modulating signal');

xlabel('time'), ylabel('amplitude');

grid on;

%Plot carrier signal

figure;

plot(t, Vc);

title('AM carrier signal');

xlabel('time'), ylabel('amplitude');

grid on;

clear; Result:

Thus the waveform for Amplitude Modulation is generated using Mat lab

9. FREQUENCY MODULATION AIM:

To generate the waveform for frequency modulation using Matlab Simulation.

THEORY: Frequency modulation uses the information signal, Vm(t) to vary the carrier

frequency within some small range about its original value. Here are the three signals in

mathematical form

• Information: Vm(t)

• Carrier: Vc(t) = Vco sin ( 2 π fc t + φ )

• FM: VFM (t) = Vco sin (2 π [fc + (Δf/Vmo) Vm (t) ] t + φ)

We have replaced the carrier frequency term, with a time-varying frequency. We have

also introduced a new term: Δf, the peak frequency deviation. In this form, you should be

able to see that the carrier frequency term: fc + (Δf/Vmo) Vm (t) now varies between the

extremes of fc - Δf and fc + Δf. The interpretation of Δf becomes clear: it is the farthest

away from the original frequency that the FM signal can be. Sometimes it is referred to as

the "swing" in the frequency.

We can also define a modulation index for FM, analogous to AM:

β = Δf/fm , where fm is the maximum modulating frequency used.

The simplest interpretation of the modulation index, β, is as a measure of the peak

frequency deviation, Δf. In other words, β represents a way to express the peak deviation

frequency as a multiple of the maximum modulating frequency, fm, i.e. Δf = β fm.

Example: suppose in FM radio that the audio signal to be transmitted ranges from 20 to

15,000 Hz (it does). If the FM system used a maximum modulating index, β, of 5.0, then

the frequency would "swing" by a maximum of 5 x 15 kHz = 75 kHz above and below

the carrier frequency. Here is a simple FM signal:

Here, the carrier is at 30 Hz, and the modulating frequency is 5 Hz. The modulation index

is about 3, making the peak frequency deviation about 15 Hz. That means the frequency

will vary somewhere between 15 and 45 Hz. How fast the cycle is completed is a

function of the modulating frequency. PROGRAM: The frequency modulation (FM) waveform in time and frequency domain . fm=250HZ,fc=5KHZ,Vm=1V,Vc=1V,m=10,t=0:0.00001:0.09 999 % setting

vc=1;

vm=1;

fm=250;

fc=5000;

m=10;

% x-axis: Time(second)

t=0:0.00001:0.09999;

f=0:10:99990;

% y-axis: Voltage(volt)

wc=2*pi*fc;

wm=2*pi*fm;

sc_t=vc*cos(wc*t);

sm_t=vm*cos(wm*t);

kf=1000;

s_fm=vc*cos((wc*t)+10*sin(wm*t));

vf=abs(fft(s_fm,10^4))/5000;

% Plot figure in time domain

figure;

plot(t,s_fm);

hold on;

plot(t,sm_t,'r');

axis([0 0.01 -1.5 1.5]);

xlabel('time(second)'),ylabel('amplitude');

title('FM time-domain');

grid on;

% Plot figure in frequency domain

figure;

plot(f,vf);

axis([ 0 10^4 0 0.4]);

xlabel('frequency'), ylabel('amplitude');

title('FM frequency-domain');

grid on;

%Plot modulating signal

figure;

plot(t,sm_t);

axis([0 0.1 -1.5 1.5]);

title('FM modulating signal');

RESULT: Thus the waveform for frequency Modulation is generated using Mat lab