communicatin system 1 lab manual 2011

PRACTICAL WORK BOOK

For Academic Session 2011

COMMUNICATION SYSTEMS -I

(TC-391)

For

TE (EE/EL)

Name:

Roll Number:

Batch:

Department:

Term/Year:

Department of Electronic Engineering

NED University of Engineering & Technology

LABORATORY WORKBOOK

For The Course

TC- 391 Communication Systems- I

Prepared By: Ms. Sundus Ali, Lecturer, Telecommunications Engineering

Reviewed By: Ms. Saba Ahmed, Assistant Professor and

Ms. Uzma Butt, Lecturer, Telecommunications Engineering

Approved By: The Board of Studies, Department of Electronic Engineering

INTRODUCTION

Communication Systems - I Practical Workbook covers a wide

range of Laboratory Sessions that can prove highly helpful in

allowing the students to grasp the core objective of this subject.

These labs will assist in solidifying the practical concepts that

are very essential for the engineering students.

This La b work book comprises of the following port ions:

1. Lab Sessions related to Signals’ Time and Frequency

Domain Analysis using MATLAB

2. Lab Sessions related to linear modulation techniques

(AM, SSB and FM), performed on modern trainer

board

3. Small Lab Projects, involving hands-on

implementation of modulation circui ts

Each Lab session also contains a brief account of relevant theory

about the topic being covered in the said session.

Credits: Lab Manual on Analog Communication (EC-205), Department

of Electronics and Communication Engineering, School of Engineering,

Udaipur, India.

Contents NED University of Engineering And Technology-Department of Electronic Engineering

Telecommunications Laboratory

CONTENTS

Lab No.

Date

Experiment

Page No.

Remarks

MATLAB

1 Introduction to MATLAB, basic Mathematical operations,

variables, vectors, matrices basic plotting and MATLAB Programming

1

2

Continuous Time Scaling And Shifting Transformation Techniques Study and check the linearity and time invariance of a system

13

3

Generation of an algorithm that gives the output obtained by

convolution of two signals

Generation of an algorithm that gives the output obtained by

correlation between two signals

To explore the application of correlation for identifying a

periodic signals corrupted by/buried in noise

18

MODULATION

4

To understand the concepts of Fourier Transform

To carryout Fourier Synthesis of a square wave

22

5 To examine the main parameters of amplitude modulated signal

Demodulation of AM signal using Envelope Detector

25

6.

LAB PROJECT (1)

33

7

To observe the operation of Product detector To check the operation of the balanced amplitude modulator

with suppressed carrier.

34

8.

LAB PROJECT (2)

42

Lab No.

Date

Experiment

Page No.

Remarks

9 To examine the main parameters of the single sideband

modulation To Check the use of filters to generate the SSB

43

10

To examine the operation of RF (Radio Frequency) receiver

To examine the operation of RF (Radio Frequency) receiver

49

11

LAB PROJECT (3)

53

12

To examine the operation of AM-RF (Radio Frequency) transmitter

To examine the operation of SSB-RF (Radio Frequency) transmitter

54

13

LAB PROJECT(4)

58

14

LAB EXAM AND PERFORMANCE EVALUATION

Lab Manual Communication Systems-I (TC-391) NED University Of Engineering And Technology-Department of Electronic Engineering

1

LAB SESSION 01

INTRODUCTION TO MATLAB

OBJECTIVES:

(a) To get acquainted with MATLAB’s environment

(b) To use MATLAB as a calculator

(c) To learn to create MATLAB variables and basic plots

(d) To learn to initialize and manipulate vectors, matrices

PRE-LAB THEORY: What is MATLAB? MATLAB is a software program that allows you to do data manipulation and visualization, calculations, math and

programming. It can be used to do very simple as well as very sophisticated tasks. It can solve large systems of

equations efficiently and it is therefore useful for solving differential equations and optimization problems. It also

provides excellent means for data visualization and has symbolic capabilities. Whilst simple problems can be solved

interactively with MATLAB, its real power shows when given calculations that are cumbersome or extremely

repetitive to do by hand!

Where can we use MATLAB? MATLAB is recognized as the interactive program for numerical linear algebra and matrix computation. In

industries, MATLAB is used for research and to solve practical engineering and mathematical problems. Also, in

automatic control theory, statistics and digital signal processing (Time-Series Analysis) one can use MATLAB. The

following tool boxes make it useful in soft computing at various industrial and scientific areas:

(i) Neural Networks (ii) Optimization (iii) Genetic Algorithms

(iv) Wavelets (v) Fuzzy Logic (vi) Control systems

(vi) Signal processing (vii) Communication Systems

Starting MATLAB After logging into your account, you can enter MATLAB by double-clicking on the MATLAB shortcut icon on the

Window.

After starting MATLAB, the MATLAB Desktop will appear. The default three-part window looks similar to the

figure below. The Desktop is also known as an Integrated Development Environment (IDE). Clicking the Start

button in the lower left-hand corner will display a list of MATLAB tools, shortcuts, and documentation.


2

Figure 1.1 GUI view of MATLAB

Using MATLAB as a Calculator: As an example of a simple interactive calculation, just type the expression you want to evaluate. Let’s start at the

very beginning. For example, in order to calculate the following expression

1+2*3, one needs to type it at the prompt command (>>) as follows:

>> 1+2*3

ans=

7

Lab Task:

Try the following and record your observations:

>>3+7.5

>>18/4

>>3*7


3

>>3^2

Observations:

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

Mathematical Functions:

The following table lists some commonly used standard MATLAB functions:

Function MATLAB Syntax

ex exp(x)

√x sqrt (x)

Ln x log(x)

Log10 (x) log10(x)

Cos x cos(x)

Sin x sin(x)

Tan x tan(x)

Cos-1 x acos(x)

Sin-1 x asin(x)

Tan-1 x atan(x)

Table 1.1 Standard MATLAB functions

Example 1: y= e-a sin(x) + 10√y for a=5, x=2, and y=8 is computed by

>>a=5; x=2; y=8;

>>y=exp (-a)*sin(x) +10*sqrt(y)

y=

28.2904

Example 2:

To calculate sin (pi/4) and e10

>>sin(pi/4)

ans=

0.7071

>>exp(10)

ans=


4

2.2026e+004

Variables, Expressions & Statements: Variable names must be a single word containing no space and up to 31 characters. Variables names are case

sensitive and a variable name must start with a letter. Punctuation characters are not allowed.

MATLAB’s statements are of the form:

>> variables = expression or

>> expression

Expressions are composed of operators, function and variable names. After evaluation the value is assigned

to the variable and displayed. If the variable name and = sign are omitted, a variable ans (for answer) is

automatically created and the result is assigned to it.

Rules of Precedence:

Expressions are evaluated from left to right with exponential operation having the highest precedence,

followed by multiplication and division having equal precedence, followed by addition and subtracting

having equal precedence. Parentheses can be used to alter this ordering in which case these rules of

precedence are applied within each set of parentheses starting with the innermost set and proceeding

outward.

The most recent values assigned to the variables you used in the current session are available. For example,

if you type a at the prompt you get the output as:

>> a

a =

7

The display of numerical values can have different format as we see below:

>> e= 1/3

e =

0.3333

>> format long (long decimal format)

>> e

e =

0.33333333333333

>> format short e (short exponential format)

>> e

e =

3.3333 e-01

>>format long e (long exponential format)

e =

3.33333333333333 e-04


5

>>format ( default format)

>>e

e =

0.333

Entering Matrices: You can enter matrices in various different ways:

• Enter an explicit list of elements.

• Load matrices from external data files.

• Generate matrices using built-in functions.

• Create matrices with your own functions and save them in files.

MATLAB works with essentially only one kind of objects, i.e. a rectangular numerical matrix with possibly

complex entries. All variables represent matrices. If you want to store a matrix 1 2 3 in a

variable a in the MATLAB’s current memory, you type the following:

4 5 6

7 8 9

>> a = [1 2 3; 4 5 6; 7 8 9]

a =

1 2 3

4 5 6

7 8 9

The rows are separated by semicolons and elements are separated by space or by comma. That is, the above

matrix can also be stored by the following command.

>> a = [1,2,3;4,5,6;7,8,9];

or by the statement

>> a = [

1 2 3

4 5 6

7 8 9 ]

The semicolon at the end of a command suppresses the output.

The matrix a is transposed and is stored in b by the following command

>> b = a’

b =

1 4 7

2 5 8

3 6 9

The following matrix operations are available in MATLAB:


6

Table 1.2 Matrix Operations

Matrix Initialisation and Addressing:

>>A=[1 2 3 4;5 6 7 8]

A=

1 2 3 4

5 6 7 8

>>A( : , : )

Ans=

1 2 3 4

5 6 7 8

>>A(1,2)

Ans=

2

>>A(1, :)

Ans=

1 2 3 4

>>A(: ,[1,3])

Ans=

1 3

5 7

>>A=(: ,end)

Ans=

3

7

Selected portion of one matrix can be assigned to a new matrix

>>B= A(: , 3 :end)

B=


7

3 4

7 8

To select the elements along main diagonal,

>>Diag(B)

Ans=

3

8

Changing and deleting matrix elements:

>>A=1:5

A=

1 2 3 4 5

>>A(2) = 6

A=

1 6 3 4 5

>>A([1 3]) = 0

A=

0 6 0 4 5

Manipulating Matrix:

>>A=[1 3 4; 5 7 8]

A=

1 3 4

5 7 8

For transpose,

>>A’

Ans=

1 5

3 7

4 8

>>fliplr (A)

Ans=

4 3 1

8 7 5


8

>>flipud(A)

Ans=

5 7 8

1 3 4

Matrix Operation:

>>A=[1 3 4 ; 5 7 8]

A=

1 3 4

5 7 8

>>2+A

Ans=

3 5 9

7 9 10

Element-by-element operations vs. Matrix Operations:

Matrix multiplication has a special procedure, and it is different from simple element-to-element

multiplication. This is unlike addition operation, where two matrices cab be added by simple element-by-

element addition, this is adding corresponding elements of the two matrices in question.

Thus, we have a special set of operators that distinguish Matriz operations from element-by-element

operations. When you intend to perform matrix operation, you simply use, * for multiplication, / for

division, ^ for exponentiation and so on.

But if you intend to perform element-by-element operations, you have to use, .* for multiplication, ./ for

division and .^ for exponentiation.

Example,

>>A=[2 3 4;5 6 7;2 1 0];

>>B=[1 2 3;5 6 2;0 0 5];

>>A*B

Ans=

17 22 32

40 52 59

7 10 8

>>A.*B

Ans=

2 6 12

25 42 12


9

0 0 0

Basic Plotting:

MATLAB has an excellent set of graphics tools. Plotting a given data set or results of computation is possible with a

very few commands. Trying to understand mathematical functions with graphics is an enjoyable and very ancient

way of learning mathematics. Being able to plot mathematical functions and data freely is the most important step,

and this section is written to assist you to do just that.

The MATLAB command used to plot the graph is “plot(x,y)”. For example, if the vector x= (1; 2; 3; 4; 5; 6) and y=

(3; -1; 2; 4; 5; 1)

>> x= [1 2 3 4 5 6];

>> y= [3 -1 2 4 5 1];

>>plot(x,y)

Figure 1.2 Output Plot

Now try the following:

>>x = 0: pi/100: 2*pi;

>>y= sin (x);

>>plot (x,y)

Adding titles and axis labels:

MATLAB enables you to add axis labels and titles. For example, using the graph from the previous

example, add x- and y-axis labels.

>> xlabel (‘x = 0:2\pi’);

>>ylabel (‘sine of x’);

>>title (‘Plot of the Sine Function’)


10

Multiple data sets in one plot: Multiple (x;y) pairs arguments create multiple graphs with a single call to plot. For example, these statements plot

three plots related functions of x: y1=2 cos(x), y2=cos(x), and

y3=0.5*cos (x) in the interval 0 ≤ x ≤ 2∏

>> x= 0:pi/100:2*pi;

>>y1= 2*cos(x);

>>y2= cos (x);

>>y3= 0.5*cos (x);

>>plot (x, y1, ‘- -‘,x, y2, ‘- - ‘, x, y3, ‘:’)

>>xlabel (‘0 \leq x \leq 2\pi’)

>>ylabel (‘Cosine functions’)

>>legend (‘2*cos(x)’, ‘cos(x)’, ‘0.5*cos(x)’)

>>axis ([0 2*pi -3 3])

The result of the multiple data in one graph is shown below:

Figure 1.3 Output Plot

MATLAB Programming:

To make a file in MATLAB, click on “New” in the file menu in menu bar. It will open a new file.

After writing the code, if we want to save this file (called m-file), it will be saved in “bin” folder of

MATLAB by default.

Such files are called “M-files” because they have an extension of “.m” in its filename. There are two types of

M-files: Script Files and Function Files

% This is a sample m-file

a = [1,2,3;0,1,1;1,2,3]

b =a’;

c = a+b

d = inv(c)


11

Save this file as “rkg.m”.

Then in order to execute this file, type

>> rkg <ENTER>

Write the output:

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

The % symbol indicates that the rest of the line is a comment. MATLAB will ignore the rest of the line.

However, the first comment lines which document the m-file are available to the online help facility and will

be displayed.

An M-file can also reference other M-files, including referencing itself recursively.

Function Files: Function files provide extensibility to MATLAB. You can create new functions specific to your problem

which will then have the same status as other MATLAB functions. Variables in a function file are by default

local. However, you can declare a variable to be global if you wish.

Example: Function y = prod (a,b)

y = a*b;

Save this file with filename prod.m then type on the MATLAB prompt

>> prod (3,4)

ans =

12

In MATLAB to get input from user the syntax rule is:

>>variable name = input (‘variable’);

Example:

>>a=input (‘ Enter a number ‘);

And on command prompt if we run this program it will be displayed like:

>> Enter a number

If we type 2 at the cursor then we get the output as

a=2

Control Flow: MATLAB offers the following decision making or control flow structures:

1. If- Else –End Construction

2. For loops


12

3. While loops

4. Switch-Case constructions etc.

Script Files:

A script file consists of a sequence of normal MATLAB statements. If the file has the filename, say, rkg.m,

then the MATLAB command >> rkg will cause the statements in the file to be executed. Variables in a

script file are global and will change the value of variables of the same name in the environment of the

current MATLAB session.

Script files are often used to enter data into a large matrix; in such a file, entry errors can be easily edited out.

The load Function: The load function reads binary files containing matrices generated by earlier MATLAB sessions, or reads

text files containing numeric data. The text file should be organized as a rectangular table of numbers,

separated by blanks, with one row per line, and an equal number of elements in each row. For example,

outside of MATLAB, create a text file containing these four lines:

16.0 3.0 2.0 13.0

5.0 10.0 11.0 8.0

9.0 6.0 7.0 12.0

4.0 15.0 14.0 1.0

Save the file as magik.dat in the current directory. The statement load magik.dat reads the file and creates a

variable, magik, containing the example matrix. An easy way to read data into MATLAB from many text or

binary formats is to use the Import Wizard.

EXERCISE:

1. Given that x=-5+9i and y=6-2i, use MATLAB to show that x-y=1+7i, xy= -12+64i and x/y= -1.2 + 1.1i 2. Use MATLAB to compute: 6(351.4 )+140.35 3. Use MATLAB to plot the functions y=4√(6x+1) and z=5e0.3x – 2x over the interval 0<x<1.5 4. Use MATLAB to plot function s= 2 sin (3t+2) + √(5t+1) over the interval 0<t<5. 5. Plot exp(-x) sin(8x) for 0≤ x ≤2Π 6. Plot the function y= (4/Π) [(sin x) + (sin 3x)/3 + (sin 5x)/5 + (sin7x)/7] over the interval –Π<x<Π. These are the

first four terms of the Fourier Series representation of a square wave. 7. Define a 4x3 matrix with zeros everywhere except the first line that is filled with 1. Hint= use ones and zeros

command 8. Use MATLAB to solve following set of equations.

6x - 4y + 8z = 112 -5x – 3y +7z = 75

14x + 9y – 5z = -67 9. Use MATLAB to find roots of 13x3 + 182x2 – 184x +2503 = 0


13

LAB SESSION 02

SIGNAL TRANSFORMATION TECHNIQUES, LINEAR AND TIME VARIANT PROPERTIES OF A SYSTEM

OBJECTIVES:

(a) Learn time domain representation of various discrete signals

(b) Learn and implement amplitude and continuous time scaling techniques

(c) Learn and implement various time shifting techniques

PRE-LAB THEORY (a): Amplitude Scaling: Amplitude scaling: the function g(t) is multiplied by the constant A for all values of t. This transformation can be

indicated by the notation:

g(t) → Ag(t)

Figure shows two examples of amplitude-scaling of the function g(t)

Figure 2.1 examples- amplitude-scaling of the function g(t)


14

Time Shifting:

Time shifting by t0 corresponds to the transformation:

g(t) → g(t-t0)

where, t0 is the time shift. If t0>0, the waveform g(t) is obtained by shifting towards the right, relative to the time

axis. If to<0, g(t) is shifted to the left.

Figure 2.2 Time shifting example

Time Scaling: Time scaling corresponds to the transformation:

g(t) → g(t/a)

If a>0, t→ t/a expands the function horizontally by a factor of a.

The figure compares g(t) and g(t/2)

PROCEDURE (a):

Problem 1: Use MATLAB, plot the following function defined by

G(t)= 0 t<-2

-4-2t -2<t<0

-4+3t 0<t<4

16-2t 4<t<8

0 t>8


15

Then plot the transformed functions:

3G(t+1)

0.5G(3t)

-2G((t-1)/2)

Problem 2: Draw the following built-in funtions:

a. Diric b. Sawtooth c. Square d. tripuls e. sinc

Continuous-Time Even and Odd Functions:

A Continuous-time even function verifies the relation:

g(t) = g(-t)

Even functions are symmetrical about the y-axis

A continuous-time odd function verifies the relation:

g(t) = -g(-t)

Odd functions are symmetrical about the origin O

Figure 2.3 Even and Odd Signal

Linear and Non-Linear Systems: We now investigate the linearity property of a causal system of the type described. Consider the system given by:

y[n]-0.4y[n-1]+0.75y[n-2]=2.2403x[n]+2.4908x[n-1]+2.2403x[n-2] Eq. (A)


16

MATLAB program is used to simulate the system to generate three different input sequences x1[n], x2[n] and x[n]=

a* x1[n]+b*x2[n], and to compare and plot the corresponding output sequences y1[n], y2[n] and y[n].

PROCEDURE (b):

Problem 1:

1. Generate following signals: a. x1= cos (2pi(0.1n)) b. x2= cos (2pi(0.4n)) for n=0 to 40

2. Now, apply operation: x=a*x1+b*x2 such that a= 2 and b= -3

3. Assume y1 and y2 to be the outputs of the system when x1 and x2 are applied as inputs respectively. The system

has following response (refering to eq A):

y[n]-0.4y[n-1]+0.75y[n-2]=2.2403x[n]+2.4908x[n-1]+2.2403x[n-2]

Mathematically, y1=h convolved with x1 y2=h convolved with x2

4. Also compute y such that, y=h convolves with x where x was computed in point 2.

5. Now compute, yt such that yt=a*y1+b*y2 where a and b are defined in point 2.

6. Now, find out d, the difference signal by subtracting yt from y.

7. Plot the outputs and difference signal.

Time and Time-Invariant Systems:

We next investigate the time-invariance property of a causal system. Consider again the system given in Eq

(A). MATLAB program is used to simulate the system of Eq (A), to generate two different input sequences

x[n] and x[n-D], and to compute and plot the corresponding and plot the corresponding output sequences

y1[n], y2[n] and the difference y1[n] – y2[n+D]

Figure 2.4 Time-Invariant System

PROCEDURE (c): Problem 1: 1. Use the same values of input signals x1 and x2, a and b, as well the same system response values used

in the above problem in procedure (b).

2. Assume the value of D to be 10


17

3. Compute x= a*x1+b*x2

4. Compute y= system convolves with x.

5. Compute yd such that yd=system convolves with xd, where xd is a delayed version of x byD.

(Hint: xd=[zeros(1,D) x])

6. Compute the difference signal d= y-yd

7. Plot the outputs y[n] and yd[n] and difference signals

Exercise:

1. Generate a code which uses input to shift unit-sample right or left using function.

2. Using MATLAB, graph the following discrete-time function:

g[n]= 10 e-∏/4 sin(3∏n/16) u[n] then graph the following functions: a. g[2n] b. g[n/3]

3. Using MATLAB, find the energy or power of the following signals:

a. x(t) = tri ((t-3)/10) b. X[n] = e-[n/10] sin (2∏n/4)

4. find and sketch the even and odd parts of these signals:

g[n]= u[n]- u[n-4] g[n]= cos (2∏n/4) g[n]= sin(2∏n/4) g[n]= e-(n/4)u[n]

5. Compute and plot the impulse response of the system described below: y[n] - 0.4[n-1]+0.75y[n-2]= 2.2403x[n]+2.24908x[n-1]+2.2403x[n-2]


18

LAB SESSION 03

CONVOLUTION AND CORRELATION

OBJECTIVES:

(a) To learn the generation of an algorithm that gives the output obtained by convolution of two signals

(b) To learn the generation of an algorithm that gives the output obtained by correlation between two

signals

(c) To explore the application of correlation for identifying a periodic signals corrupted by/buried in

noise

PRE-LAB THEORY (a):

CONVOLUTION:

The response y (n) of the LTI system as a function of the input signal x(n) and the unit sample response h(n),

is a convolution sum between x(n) and h(n). The input x(n) is convolved with the impulse response h(n) to

yield the output y(n).

The equation of the correlation: Y (n) =h (n) * X (n)

It is implemented in MATLAB by the command “Conv”, provided that two sequences be convolved are of

finite length.

PROCEDURE (a):

Problem 1:

Two signals are provided below. Apply convolution on the two signals and plot the output.

N 0 1 2 3 4 5 6 7 8

h(n) 4 2 -1 3 -2 -6 -5 4 5

x(n) -4 1 3 7 4 -2 -8 -2 -1

Problem 2:

1. Given a discrete-time sine wave (frequency and amplitude of your choice). Plot the waveform.

2. Generate AWGN signal. Plot the waveform.

3. Add White Gaussian Noise (random in nature) to the sine signal in order to have it corrupted.

4. Plot the corrupted signal.

5. Generate a moving average FIR filter (number of taps should be user-defined). Plot the response of the

filter.

6. Now, convolve the corrupted signal (noise + sine) with the FIR filter. Plot the output and provide the

conclusion.


19

Figure 3.1 Projected output of the convolution problem

CORRELATION:

Correlation is the process that qualifies the degree of inter-dependence of one process upon another or

measures the similarity between one set of data and other. Correlation is of two types: Cross-correlation and

Auto Correlation. It is implemented in MATLAB by the command “xcorr”, provided that two sequences be

convolved are of finite length.

Problem 3:

Two signals are provided below. Apply correlation on the two signals and plot the output.

n 0 1 2 3 4 5 6 7 8

x1(n) -4 1 3 7 4 -2 -8 -2 -1

x2(n) 4 2 -1 3 -2 -6 -5 4 5

Problem 4:

Calculation of Auto-correlation sequence (ACS) and effect of time-shifting

1. Provided a signal x1= [1 2 3 4]

2. Apply auto-correlation on x1 and plot the output.

3. Apply time shifting (n0=2 or 4) to the signal and save the output in vector x2. Plot the output.

4. Now, apply autocorrelation on x2 and x3 and plot the output graphs.

5. Observe and write down your conclusion.


20

Figure 3.2 Projected output of Correlation problem

Application of Correlation: Detection of periodic signal buried in noise!

Problem 5:

1. Given a sine wave signal (amplitude and frequency of your choice and 100 samples on x-axis).

2. Apply auto-correlation of the sine wave signal and plot the output.

3. Generate AWGN signal of equal number of samples (100). Plot the noise signal.

4. Apply auto-correlation of noisy signal and plot the output. Write your observation regarding the output.

5. Now, add the sine wave signal (generated previously) to the noisy signal and plot the graph showing the

corrupt signal.

6. Now, apply auto-correlation of the sine + noise signal and plot the output. What do you observe? Write

your conclusion.


21

Figure 3.3 Projected output of Application of Correlation Problem

LAB ASSIGNMENT:

Compute and plot the convolution x(n)*h(n) for the pairs of signals given below:

1. X(n)={1 1 1 1}; Y(n)={6 5 4 3 2 1}

↑ ↑

2. X(n)={1 1 1 1}; Y(n) {6 5 4 3 2 1}

↑ ↑

3. X(n)= 13n 0≤n≤6, 0 elsewhere

h (n)= 1 -2≤n≤2, 0 elsewhere

4. Determine the autocorrelation sequences of the following signals and write your observations:

a. X(n)= {1 2 1 1}

↑

b. Y(n)= {1 1 2 1}

↑


22

LAB SESSION 04

FOURIER SYNTHESIS OF SQUARE WAVE OBJECTIVES:

(a) To understand the concepts of Fourier Transform

(b) To carryout Fourier Synthesis of a square wave

EQUIPMENT REQUIRED:

Modules T10H.

+/- 12Vdc Supply

Oscilloscope

PRE-LAB THEORY: A square wave spectrum is made up of the sum of all the harmonics being odd of the fundamental

frequency with decreasing amplitude according to the law of trigonometric Fourier series. In other

words, the square wave shown in fig 4.1 can be obtained by summing up the infinite sine waves as per

the following relation:

S(t) = sin(2ΠFt)/1 + sin(2Π3Ft)/3 + sin(2Π5Ft)/5 + sin(2Π7Ft)/7 + sin(2Π9Ft)/9 + ……..

Figure 4.1 Square wave time and frequency domain representation

Frequency spectrum of a square wave can be mathematically represented by:

∞

f(t) = a0 + ∑( an cos nwt + bn sin nwt)

n=1


23

PROCEDURE:

For Odd harmonics (1, 3, 5, 7, 9): set two way switches “-/0/+” on “+” and two way

switches “sin/cos” on “sin”.

Even harmonics (2, 4, 6, 8): two way switches “-/0/+” on “0”.

Connect the oscilloscope with the amplifier output of the fundamental (1st) and adjust the

amplitude at 10Vp-p.

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Connect the oscilloscope with the output of the third harmonic amplifier (3RD) and adjust the

amplitude at 10/3 ≈ 303Vp-p.

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Connect the oscilloscope with the output of the 5TH harmonic amplifier (5TH) and adjust

the amplitude at 10/5 = 2Vp-p.

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Connect the oscilloscope with the output of the seventh harmonic amplifier (7TH) and

adjust the amplitude at 10/7 ≈ 1.4Vp-p.

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Connect the oscilloscope with the output of the 9th harmonic amplifier (9TH) and adjust the

amplitude at 10/9 ≈ 1.1Vp-p-

----------------------------------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------------------------

Connect the oscilloscope with OUT and check that there is the signal corresponding to

the components sum.

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------------------------------------------------------------------------------------------------------------

Remove some harmonics (put the relating two way switch on 0) and check the O/P

signal.

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24

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RESULT AND WAVEFORMS:


25

LAB SESSION 05

AMPLITUDE MODULATION AND DEMODULATION

OBJECTIVES: (a) To examine the main parameters of an Amplitude Modulated signal

(b) To check the operation of an amplitude modulator

(c) To carry out characteristic measurement on Amplitude Modulated signal

(d) To investigate the Demodulation of an AM signal with an Envelope Detector

EQUIPMENT REQUIRED:

Amplitude Modulation Work board 53-130 which comprises the following blocks:

Signal Generation

Modulation

Filters

Demodulation

PRE-LAB THEORY (a):

Modulation: The modulation is simply a method of combining two different signals and is used in the transmitter

section of a communication system. The two signals that are used are the information signal and the

carrier signal. The information signal is the signal that is to be transmitted and received and is sometimes

referred to as the intelligent signal. The carrier signal allows the information signal to be transmitted

efficiently through the transmission media. The carrier signal is normally generated by an oscillator

and has a constant frequency and amplitude. The information signal that is fed into the transmitter

modifies the carrier signal.

Figure 5.1 Modulation Techniques


26

Amplitude Modulation: It is the simplest form of signal processing in which the carrier amplitude is simply changed

according to the amplitude of the information signal, hence the name Amplitude Modulation. When the

information signal’s amplitude is increased the carrier signal’s amplitude is increased and when the

information signal’s amplitude is decreased the carrier signal’s amplitude is decreased. In other

words, the ENVELOPE of the carrier signal’s amplitude contains the information signal.

Modulation index “m” = Vmax – Vmin Vmax + Vmin

Figure 5.2 Amplitude Modulation

Modulation Mathematics: The equation of a sinusoidal voltage waveform is given by: v = Vmax.sin(ωt+Ø) where:

• v is the instantaneous voltage

• Vmax is the maximum voltage amplitude

• ω is the angular frequency

• Ø is the phase

Amplitude modulation uses variations in amplitude (Vmax) to convey information. The wave whose

amplitude is being varied is called the carrier wave. The signal doing the variation is called the

modulating signal. For simplicity, suppose both carrier wave and modulating signal are sinusoidal; i.e.,

vc = Vc sin ωc t (c denotes carrier)

and vm = Vm sin ωm t (m denotes modulation)

We want the modulating signal to vary the carrier amplitude, Vc , so that:

vc = (Vc + Vm sin ωmt).sin ωc t

where (Vc + Vm sin ωm t) is the new, varying carrier amplitude.

Expanding this equation gives:

vc = Vc sin ωc t + Vm sin ωc t. sin ωm t which may be rewritten as:


27

vc = Vc [sin ωc t + m sin ωc t. sin ωm t] where m = Vm/Vc and is called the modulation index.

Now:

sin ωc t.sin ωm t = (1/2) [cos(ωc - ωm) t - cos(ωc + ωm) t]

so, from the previous equation:

vc = Vc [sin ωc t + m sin ωc t. sin ωm t]

we can express vc as:

vc = Vc sin ωc t + (mVc/2) [cos(ωc - ωm) t] - (mVc/2) [cos(ωc + ωm) t]

This expression for vc has three terms:

1. The original carrier waveform, at frequency ωc, containing no variations and thus carrying no

information.

2. A component at frequency (ωc - ωm) whose amplitude is proportional to the modulation index. This is

called the Lower Side Frequency.

3. A component at frequency (ωc + ωm) whose amplitude is proportional to the modulation index. This is

called the Upper Side Frequency.

It is the upper and lower side frequencies which carry the information. This is shown by the fact that only

their terms include the modulation index m. Because of this, the amplitudes of the side frequencies vary in

proportion to that of the modulation signal.

Sidebands: If the modulating signal is a more complex waveform, for instance an audio voltage from a speech

amplifier, there will be many side frequencies present in the total waveform. This gives rise to

components 2 and 3 in the last equation being bands of frequencies, known as sidebands. Hence we have

the upper sideband and the lower sideband, together with the carrier. Clearly, for a given carrier

amplitude there are limits for the size of the modulating signal; the minimum must give zero carrier, the

maximum gives twice the unmodulated carrier amplitude. If these limits are exceeded, the modulated

signal cannot be recovered without distortion and the carrier is said to be over-modulated.

Figure 5.3 Frequency Spectrum of AM Signal


28

Experimental Determination of the Modulation Index This is most easily done by measuring the maximum and minimum values which the instantaneous

amplitude of the carrier reaches. Let us call these x and y. Taking our previous equation:

vc = Vc [sin ωc t + m sin ωct. sin ωm t]

and re-arranging it yet again, we can express vc as:

vc = Vc sin ωc t [1 + m sin ωm t]

so that the instantaneous amplitude of the carrier is:

Vc [1 + m sin ωm t]

Since sin wm t can vary between +1 and -1,

x = Vc (1 + m) and y = Vc (1 - m)

To get the value of modulation index m from x and y, we eliminate Vc between these

equations by division, giving:

y /x = (1 - m)/(1 + m).

Solving for m gives:

m = (x - y)/(x + y)

PROCEDURE (a):

In this practical the hardware is configured as shown in figure 5.4.

Figure 5.4 Experiment (a)

Set the carrier level to maximum. Set modulation level to zero. Observe the signals at all monitoring

points. Write your observations.

____________________________________________________________________________________

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29

____________________________________________________________________________________

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____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

Now increase the modulation level and observe at monitor point 6

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

Increase the modulation level until the carrier amplitude just reaches zero on negative modulation

peaks. This is 100% modulation. Observe the signals at all the monitoring points both with the

oscilloscope and the spectrum analyser at various modulation levels.

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

Also, with a fixed modulation level try adjusting the carrier level.

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

QUESTIONS (a):

1. The 'envelope' of the modulated carrier wave is a curve joining its peaks. The positive envelope,

joining the positive peaks, should follow the shape of the modulating signal in one polarity and the

negative envelope, joining the negative peaks, in the opposite polarity. What happens to the positive

and negative envelopes when over-modulation occurs?

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

2. How would you recognise over-modulation on the spectrum analyser display?

____________________________________________________________________________________

____________________________________________________________________________________


30

3. What is the amplitude of the two sidebands relative to the carrier expressed in dB for 50 percent

modulation with a sine wave?

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

PRE-LAB THEORY (b):

The purpose of any detector or demodulator is to recover the original modulating signal with the minimum of

distortion and interference. The simplest way of dealing with an AM signal is to use a simple half- wave

rectifier circuit. If the signal were simply passed through a diode to a resistive load, the output would be a

series of half-cycle pulses at carrier frequency. So the diode is followed by a filter, typically a capacitor and

resistor in parallel.

Figure 5.5 Envelope Detector Circuit

The capacitor is charged by the diode almost to the peak value of the carrier cycles and the output therefore

follows the envelope of the amplitude modulation. Hence the term “envelope detector”.

The time constant of the RC network is very important because if it is too short the output will contain a

large component at carrier frequency. However, if it is too long it will filter out a significant amount of the

required demodulated output.

In this practical the output of the AM generator that is fed to an envelope detector. The output can be

monitored and compared with the original modulation source. The time constant of the filter following the

detector can be adjusted. This filter is often called a post-detection filter. It also introduces a phase shift

between the original signal and the output.

PROCEDURE (b):

Power the module and connect it with the PC

Obtain an AM modulated signal from an AM modulator and apply it the input of the envelope

detector. Here the signal from the amplitude modulator from the AM Signal generator is demodulated

using an envelope detector.

Confirm that the modulated signal is the same.


31

Use the oscilloscope to monitor the detector output 16 and adjust the time constant. If it’s too less and

too large what will happen? Also state the reason.

_______________________________________________________________________________________

_______________________________________________________________________________________

____________________________________________________________________________________

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___________________________________________________________________________________

Figure 5.6 Experiment Setup (b)

Use the spectrum analyzer to observe the carrier spectral components. Record your observations:

___________________________________________________________________________________

___________________________________________________________________________________

___________________________________________________________________________________

Compare the original modulating signal with the detector output in both shape and phase at various

time constants using the oscilloscope. Record your observations:

___________________________________________________________________________________

___________________________________________________________________________________

___________________________________________________________________________________

___________________________________________________________________________________

___________________________________________________________________________________

___________________________________________________________________________________


32

QUESTIONS (b): 1. Is the phase shift caused by the post detection filter a lead or lag?

________________________________________________________________________________

________________________________________________________________________________

________________________________________________________________________________

2. Why do you think that the filter causes a phase shift?

________________________________________________________________________________

________________________________________________________________________________

________________________________________________________________________________

3. How does the ratio of modulating frequency to carrier frequency affect the design of the detector and

the post detection filter?

________________________________________________________________________________

________________________________________________________________________________

________________________________________________________________________________

4. What problems could be caused if the range of modulating frequencies was quite large?

________________________________________________________________________________

________________________________________________________________________________

________________________________________________________________________________

RESULTS AND WAVEFORMS/SPECTRUM:


33

LAB SESSION 06

LAB PROJECT (1) TASK ASSIGNED BY THE INSTRUCTOR: SUBMISSION DEADLINE: REMARKS BY THE STUDENT: REMARKS BY THE INSTRUCTOR: MARKS OBTAINED:


34

LAB SESSION 07

PRODUCT DEMODULATOR AND DSBSC OBJECTIVES:

(a) To observe the operation of Product detector

(b) To check the operation of the balanced amplitude modulator with suppressed carrier

EQUIPMENT REQUIRED: Amplitude Modulation Work board 53-130 which comprises the following blocks:

Signal Generation

Modulator

Filters

Demodulator

PRE-LAB THEORY (a): Product detector has certain advantages over the simple envelope detector but at the expense of some

complexity. It is not often used for Amplitude Modulation but is the only type of detector that will

demodulate the suppressed carrier amplitude. It is important to appreciate that a product detector will

demodulate all forms of AM.

Product Detector:

If the AM signal is mixed with (i.e., modulated by) a frequency equal to that of its carrier, the two

sidebands are mixed down to the original modulating frequency and the carrier appears as a dc level.

The mathematics of the process shows that this will only happen if the mixing frequency is equal not

only in frequency to that of the carrier, but also in phase; i.e., the two signals are synchronous. This is

why a product detector when used for AM is sometimes called a synchronous detector. For AM, the

effect is very similar to a full-wave rectifier rather than the half-wave of the envelope detector.

Figure 7.1 Product Detector Block Diagram


35

The output still needs a post-detection filter to remove the residual ripple, but this time the ripple is

at twice the carrier frequency and is therefore further away from the modulation and hence easier

to remove. In general terms the product detector gives less distortion, partly because it uses both positive

and negative peaks of the carrier.

Generating the Mixing Frequency:

This is produced by an oscillator which is usually referred to as a Beat Frequency Oscillator or

BFO. This is because it is not at the same frequency as the carrier the output of the product detector.

It works on a frequency equal to the difference between o f the two, which is called a beat frequency.

(You will be able to see this when you adjust the BFO for synchronism).

As previously described, it is vital that the BFO be synchronised to the carrier. In practice this is achieved

with a special recovery circuit but here for simplicity a sample of the carrier is fed directly to the BFO

and when the free running frequency of the BFO is near to that of the carrier it locks into synchronism.

PROCEDURE (a):

Follow required procedure to obtain product detector demodulating AM output.

The oscilloscope shows its input at monitor point 6, which is the output of the same modulator as

before.

Monitor the BFO output with the oscilloscope and use the BFO frequency control to lock it to

the carrier. This will be indicated by a stationary TRACE.

Figure 7.2 Experiment Setup (a)

Use the oscilloscope to look at the output of the detector before the filter and note the

frequency of the ripple compared with the carrier.

_______________________________________________________________________________

_______________________________________________________________________________

_________________________________________________________________________

Use the spectrum analyzer to confirm this. Examine the output of the filter and compare it with

the modulation source. Record your observations:

_______________________________________________________________________________


36

_______________________________________________________________________________

_________________________________________________________________________

Monitor the detector output before the filter with the oscilloscope, then unlock the BFO with

the BFO frequency control and observe the result. Repeat whilst observing the filtered

output. Record your observations:

_______________________________________________________________________________

_______________________________________________________________________________

_______________________________________________________________________________

_______________________________________________________________________________

______________________________________________________________________________

QUESTIONS (a): 1. Are the design considerations for a post-detection filter different from those for the envelope detector?

_____________________________________________________________________________________

_____________________________________________________________________________________

____________________________________________________________________________________

2. Examine the filtered output, using the spectrum analyser at large size, with the BFO synchronised. The

trace should show three points where the level is above the background ripple. What do they represent?

_____________________________________________________________________________________

_____________________________________________________________________________________

____________________________________________________________________________________

3. Again examine the filtered output, using the spectrum analyser at large size. Decrease the amplitude of the

modulation signal as far as possible without the instrument trigger failing. Then vary the BFO control. How

wide is the available range of beat frequency?

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

PRE-LAB THEORY (b):

Double sideband suppressed carrier modulation: In AM modulation, two-third of the transmitted power appears in the carrier which itself conveys no

information. The real information is contained within the sidebands. One way to overcome this

problem is simply to suppress the carrier. Since the carrier does not provide any useful information,

there is no reason why it has to be transmitted. By suppressing the carrier the resulting signal is

simply the upper and lower sidebands. Such a signal is referred to as a double-sideband

suppressed carrier (DSB-SC or DSB) signal. Double sideband suppressed carrier modulation is


37

simply a special case of AM with no carrier. A circuit called balanced modulator generates double

sideband suppressed carrier signals.

Figure 7.3 DSBSC Signal Generation

Figure 7.4 DSBSC Spectrum

In the theory for the Amplitude Modulation with Full Carrier assignment, it was established that the output

signal of the AM Modulator circuit is:

vc = Vc sin ωc t + (mVc/2) [cos(ωc - ωm) t] - (mVc/2) [cos(ωc + ωm) t]

In DSB suppressed carrier modulation, the carrier term Vc sin ωc t is suppressed, leaving just:

vc =(mVc/2) [cos(ωc - ωm) t] - (mVc/2) [cos(ωc + ωm) t] as the modulated signal.

The two cosine terms represent the lower and upper sidebands respectively.


38

Figure 7.5 a. information signal b. Carrier Signal c. DSBSC modulator output

Demodulating the DSB Signal: In order to change the sideband frequencies back to the original modulating frequency, a locally-generated

carrier frequency (from the BFO) is used to modulate the DSB signal. (Remember that modulation for the

purpose of frequency changing is traditionally called mixing).

Suppose that the BFO signal is: vo = Vo sin (ωo + ø)

The modulation process will produce a signal proportional to:

[Vo sin(ωo + ø)] (Vc/2) [cos(ωc - ωm) t - cos(ωc + ωm) t]

or to: 2 sin(ωo + ø) [cos(ωc - ωm) t - cos(ωc + ωm) t]

This can be divided into two terms:

2 sin(ωo + ø) cos(ωc - ωm) t ... (1) and: - 2 sin(ωo + ø) cos(ωc + ωm) t ... (2)

but as 2 sin A cos B = sin(A + B) + sin (A - B), the first term, (1), becomes:

(1) sin(ωo + ø + ωc - ωm) t + sin(ωo + ø - ωc + ωm) t

Since ωo is supposed to be equal to ωc, (ωo + ωc - ωm) will be a frequency roughly twice of that of the carrier.

This does not contribute to the desired signal. The rest of the expression, which does

contribute, is: sin(wo + ø - ωc + ωm) t

If ωo = ωc, then sin (ωo + ø - ωc + ωm) t becomes simply


39

sin(ø + ωm) t, which is the original modulating frequency. Similarly the other term, (2), makes

contribution: (2) - sin(ωo + ø - ωc - ωm) t

which, for ωo = ωc , becomes: sin(- ø + ωm) t

We now have two terms, or components of the output signal, each of the original modulating frequency.

However, there is a problem when we combine them. The two terms are: sin(ø + ωm) t and sin(- ø + ωm) t

If the phase ø is zero, the two terms become identical, so they combine to produce the signal: 2 sin ωm t

i.e. a signal at the original modulating frequency. Now suppose that the phase now changes through π/2

radians (90 degrees).

The two sinusoids would now be π radians (180 degrees) apart in phase and would cancel each other out.

We have assumed that ωo = ωc. If this were not true, the effect would be the same as if ø were continually

changing, making the two terms alternately reinforce and cancel each other.

This may be shown mathematically thus: sin(ø + ωm) t + sin(- ø + ωm) t = 2 sin ωm t cos ø

Since cos 0 = 1, the strongest output is obtained for ø = 0. With ø = π/2, cos ø = 0, so no output is obtained.

PROCEDURE (b):

Use the oscilloscope and spectrum analyser to examine the signals at monitor point 4 and monitor

point 5.

Figure 7.6 Experiment Setup

Set the carrier balance to mid-scale. Note that they are the same as for simple AM.

Now examine at monitor point 6 and observe the wave shape.

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________


40

Use the spectrum analyser to observe that there are two sidebands but no carrier. Record your

observations:

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

Adjust the carrier balance and observe the effect on carrier amplitude

___________________________________________________________________________________

___________________________________________________________________________________

___________________________________________________________________________________

Adjust modulation level and carrier level and observe the effects

_____________________________________________________________________________________

_____________________________________________________________________________________

_____________________________________________________________________________________

Monitor at monitor point 13 and adjust the BFO frequency for a stable trace, so that the

BFO is in phase with the original carrier. Observe the changes

__________________________________________________________________________________

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__________________________________________________________________________________

Unlock the BFO and observe the result

___________________________________________________________________________________

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___________________________________________________________________________________

QUESTIONS (b): 1. Why does AM have a low efficiency when the full carrier is transmitted?

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

2. How can you tell whether the modulator is balanced when using the oscilloscope? and when

using the spectrum analyser?

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________


41

3. Measure the carrier suppression ratio for the system in Practical 1 when set for maximum

modulation and minimum carrier amplitude.

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

4. Does the term ‘over modulation’ have any meaning in a DSB system?

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

RESULTS AND WAVEFORMS/SPECTRUM: (attach)


42

LAB SESSION 08

LAB PROJECT (2)

TASK ASSIGNED BY THE INSTRUCTOR: SUBMISSION DEADLINE: REMARKS BY THE STUDENT: REMARKS BY THE INSTRUCTOR: MARKS OBTAINED:


43

LAB SESSION 09

SINGLE-SIDE BAND SIGNAL GENERATION AND DETECTION

OBJECTIVES:

(a) To examine the main parameters of the single sideband modulation

(b) To check the use of filters to generate the SSB

(c) Demodulation of SSB signals using Product/ Synchronous detection

EQUIPMENT REQUIRED: Amplitude Modulation Work board 53-130 which comprises the following blocks:

Signal Generation

Modulator

Filters

Demodulator PRE-LAB THEORY (a): Single sideband modulation: A modulation technique in which only one sideband out of the two is transmitted is known as

Single Side band transmission. In double sideband transmission, the basic information is

transmitted twice once in each sideband. Therefore, transmitting both signals is redundant. The

information can be transmitted through one sideband by further suppressing the one sideband. The

generated signal is termed as single sideband suppressed carrier.

Figure 9.1 Generation of SSB signal


44

Generating SSB:

The generator in the practical is a balanced modulator, producing DSB, followed by a bandpass filter for

the required sideband. There are other methods but this filter method is the simplest to understand and is

in very common usage in communication systems. It may be necessary for the bandpass filter to have a

very good shape factor because, at normal carrier and audio frequencies, the upper and lower sidebands

are quite close in frequency.

Another consideration is that the sideband filter should offer significant attenuation to the carrier, so that

the balanced modulator need not be so accurately balanced. In practice the balanced modulator might

provide 30 db of carrier suppression and the filter a further 10db. The other sideband would normally be

about 30 to 40 db down on the wanted one. In order to achieve this, the SSB filter has several poles and

is, in most cases a ceramic filter or crystal filter. Various filters are commercially available with

different specifications depending on the application.

In the practical, a high modulating frequency is used, so one can see clearly the relationship between the

various frequency components. This means that the filter specification can be relaxed and here a single

tuned circuit is used. Separate filters are provided for upper and lower sidebands and the means is

provided to monitor the output of both.

Figure 9.2 Frequency Spectrum Representations

Upper or Lower Sideband? There is no reason why one sideband gives better results than the other, but general practice seems to

favour the upper sideband. One convention is that with carrier frequencies below 10 MHz the lower

sideband should be used, but this is not always the case. The result of this is that many pieces of

communication equipment have to be able to deal with both.


45

PROCEDURE (a):

Use the spectrum analyser and oscilloscope to observe at monitor point 6. Note that the signal is

DSB. Adjust the carrier balance as before.

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

Monitor at monitor point 8, and at monitor point 9, and note the observations.

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

Figure 9.3 Experiment Setup (a)

Use the oscilloscope to observe that the SSB output.

Use the spectrum analyser to note the sidebands. Write your observations.

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

QUESTIONS (a):

1. Why is the balance of the modulator less important in a filter method SSB generator than for a DSB

generator?

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________


46

2. How is the width of the SSB filter related to the maximum and minimum modulating frequencies?

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

PRE-LAB THEORY (b): Single sideband demodulation:

In the double sideband suppressed carrier practical, we saw how DSB is demodulated using the BFO to

reinsert the carrier. In the case of DSB, the BFO must be in phase with the original carrier or the process

will not work correctly. Since SSB is transmitted without a carrier it is not surprising that a similar

method is employed.

The main difference is that, for SSB, the BFO need not be in phase with the carrier. It does need to be

at the same frequency but even a small error in the frequency results only in a small error in the frequency

of the demodulated output. This means that in non-critical applications, such as speech, a small overall

frequency error does not make the system useless. The effect on speech is to raise or lower the tone of the

voice, which within limits does not reduce intelligibility.

The fact that the BFO need not be locked, greatly simplifies the design of the receiver, and makes SSB

one of the most powerful techniques for transmitting audio frequencies over radio links with its narrow

bandwidth and efficient use of available transmitter power.

In the practical, you can use both upper and lower sidebands and see that with the BFO set correctly, near

to the original carrier frequency, even though the two sidebands are at different frequencies the

demodulated output is the same. You can also see that changing the BFO frequency causes the

demodulated output to change in frequency by a similar amount.

PROCEDURE (b):

Figure 9.4 Experiment Setup (b)


47

Monitor at monitor point 6, and observe the DSB signal

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

Move to monitor point 10 and note the upper sideband signal

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

Use the spectrum analyser to confirm the frequency. What do you observe?

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

Change to lower sideband (by pressing the button) and repeat.

__________________________________________________________________________________

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__________________________________________________________________________________

Now monitor at point 14 and compare the output with the modulation input.

__________________________________________________________________________________

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__________________________________________________________________________________

Use either the oscilloscope or analyser to set the BFO frequency to that of the carrier, by

monitoring at monitor point 13

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__________________________________________________________________________________

__________________________________________________________________________________

QUESTIONS (b):

1. Why is SSB more efficient than either simple AM or DSB?

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

2. If the BFO frequency rises, what happens to the frequency: a) of the upper sideband? b) of the

lower sideband?

__________________________________________________________________________________

__________________________________________________________________________________


48

__________________________________________________________________________________

3. Calculate the bandwidth of the transmitted signal when the modulation frequency band extends

from 500 Hz to 50 kHz for simple AM, DSB and SSB.

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

4. If a SSB channel has no modulating signal, what is the modulated signal like?

___________________________________________________________________________________

___________________________________________________________________________________

________________________________________________________________________________

RESULTS AND WAVEFORMS:


49

LAB SESSION 10

RF TRANSMITTER AND RECEIVER OBJECTIVES:

(a) To examine the operation of RF(Radio Frequency) transmitter

(b) To examine the operation of RF (Radio Frequency) receiver

EQUIPMENT REQUIRED:

Power supply mod. PSI-PSU/ EV

Experiment Mod MCM24/EV

Dual-trace Oscilloscope

Function generator

PRE-LAB THEORY (a):

The purpose of the transmitter is to convert the information that is to be transmitted into modulated

radio frequency signal. Through a transmission line, this signal is applied to the antenna that

radiates the information is space as electromagnetic waves. Transmission can be obtained not only

via radio but also from copper & optical fiber.

In the past direct modulation was used which guarantee a circuit simplicity to the detriment of the

quality in the communication. Today modulation is achieved on a fixed and well stabilized frequency

(intermediate frequency) signal that translates at frequency of channel to be used & makes circuit

complexity balanced & easily controlled.

Figure 10.1 Basic Block diagram of RF Transmitter

Functions of the blocks of transmitter: The local oscillator provides the modulator with a signal whose frequency is stabilized through

a PLL circuit. The modulator changes the frequency spectrum of the signal provided by the local

oscillator, according to the kind of modulation used and the provided information and generates the IF


50

signal. The IF frequency is always equal and does not depend on the frequency of the RF channel that is

to be used. This enables the optimisation of the modulation and filtering circuits. The IF filter cleans the

useful signal from any inter modulation products or noise. As the IF is always the same the filter does

not need any regulation or calibration. The frequency converter has the purpose to translate the

frequency from IF to RF and so to the frequency of the channel that is to be used. The RF filter or

output filter, cleans the useful signal from the inter modulation products that was added during

frequency conversion. As the RF can be changed the filter must be calibrated again.

RF RECEIVER: The purpose of receiver is to convert the modulated radio frequency signal into information

that is to be received. Using an antenna RF signal is picked up by the space in which it travelled

electromagnetic wave and sent through a transmission line to the electronic circuit of the receiver

in order to be demodulated. The reception cannot be only radio but also from other supports like

copper & optical fiber.

In the past, direct modulation was used which guarantee a circuit simplicity to the detriment of

the quality in the communication. Today, demodulation is achieved on a fixed and well stabilised

frequency (intermediate frequency) signal that translates at frequency of channel to be used &

makes circuit complexity balanced & easily controlled.

Figure 10.2 RF Receiver Block Diagram

Operation of block of receiver (Super heterodyne receiver):

The filter and RF amplifier remove the channel we do not want to receive from the useful signal

and increases its amplitude level. As the RF signal can be different, the input filter must change its

characteristics. Typically this occurs automatically without the user intervention, by means of

D.C control circuits. The frequency converter translates the frequency from RF channel

frequency that is to be received to IF. It employs a frequency stabilised oscillator with a PLL

circuit. The filter and IF amplifier clean the useful signal from any inter-modulation products or

noise and increase it’s amplitude level. As the IF is always the same, the filter does not need

regulation or calibration and can be a commercial component optimised for this purpose. The

demodulator must receive or extract the information contained into the IF signal. The frequency

spectrum of the IF signal depends on the kind of modulation technique used and on the

information. The IF frequency is always equal and does not depend on the frequency of the RF

channel that is to be used. This optimises the modulation and filtering circuits.


51

PROCEDURE:

Following section is used for the converter 1: VCO1, 2: modulator, 3: IF filter

Set the SW1 on the modulation selectors section to AM/DSB/FM

Turn the trimmer Level completely Clockwise to obtain the maximum amplitude on the signal

VCO1 out provided by the local Oscillator VCO1

Set switch SW6 to DC to obtain the manual control of the local Oscillator frequency

Adjust trimmer DC source of VCO1 out to obtain a frequency of about 10750 kHz

Connect the input AM/DSB/MOD into a sine signal with amplitude of 1Vpp and frequency of

50 kHz using an external generator

Set switch SW4 to Mix out so that the signal of the local oscillator reaches the input

CARRIER IN of the mixer

Set switch SW3 to DSB: in this condition the mixer is perfectly balanced and does not show the

signal with higher frequency (carrier) across the output

Connect and set the oscilloscope as follows: a. Channel 1 to the input AM/DSB/MOD IN

b. Channel 2 to the output of the mixer (TP2)

Check that the signal across TP2 which is the product of the carrier and modulating

signal is of DSB shape

With the current setting of SW4, the signal produced by the mixer reaches the section IF filter

Set switch SW5 to QUARTZ so that the signal crosses the quartz band pass filter

Turn the trimmer LEVEL completely clockwise to obtain the maximum amplitude of the

signal across TP3 at the output of the section IF FILTER

Connect the oscilloscope to TP3 & check there is a sine signal. Adjust modulating

signal to display max amplitude

Connect the oscilloscope to TP3 and measure the frequency of the present signal which is equal

to 10.7MHz

Set switch SW5 to CERAMIC so that the signal crosses the ceramic band pass filter

Connect one probe of the Oscilloscope to TP3 of IF filter

Following section used for converter2: VCO1&2, modulator, IF filter, RF mixer, RF filter, RF

power amplifier

Remove external generator

Set the sw1 on the modulation selectors section to AM/DSB/FM



Set switch SW6 to PLL to obtain automatic control of the fixed local Oscillator frequency


52

Set switch SW4 to VCO1 out so that the signal of the local oscillator reaches the IF filter

Set switch SW5 to QUARTZ

Turn the trimmer LEVEL completely clockwise to obtain the maximum amplitude of the

signal across TP3

Connect the oscilloscope to TP3 and measure the frequency of the present signal which is equal

to 10.7MHz

Set switch SW7 to PLL to obtain automatic control of the fixed local Oscillator frequency.

With these setting choice of ceramic or quartz filter does not change received signal

substantially

Connect the oscilloscope to VCO2 OUT and measure the frequency which is equal to

11.7MHz

Set switch SW10 to LPF that corresponds to the low pass filter with cut-off frequency of

1.5MHz

Set switch SW11 to OFF in order to turn off antenna amplifier

Connect one probe of the oscilloscope to the out put CABLE OUT and check there is the

signal of frequency equal to 1MHz

Adjust the LEVEL of the section RF FILTER for the best display

Now Set switch SW7 to DC to obtain the manual control of the local Oscillator frequency

Connect oscilloscope to VCO2 out & adjust DC source to get freq of 12MHz. Check frequency

of output signal



53

LAB SESSION 11

LAB PROJECT (3)



54

LAB SESSION 12

AM-RF AND SSB-RF TRANSMITTER OBJECTIVES:

(a) To examine the operation of AM-RF (Radio Frequency) transmitter

(b) To examine the operation of SSB-RF (Radio Frequency) transmitter

EQUIPMENT REQUIRED:

Power supply mod. PSI-PSU/ EV

Experiment Mod MCM24/EV

1 Dual-trace Oscilloscope

1 Function generator

PRE-LAB THEORY (a): Amplitude Modulation: It is the simplest form of signal processing in which the carrier amplitude is simply changed

according to the amplitude of the information signal hence the name amplitude modulation.

When the information signals amplitude is increased the carrier signals amplitude is increased and

when the information signals amplitude is decreased the carrier signals amplitude is decreased .In other

words the ENVELOPE of the carrier signals amplitude contains the information signal.

Generation of Amplitude Modulation in RF TX:

In the module, BALUN (impedance match transformer) converts balanced signal at o/p into unbalance

signal or vice versa to match mixer operation. Mixer is calibrated to operate in balance mode. DSB/AM

enables to balance or unbalance the circuit. IF filter is used in particular as ceramic BPF is centred on

IF frequency equal to 10.7MHz. Here we use transmission channel with 1MHz frequency via PLL or

variable tuning. SW7 enables TWO modes: fixed tuning at RF channel at 1MHz using VCO, variable

tuning using DC Source. To filter output signal we use LPF of 1.5MHz. Amplifiers present in signal

path (buffer) are used to match output or input impedance of filters. RF power amplifier consist of 2

stages: antenna amplifier tuned on frequency of 1MHz to operate with ferrite antenna, cable amplifier

gives output via coaxial cable.


55

PROCEDURE (a):

Following sections are used: VCO1, Low frequency, and modulator blocks





Connect output OUT2 to input AM/DSB MOD IN

Set switch SW3 to AM to carryout AM modulator with unbalance mixer

Set switch SW4 to Mix out to take signal of the mixer & not the one of VCO

Set switch SW5 to CERAMIC so that the signal crosses the ceramic band pass filter

Connect and set the oscilloscope as follows:

Channel 1 to the input AM/DSB/MOD IN

Channel 2 to the output of the mixer (TP2)

Adjust the trimmer LEVEL of modulating signal for the best display

Vary the amplitude of the modulating signal and check the 3 following conditions: a. Modulation

percentage lower than the 100%, b. Equal to the 100%, c. Superior to 100% over modulation

Remove modulating signal & check presence of carrier signal.

Remove carrier signal & check complete absence of signal across output

Frequency Response of the Ceramic Filter:

Provide a signal of 1kHz-1 Vpp to modulating input

Move probe from TP2 to TP3 & see no changes are visible. This is because filter removes all

unnecessary components & displayed signal represent only single IF component.

Remove modulator & measure amplitude of IF signal

Connect modulator again & increase its frequency

AM Radio Transmitter:

Set switch SW10 to LPF to use output LPF

Set switch SW11 to TX ON to enable antenna power amplifier

Adjust the trimmer LEVEL of the section RF MIXER to maximum amplitude & check signal

after conversion made by RF MIXER at TP6

Observe signals at output via cable & antenna

At TP7 observe signal higher than power supply voltage due to effect of tuned circuits.

With sine signal of 1kHz if output is distorted then reduce level of IF signal to reduce saturation

of mixer


56

PRE-LAB THEORY (b): DSB to SSB (Double side Band to Single side band): The carrier does not carry any information as it has constant amplitude & frequency independently

from modu la t ing s i gna l . The s igna l is called s uppressed carrier modulation or DSB modulation.

The two side bands are exactly same. It follows that information can be transmitted using single side

band: carrier is superfluous & the other sideband redundant. We can generate SSB by using filters.

First amplitude modulation with suppressed carrier DSB is generated using balanced modulator then a

BPF extracts one of two side bands.

Transmit SSB signal it is necessary to convert SSB signal with IF on RF channel suppression is made

through quartz filter. This becomes IF signal that must be converted into RF using conversion stage. To

filter o/p signal from RF mixer & take single component use RF BPF with center frequency 3.5MHz.

Amplifiers present in signal path (buffer) are used to match output or input impedance of filters. RF

power amplifier consist of 2 stages: antenna amplifier tuned on freq of 1MHz to operate with ferrite

antenna, cable amplifier gives o/p via coaxial cable. Here o/p via cable is used as it is wide band.

PROCEDURE:





Connect o/p OUT2 to i/p AM/DSB MOD IN

Set switch SW3 to DSB to carryout AM modulator with unbalance mixer

Set switch SW4 to Mix out to take signal of the mixer & not the one of VCO.1

Connect and set the oscilloscope as follows: a. Channel 1 to the input AM/DSB/MOD IN

b.Channel 2 to the output of the mixer (TP2)

See waveforms of the signals. Adjust the trimmer LEVEL of modulating signal for the best

display.

Single Side Band Generation (SSB):

Set switch SW1 to SSB & SW2 to LSB

Set switch SW5 to QUARTZ so that the signal crosses the band pass filter

Connect o/p OUT3 to i/p AM/DSB MOD IN

Connect and set the oscilloscope as follows: a. Channel 1 to the input of modulating signal

AM/DSB/MOD IN b. Channel 2 to the output of the mixer (TP2)

Move probe from TP2 to TP3 & check the presence of sine signal: we can state filter extracts

only one of two components generated by balanced modulator so there is a SSB signal across this


57

TP

Measure the following IF frequencies: a. frequency of carrier (VCO1 OUT) b. frequency of

modulating signal (OUT3) c. frequency of SSB signal across filter output (TP3)

Single Side Band Radio Transmitter (SSB):

Analyze signal during path from IF freq up to RF frequency o/p on CABLE OUT

Set switch SW1 to SSB & SW2 to USB

Check freq relation b/w RF, IF, modulating signal & carrier

Check signal transmitted via CABLE

Compare operation of TX via cable between SSB & AM

Frequency Response to Quartz IF Filter:

Set switch SW1 to SSB & SW2 to LSB

Set switch SW5 to QUARTZ to use quartz band pass filter

Provide a signal of 2kHz-1Vpp to modulating input

The frequency of IF signal in these conditions is equal to centre band frequency of quartz filter

i.e. 10.7 MHz



58

LAB SESSION 13

LAB PROJECT (4)


communicatin system 1 lab manual 2011

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