com sys lab 2 n 3

8/2/2019 com sys lab 2 n 3

http://slidepdf.com/reader/full/com-sys-lab-2-n-3 1/16

[Type the document title]

Prepared by Engr. Faiza Mushtaq Page 1

Lab : 02

Amplitude Modulation.

1. Amplitude modulation in matlab.2. Amplitude Modulation with Simulink

3. Hard ware implementation of Amplitude Modulation

OBJECTIVES:

To verify the principles of amplitude modulation (AM).

EQUIPMENT:

PC

Matlab 7.xx

C-pal 27 trainer .

THEORY:

A message signal (audio or video or pulses) cannot be transmitted over long distances directly

due to attenuation, distortion, interference and noise in any medium. A suitable high

Frequency signal needs to be modulated by the low frequency band so that the message can betransmitted efficiently over long distances.

Why Modulate?Why do we have to modulate a signal for transmission? Why can't the signal be sent as it is?

There are two main reasons for modulation. The 1st reason has to do with the laws of

electromagnetic propagation, which dictate that the size of the radiating element, the antenna,

be a significant fraction of the wavelength of the signal being transmitted. For example, if we

want to transmit a 1 kHz signal by a quarter wave antenna, the size of the antenna would need

to be 75 km. On the other hand, if the signal is being transmitted on a high frequency carrier,

say 630 kHz, the corresponding size of the radiating antenna needs to be only 119 m.

The second reason is for the simultaneous transmission of different signals. As audio signals

relevant to humans lie from a few hertz to a few thousand hertz, we could broadcast only one

base band signal at a time. Simultaneous transmission would cause the overlap of signals andwe would not be able to separate them. However, through modulation, we can transmit many

signals simultaneously by shifting their spectra using different carrier frequencies. This is called

frequency division multiplexing (FDM).

Amplitude Modulation (AM):

8/2/2019 com sys lab 2 n 3




It is a process in which the maximum amplitude of the carrier Wave is varied linearly in

accordance with instantaneous amplitude of modulating signal or base band signal. The waves

can be voltage or current signals. The waveforms of the carrier wave, modulating wave and the

resultant modulated wave are shown below. A carrier signal is represented by,

In amplitude modulation a voltage proportional to the modulating signal is added to the carrier

amplitude. Then modulated carrier wave is given by,

() [ ()] ( )

The term [Ecmax +em(t)] describes the envelop of the modulated wave.

Amplitude modulation is a type of modulation where the carrier signal’s amplitude is varied in

accordance with the information bearing signal. The amplitude modulated signal’s envelope or

boundary embeds the information bearing signal. The total power of the transmitted signal

varies with the modulating signal, whereas the carrier power remains constant.

8/2/2019 com sys lab 2 n 3




LAB TASK 1

Steps to Follow:

1. Generate a message signal of frequency 1Hz with V(t) = Vo sin (2 p f t ) and |V(t)|>0

2. Generate a carrier signal of frequency 20Hz

3. Plotting both the signals.4. Plotting the modulated signal.

Description:

Message Signal: generate the message signal

Information: V (t) = Vo sin (2 p f t )

Carrier Signal: For generation of carrier signal fc is taken as specified.

Carrier: Vc(t) = Vco sin (2 p f c t )

Spectrum Plot: Since the spectrum plot is generated to find the frequency components of a

signal, therefore, to plot the spectrum of message and carrier signal, their zero shifted Fourier

transforms have been taken. Only their positive side has been taken and plotted against linear

frequency.

Amplitude Modulation: As in case of Amplitude Modulation, the message signal is simply

multiplied by the carrier signal, therefore here the amplitude modulated signal has been

generated by using the dot multiplication operator (.*) in MATLAB.

AM: VAM(t) = { Vco + Vm(t) }sin (2 p f c t ) s(t) = V(t). xV c(t)

8/2/2019 com sys lab 2 n 3




OUTPUT:

8/2/2019 com sys lab 2 n 3




LAB TASK 2

Amplitude Modulation with Simulink

Steps to Follow:First start new model.

We need to build the following function.

)cos()](1[)(cc

wt m At s

Where m(t) is a sinusoidal function. So we got four inputs.AC and 1 which are constants, and two

sinusoidal for our message and carrier.

1- From our library browser we go to Simulink > Sources, we drag two constants and two sine

waves. And name them with appropriate names.

2- Double click on the Signal wave. Change the Frequency to 5 (rad/sec) and change sample time to

1/100.3- Do the same for the carrier but with Frequency =1000.

Let's link them to gather.

4- Go to Simulink > Math Operations. Drag one Product block and one Sum block.

5- Now link the Signal and the Constant 1 to the Sum operation inputs.

6- Go to Simulink > Sinks. Drag one Scope.

7- Link the output of the Product to the Scope.

8- Now run your model and double click one the scope. You will see your modulated signal.

8/2/2019 com sys lab 2 n 3




.

8/2/2019 com sys lab 2 n 3




3. AIM:

To generate Amplitude Modulation (AM) wave

2. APPARATUS REQUIRED:

S.N. Item required Specification(Range ) Quantity

1 communication

trainer kit

_ _ one

2 CRO 30 MHz one

3 Connecting leads _ _ 3 to4

8/2/2019 com sys lab 2 n 3




BLOCK DIAGRAM

Methods of modulation & demodulation:

Amplitude modulation is carried out by a circuit utilizing the nonlinear characteristic of solid

state device like a diode called Square law modulator or collector circuit of a transistor called ascollector modulator. It can be seen from the waveforms of the carrier wave, modulating wave

and the resultant modulated wave that the envelope of the resultant waveform is identical to themodulating wave and thus utilized by the AM receiver for recovery of original message i.e. the

modulating signal.

EXPERIMENTAL PROCEDURE :

1. Make the connection according to the block diagram

2. Switch on the power supply .Connect AF Generator to CRO and note the Frequency andamplitude of AF output. .

3. Similarly note the output amplitude and frequency of RF Carrier wave.4. Set the amplitude of AF generator and amplitude of carrier wave so that modulated

Output can be observed conveniently by easy manipulation of CRO.5.Draw wave forms as observed on CRO

CONCLUSION:

8/2/2019 com sys lab 2 n 3




LAB 3

MODULATING INDEX OF AM.

To generate Amplitude Modulation (AM) wave and determine its modulationIndex ‘ma

1. OBJECTIVE:To generate AM wave for different value of modulation index (m<1, m=1 &

m>1) using Matlab

2. APPARATUS REQUIRED

Computer

Matlab

3. AMIn AM the transmitter uses the information signal, Vm(t) to vary the amplitude of the

carrier, Vco to produce a modulated signal, VAM(t). Here are the three signals in

mathematical form:

Information: Vm(t) = Vmo sin (2 p fm t)

Carrier: Vc(t) = Vco sin (2 p fc t)

AM: VAM(t) = { Vco + Vm(t) }sin (2 p fc t + f)

4. MODULATION INDEX :If the information signal is also a simple sine wave the modulation index has a simple

form:

m = Vmo/Vco

The interpretation of the modulation index, m, may be expressed as: The fraction

(percentage if multiplied by 100) of the carrier amplitude that it varies by. If m = 0.5,

the carrier amplitude varies by 50 % above and below its original value. If m= 1.0

then it varies by 100%.

Here is a typical AM signal, showing the parts. Note that the information modulates

the envelope of the carrier signal.

8/2/2019 com sys lab 2 n 3




In this example, the modulation index is < 1.0

5. LAB TASKPerform the AM WITH:

100% Modulation

50% Modulation

25% Modulation

150% Modulation

In matlab and simulink.

8/2/2019 com sys lab 2 n 3




6. OUTPUT:

8/2/2019 com sys lab 2 n 3




7. CONCLUSION:

8. QUESTIONS:

1. Write down AM equation in terms of modulation index.

2. Draw AM envelope.

3. Write down AM transmitted power equation in terms of modulation index.

4. Why we have not addressed the meaning modulating by more than 100% andwhy AM has a maximum practical efficiency of about 33 %.

8/2/2019 com sys lab 2 n 3




2. AIM:

To generate Amplitude Modulation (AM) wave and determine its modulation

Index ‘ma

2. APPARATUS REQUIRED:

S.N. Item required Specification(Range ) Quantity

1 comunication

trainer kit

_ _ one

2 CRO 30 MHz one

3 Connecting leads _ _ 3 to4

8/2/2019 com sys lab 2 n 3




BLOCK DIAGRAM

Modulation index:

The ratio of maximum amplitude of the modulating signal to the maximum amplitude of carrier

wave is defined as the amplitude modulation index and denoted by ‘ma ’. The modulation index

is also known as depth of modulation or degree of modulation or modulation factor. Normally

the value of ‘ma’ lies between 0 and 1. The modulation index is given by expression.

It can be seen from the resultant of AM(single tone modulation) waveform

From (1) & (2)

Clearly, the Envelope of the modulated signals has the same shape as m(t) when m < 1. If the

Value of ‘ ma’ exceeds 1, then the percentage modulation is greater than 100 and the base band

signal is not preserved in the envelope. In this case the base band signal recovered from theenvelope by the demodulator of a receiver will be distorted. This type of distortion is called

envelope distortion and AM signal is called over modulated.

Methods of modulation & demodulation:

Amplitude modulation is carried out by a circuit utilizing the nonlinear characteristic of solid

state device like a diode called Square law modulator or collector circuit of a transistor called as

collector modulator. It can be seen from the waveforms of the carrier wave, modulating waveand the resultant modulated wave that the envelope of the resultant waveform is identical to the

modulating wave and thus utilized by the AM receiver for recovery of original message i.e. the

modulating signal.

8/2/2019 com sys lab 2 n 3




EXPERIMENTAL PROCEDURE :

1. Make the connection according to the block diagram2. Switch on the power supply .Connect AF Generator to CRO and note the Frequency and

amplitude of AF output. .

3. Similarly note the output amplitude and frequency of RF Carrier wave.4. Set the amplitude of AF generator and amplitude of carrier wave so that modulated

Output can be observed conveniently by easy manipulation of CRO.

5. Note down the values of Vmax and Vmin

6. Change the amplitude of AF Generator to different values, keeping RF output constant

And note down the corresponding values of Vmax and Vmin.

Calculate amplitude modulation index ma =.(Vmax – Vmin) / (Vmax+ Vmin)

8. Repeat step No.6 and tabulate the results.

9. Draw wave forms as observed on CRO and label the different waveformsAppropriately.

8/2/2019 com sys lab 2 n 3




OBSERVATION TABLE:

CONCLUSION:

com sys lab 2 n 3

Documents