Comm lab manual_final

Experiment No.1
AMPLITUDE SHIFT KEYING
Aim: To generate and demodulate amplitude shift keyed (ASK) signal using MATLAB
Theory
Generation of ASK
Amplitude shift keying - ASK - is a modulation process, which imparts to a sinusoid two or more
discrete amplitude levels. These are related to the number of levels adopted by the digital message.
For a binary message sequence there are two levels, one of which is typically zero. The data rate is
a sub-multiple of the carrier frequency. Thus the modulated waveform consists of bursts of a
sinusoid. One of the disadvantages of ASK, compared with FSK and PSK, for example, is that it
has not got a constant envelope. This makes its processing (eg, power amplification) more
difficult, since linearity becomes an important factor. However, it does make for ease of
demodulation with an envelope detector.
Demodulation
ASK signal has a well defined envelope. Thus it is amenable to demodulation by an envelope
detector. Some sort of decision-making circuitry is necessary for detecting the message. The signal
is recovered by using a correlator and decision making circuitry is used to recover the binary
sequence.
Algorithm
Initialization commands
ASK modulation
1. Generate carrier signal.
2. Start FOR loop
3. Generate binary data, message signal(on-off form)
4. Generate ASK modulated signal.
5. Plot message signal and ASK modulated signal.
6. End FOR loop.
7. Plot the binary data and carrier.
ASK demodulation
1. Start FOR loop
2. Perform correlation of ASK signal with carrier to get decision variable
3. Make decision to get demodulated binary data. If x>0, choose ‘1’ else choose ‘0’
4. Plot the demodulated binary data.
1

Program
%ASK Modulation
clc;
clear all;
close all;
%GENERATE CARRIER SIGNAL
Tb=1; fc=10;
t=0:Tb/100:1;
c=sqrt(2/Tb)*sin(2*pi*fc*t);
%generate message signal
N=8;
m=rand(1,N);
t1=0;t2=Tb
for i=1:N
t=[t1:.01:t2]
if m(i)>0.5
m(i)=1;
m_s=ones(1,length(t));
else
m(i)=0;
m_s=zeros(1,length(t));
end
message(i,:)=m_s;
%product of carrier and message
ask_sig(i,:)=c.*m_s;
t1=t1+(Tb+.01);
t2=t2+(Tb+.01);
%plot the message and ASK signal
subplot(5,1,2);axis([0 N -2 2]);plot(t,message(i,:),'r');
title('message signal');xlabel('t--->');ylabel('m(t)');grid on
hold on
subplot(5,1,4);plot(t,ask_sig(i,:));
title('ASK signal');xlabel('t--->');ylabel('s(t)');grid on
hold on
end
hold off
%Plot the carrier signal and input binary data
subplot(5,1,3);plot(t,c);
title('carrier signal');xlabel('t--->');ylabel('c(t)');grid on
subplot(5,1,1);stem(m);
title('binary data bits');xlabel('n--->');ylabel('b(n)');grid on
2

% ASK Demodulation
t1=0;t2=Tb
for i=1:N
t=[t1:Tb/100:t2]
%correlator
x=sum(c.*ask_sig(i,:));
%decision device
if x>0
demod(i)=1;
else
demod(i)=0;
end
t1=t1+(Tb+.01);
t2=t2+(Tb+.01);
end
%plot demodulated binary data bits
subplot(5,1,5);stem(demod);
title('ASK demodulated signal'); xlabel('n--->');ylabel('b(n)');grid on
3

Model Graphs
Result
The program for ASK modulation and demodulation has been simulated in MATLAB and
necessary graphs are plotted.
4

Experiment No.2
PHASE SHIFT KEYING
Aim: To generate and demodulate phase shift keyed (PSK) signal using MATLAB
Generation of PSK signal
PSK is a digital modulation scheme that conveys data by changing, or modulating, the phase of a
reference signal (the carrier wave). PSK uses a finite number of phases, each assigned a unique
pattern of binary digits. Usually, each phase encodes an equal number of bits. Each pattern of bits
forms the symbol that is represented by the particular phase. The demodulator, which is designed
specifically for the symbol-set used by the modulator, determines the phase of the received signal
and maps it back to the symbol it represents, thus recovering the original data.
In a coherent binary PSK system, the pair of signal S1(t) and S2 (t) used to represent binary
symbols 1 & 0 are defined by
S1 (t) = √2Eb/ Tb Cos 2πfct
S2 (t) =√2Eb/Tb (2πfct+π) = - √ 2Eb/Tb Cos 2πfct where 0 ≤ t< Tb and
Eb = Transmitted signed energy for bit
The carrier frequency fc =n/Tb for some fixed integer n.
Antipodal Signal:
The pair of sinusoidal waves that differ only in a relative phase shift of 180° are called antipodal
signals.
BPSK Transmitter
Binary Wave Product BPSK signal
(Polar form)
Product
Modulator
c1 (t) = √2/Tb cos 2πfct
5

The input binary symbols are represented in polar form with symbols 1 & 0 represented by
constant amplitude levels √Eb & -√Eb. This binary wave is multiplied by a sinusoidal carrier in a
product modulator. The result in a BSPK signal.
BSPK Receiver:
PSK signal x Choose ‘1’ if x > 0
Choose ‘0’ if x < 0
c1 (t).
X ∫ dt
Decision
device
The received BPSK signal is applied to a correlator which is also supplied with a locally generated
reference signal c1 (t). The correlated o/p is compared with a threshold of zero volts. If x> 0, the
receiver decides in favour of symbol 1. If x< 0, it decides in favour of symbol 0.
Algorithm
PSK modulation
1. Generate carrier signal.
2. Start FOR loop
3. Generate binary data, message signal in polar form
4. Generate PSK modulated signal.
5. Plot message signal and PSK modulated signal.
6. End FOR loop.
7. Plot the binary data and carrier.
PSK demodulation
1. Start FOR loop
Perform correlation of PSK signal with carrier to get decision variable
2. Make decision to get demodulated binary data. If x>0, choose ‘1’ else choose ‘0’
6

Program
% PSK modulation
clc;
clear all;
close all;
Tb=1;
t=0:Tb/100:Tb;
fc=2;
c=sqrt(2/Tb)*sin(2*pi*fc*t);
N=8;
m=rand(1,N);
t1=0;t2=Tb
for i=1:N
t=[t1:.01:t2]
if m(i)>0.5
m(i)=1;
else
m(i)=0;
m_s=-1*ones(1,length(t));
end
message(i,:)=m_s;
%product of carrier and message signal
bpsk_sig(i,:)=c.*m_s;
%Plot the message and BPSK modulated signal
title('message signal(POLAR form)');xlabel('t--->');ylabel('m(t)');
grid on; hold on;
subplot(5,1,4);plot(t,bpsk_sig(i,:));
title('BPSK signal');xlabel('t--->');ylabel('s(t)');
grid on; hold on;
t1=t1+1.01; t2=t2+1.01;
end
hold off
%plot the input binary data and carrier signal
title('binary data bits');xlabel('n--->');ylabel('b(n)');
grid on;
subplot(5,1,3);plot(t,c);
title('carrier signal');xlabel('t--->');ylabel('c(t)');
grid on;
7

% PSK Demodulation
t1=0;t2=Tb
for i=1:N
t=[t1:.01:t2]
%correlator
x=sum(c.*bpsk_sig(i,:));
%decision device
if x>0
demod(i)=1;
else
demod(i)=0;
end
t1=t1+1.01;
t2=t2+1.01;
end
%plot the demodulated data bits
title('demodulated data');xlabel('n--->');ylabel('b(n)');
grid on
8

Modal Graphs
Result
The program for PSK modulation and demodulation has been simulated in MATLAB and
9

Experiment No.3
FREQUENCY SHIFT KEYING
Aim: To generate and demodulate frequency shift keyed (FSK) signal using MATLAB
Theory
Generation of FSK
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is
transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK
(BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s) information. With
this scheme, the "1" is called the mark frequency and the "0" is called the space frequency.
In binary FSK system, symbol 1 & 0 are distinguished from each other by transmitting one of the
two sinusoidal waves that differ in frequency by a fixed amount.
Si (t) = √2E/Tb cos 2πf1t 0≤ t ≤Tb
0 elsewhere
Where i=1, 2 & Eb=Transmitted energy/bit
Transmitted freq= ƒi = (nc+i)/Tb, and n = constant (integer), Tb = bit interval
Symbol 1 is represented by S1 (t)
Symbol 0 is represented by S0 (t)
BFSK Transmitter
Binary wave c1 (t) = √2/Tb cos 2πƒ1t +
(On-Off signaling
Form)
10
Inverter + FSK signal
c2 (t) = √2/Tb cos 2πƒ2t
X
Σ
X
The input binary sequence is represented in its ON-OFF form, with symbol 1
represented by constant amplitude of √Eb with & symbol 0 represented by zero volts.
By using inverter in the lower channel, we in effect make sure that when symbol 1is
at the input, The two frequency f1& f2 are chosen to be equal integer multiples of the
bit rate 1/Tb.By summing the upper & lower channel outputs, we get BFSK signal.

BFSK Receiver x1
Tbƒdt
0 + x = x1-x2
c1 (t) L
FSK signal -
Tbƒdt choose ‘1’ if x >0
c2 (t) x2 choose ‘0’ if x < 0
X
X
E Decision
Device
The receiver consists of two correlators with common inputs which are supplied with
locally generated coherent reference signals c1(t) and c2 (t).
The correlator outputs are then subtracted one from the other, and the resulting difference
x is compared with a threshold of zero volts. If x >0, the receiver decides in favour of
symbol 1 and if x <0, the receiver decides in favour of symbol 0.
Algorithm
FSK modulation
1. Generate two carriers signal.
2. Start FOR loop
3. Generate binary data, message signal and inverted message signal
4. Multiply carrier 1 with message signal and carrier 2 with inverted message signal
5. Perform addition to get the FSK modulated signal
6. Plot message signal and FSK modulated signal.
7. End FOR loop.
8. Plot the binary data and carriers.
FSK demodulation
1. Start FOR loop
2. Perform correlation of FSK modulated signal with carrier 1 and carrier 2 to get two decision
variables x1 and x2.
3. Make decisionon x = x1-x2 to get demodulated binary data. If x>0, choose ‘1’ else choose ‘0’.
11

12
Program
% FSK Modulation
clc;
clear all;
close all;
Tb=1; fc1=2;fc2=5;
t=0:(Tb/100):Tb;
c1=sqrt(2/Tb)*sin(2*pi*fc1*t);
c2=sqrt(2/Tb)*sin(2*pi*fc2*t);
N=8;
m=rand(1,N);
t1=0;t2=Tb
for i=1:N
t=[t1:(Tb/100):t2]
if m(i)>0.5
m(i)=1;
invm_s=zeros(1,length(t));
else
m(i)=0;
m_s=zeros(1,length(t));
invm_s=ones(1,length(t));
end
message(i,:)=m_s;
%Multiplier
fsk_sig1(i,:)=c1.*m_s;
fsk_sig2(i,:)=c2.*invm_s;
fsk=fsk_sig1+fsk_sig2;
%plotting the message signal and the modulated signal
title('message signal');xlabel('t---->');ylabel('m(t)');grid on;hold on;
subplot(3,2,5);plot(t,fsk(i,:));
title('FSK signal');xlabel('t---->');ylabel('s(t)');grid on;hold on;
t1=t1+(Tb+.01); t2=t2+(Tb+.01);
end
hold off
%Plotting binary data bits and carrier signal
title('binary data');xlabel('n---->'); ylabel('b(n)');grid on;
subplot(3,2,3);plot(t,c1);
title('carrier signal-1');xlabel('t---->');ylabel('c1(t)');grid on;

13
% FSK Demodulation
t1=0;t2=Tb
for i=1:N
t=[t1:(Tb/100):t2]
%correlator
x1=sum(c1.*fsk_sig1(i,:));
x2=sum(c2.*fsk_sig2(i,:));
x=x1-x2;
%decision device
if x>0
demod(i)=1;
else
demod(i)=0;
end
t1=t1+(Tb+.01);
t2=t2+(Tb+.01);
end
%Plotting the demodulated data bits
title(' demodulated data');xlabel('n---->');ylabel('b(n)'); grid on;

Modal Graphs
Result
The program for FSK modulation and demodulation has been simulated in MATLAB and
14

15
Experiment No.4
QUADRATURE PHASE SHIFT KEYING
Aim: To generate and demodulate quadrature phase shifted (QPSK) signal using MATLAB
Theory
Generation of Quadrature phase shift keyed (QPSK) signal
QPSK is also known as quaternary PSK, quadriphase PSK, 4-PSK, or 4-QAM. It is a phase
modulation technique that transmits two bits in four modulation states.
Phase of the carrier takes on one of four equally spaced values such as π/4, 3π/4, 5π/4
and7π/4.
Si(t) = √2E/T cos {2 πƒct + (2i – 1) π/4} , 0≤ t ≤T
0 , elsewhere
Where i = 1,2,3,4, & E= Tx signal energy per symbol
T= symbol duration
Each of the possible value of phase corresponds to a pair of bits called dibits.
Thus the gray encoded set of dibits: 10,00,01,11
Si (t) = √2E/Tcos [(2i – 1)π/4] cos (2πfct) - √2E/Tsin [(2i –1) π/4)] sin (2πfct) ,0≤ t ≤Tb
0 , else where
There are two orthononormal basis functions
c1 (t) = √2/T cos 2πƒct, 0≤ t ≤Tb
c2 (t) = √2/T sin 2πƒct, 0≤ t ≤Tb
There are four message points
Input debits Phase of
QPSK
signal
Co-ordinates of message
signals
S1 S2
10 π/4 √E/2 -√E/2
00 3π/4 -√E/2 -√E/2
01 5π/4 -√E/2 +√E/2
11 7π/4 +√E/2 +√E/2

Block diagram of QPSK Transmitter
b1(t)
c1 (t) + QPSK signal
binary b (t)
bits
+
b2 (t) c2 (t)
16
The I/p binary sequence b(t) is represented in polar from with symbols 1 & 0 represented as
+√E/2 and -√E/2. This binary wave is demutiplexed into two separate binary waves
consisting of odd & even numbered I/P bits denoted by b1 (t) & b2 (t). b1 (t) & b2(t) are
used to modulate a pair of quadrature carrier. The result is two PSK waves .These two binary
PSK waves are added to produce the desired QPSK signal
QPSK Receiver:
x1
c1 (t)
QPSK signal
Binary o/p
x2
c2 (t).
∑Demux
X
X
ƒdt Decision
Device
ƒdt
Multiplexer
X
X
Decision
Device
QPSK receiver consists of a pair of correlators with common I/P & supplied with locally
generated signal c1 (t) & c2 (t). The correlator output, x1, & x2 are each compared with a
threshold of zero volts.If x1 > 0, decision is made in favour of symbol ‘1’ for upper channel
and if x1 > 0, decision is made in favour of symbol 0. Parallely if x2 >0, decision is made in
favour of symbol 1 for lower channel & if x2 <0, decision is made in favour of symbol 0.
These two channels are combined in a multiplexer to get the original binary output.

17
Algorithm
QPSK modulation
1. Generate quadrature carriers.
2. Start FOR loop
3. Generate binary data, message signal(bipolar form)
4. Multiply carrier 1 with odd bits of message signal and carrier 2 with even bits of message
signal
5. Perform addition of odd and even modulated signals to get the QPSK modulated signal
6. Plot QPSK modulated signal.
7. End FOR loop.
8. Plot the binary data and carriers.
QPSK demodulation
1. Start FOR loop
2. Perform correlation of QPSK modulated signal with quadrature carriers to get two decision
variables x1 and x2.
3. Make decision on x1 and x2 and multiplex to get demodulated binary data.
If x1>0and x2>0, choose ‘11’. If x1>0and x2<0, choose ‘10’. If x1<0and x2>0, choose ‘01. If
x1<0and x2<0, choose ‘00’.
4. End FOR loop
5. Plot demodulated data
Program
% QPSK Modulation
clc;
clear all;
close all;
%GENERATE QUADRATURE CARRIER SIGNAL
Tb=1;t=0:(Tb/100):Tb;fc=1;
c1=sqrt(2/Tb)*cos(2*pi*fc*t);
c2=sqrt(2/Tb)*sin(2*pi*fc*t);
N=8;m=rand(1,N);
t1=0;t2=Tb
for i=1:2:(N-1)
t=[t1:(Tb/100):t2]
if m(i)>0.5
m(i)=1;
else
m(i)=0;
end
%odd bits modulated signal
odd_sig(i,:)=c1.*m_s;
if m(i+1)>0.5

18
m(i+1)=1;
else
m(i+1)=0;
end
%even bits modulated signal
even_sig(i,:)=c2.*m_s;
%qpsk signal
qpsk=odd_sig+even_sig;
%Plot the QPSK modulated signal
subplot(3,2,4);plot(t,qpsk(i,:));
title('QPSK signal');xlabel('t---->');ylabel('s(t)');grid on; hold on;
t1=t1+(Tb+.01); t2=t2+(Tb+.01);
end
hold off
%Plot the binary data bits and carrier signal
title('binary data bits');xlabel('n---->');ylabel('b(n)');grid on;
% QPSK Demodulation
t1=0;t2=Tb
for i=1:N-1
t=[t1:(Tb/100):t2]
%correlator
x1=sum(c1.*qpsk(i,:));
x2=sum(c2.*qpsk(i,:));
%decision device
if (x1>0&&x2>0)
demod(i)=1;
demod(i+1)=1;
elseif (x1>0&&x2<0)
demod(i)=1;
demod(i+1)=0;
elseif (x1<0&&x2<0)
demod(i)=0;
demod(i+1)=0;
elseif (x1<0&&x2>0)
demod(i)=0;
demod(i+1)=1;
end
t1=t1+(Tb+.01); t2=t2+(Tb+.01);
end
title('qpsk demodulated bits');xlabel('n---->');ylabel('b(n)');grid on;

Modal Graphs
Result
The program for QPSK modulation and demodulation has been simulated in MATLAB and
19

20
Experiment No.5
BER Simulation of DPSK modulation
Aim: To simulate bit error rate performance of DPSK modulation using Matlab.
Theory
DPSK involves 2 basic operations at the transmitter, differential encoding of the i/p binary wave
and phase shift keying, hence the name DPSK. To send symbol 0 we phase advance the current
signal waveform by 1800 and to send symbol 1 we leave the phase of the current signal
unchanged.
In the differential encoding at the transmitter input starts with an arbitrary first bit serving as
reference and thereafter the sequence is generated using
dk-1' previous value of differentially encoded digit.
bk' i/p binary digit at time kTb.
dk-1 ,bk' logical inversion.
Assuming reference bit added to {dk} is a'1'. {dk} is thus generated and used to phase shift key a
carrier with phase angles 0 and ?.
BER -Bit Error Rate
In digital transmission, the number of bit errors is the number of received bits of a data stream over
a communication channel that have been altered due to noise, interference, distortion or bit
synchronization errors.The bit error rate or bit error ratio (BER) is the number of bit errors divided
by the total number of transferred bits during a studied time interval. BER is a unitless
performance measure, often expressed as a percentage.In a communication system, the receiver
side BER may be affected by transmission channel noise, interference, distortion, bit
synchronization problems, attenuation, wireless multipath fading, etc.
The BER may be analyzed using stochastic computer simulations. If a simple
transmission channel model and data source model is assumed, the BER may also be calculated
using Binary symmetric channel (used in analysis of decoding error probability in case of non-
bursty bit errors on the transmission channel) and Additive white gaussian noise (AWGN) channel
without fading.

21
Algorithm
1. Generate the input data randomly
2. Implement differential encoding
3. Do BPSK modulation
4. Add AWGN noise
5. Calculate the no of bits in error
6. Plot the BER graph
Program
N = 10^4 % number of bits or symbols
rand('state',100); % initializing the rand() function
randn('state',200);% initializing the randn() function
ip = rand(1,N)>0.5;% generating 0,1 with equal probability
ipD = mod(filter(1,[1 -1],ip),2); % %differential encoding y[n]=y[n-1]+x[n]
s = 2*ipD-1; % BPSK modulation 0 -> -1; 1 -> 0
n = 1/sqrt(2)*[randn(1,N) + j*randn(1,N)]; % white gaussian noise, 0dB variance
Eb_N0_dB = [-3:10]; % multiple Eb/N0 values
for ii = 1:length(Eb_N0_dB)
y = s + 10^(-Eb_N0_dB(ii)/20)*n; % additive white gaussian noise
ipDHat_coh = real(y) > 0; % coherent demodulation
ipHat_coh = mod(filter([1 -1],1,ipDHat_coh),2); %differential decoding
nErr_dbpsk_coh(ii) = size(find([ip - ipHat_coh]),2); % counting the number of errors
end
simBer_dbpsk_coh = nErr_dbpsk_coh/N;
theoryBer_dbpsk_coh = erfc(sqrt(10.^(Eb_N0_dB/10))).*(1 - .5*erfc(sqrt(10.^(Eb_N0_dB/10))));
close all
figure
semilogy(Eb_N0_dB,theoryBer_dbpsk_coh,'b.-');
hold on
semilogy(Eb_N0_dB,simBer_dbpsk_coh,'mx-');
axis([-2 10 10^-6 0.5])
grid on
legend('theory', 'simulation');
xlabel('Eb/No, dB')
ylabel('Bit Error Rate')
title('Bit error probability curve for coherent demodulation of DBPSK')
Modal Graphs

Result
The Bit Error rate simulation of DPSK modulation was done using Matlab.
Experiment No.6
PRE-EMPHASIS AND DE-EMPHASIS
Aim: To study the characteristics of pre-emphasis and de-emphasis circuits using PSPICE
Theory
In telecommunication, a pre-emphasis circuit is inserted in a system in order to increase the
magnitude of one range of frequencies with respect to another. Pre-emphasis is usually
employed in FM or phase modulation transmitters to equalize the modulating signal drive
power in terms of deviation ratio. In high speed digital transmission, pre-emphasis is used to
improve signal quality at the output of a data transmission. In transmitting signals at high
data rates, the transmission medium may introduce distortions, so pre-emphasis is used to
distort the transmitted signal to correct for this distortion. When done properly this produces
a received signal which more closely resembles the original or desired signal, allowing the
use of higher frequencies or producing fewer bit errors.
In telecommunication, de-emphasis is the complement of pre-emphasis. It is designed to decrease,
(within a band of frequencies), the magnitude of some (usually higher) frequencies with respect to
the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise
ratio by minimizing the adverse effects of such phenomena as attenuation differences
Circuit Diagram
PRE-EMPHASIS
22

DE-EMPHASIS
Program
PRE-EMPHASIS:
Vcc 1 0 dc 10V
P1 1 3 100k
L1 1 5 1H
P3 5 2 10k
P2 3 0 68k
P4 4 0 1k
P5 7 0 1.5k
C1 3 6 0.01µf
C2 2 7 0.01µf
V1 6 0 ac 100mv sin ( 0 100mv 1KHz)
q2 2 3 4 q2N2222
. lib
. AC DEC 10 1oHz 20KHz
. probe
. end
DE-EMPHASIS;
V1 1 0 ac 1V sin ( 0 1V 1KHz)
P1 1 2 10K
23

C1 2 0 0.01µf
C2 2 0 0.01µf
. lib
. AC DEC 10 10Hz 20KHz
. probe
. end
Modal Graphs
24

Result
The characteristics of pre –emphasis and de emphasis circuits were studied
Experiment No.7
ASK DEMODULATION USING PRODUCT DETECTION
Aim: To demodulate ASK signal through product detection technique using the Communication
Trainer Kit.
Theory
Amplitude shift keying is one of several digital modulation techniques used for frequency
division multiplexing of communications channel. Despite its grand name, ASK is basically
amplitude modulation (AM) with digital data for the message instead of speech and/or music.
The product detector is a method of demodulating AM and ASK signals .The below figure
shows the product detector used as an ASK demodulator.
The incoming ASK signal is one input to a functional block called a multiplier. The Multipliers
other input is a pure sine wave (called the local carrier) that is generated by the receiver and must
be the same frequency as the ASK signal’s carrier for the demodulation to be successful.
Mathematically, the multiplication of the multiplier’s inputs can be described as:
Product detector’s output = the ASK signal x the local carrier.
Equipment Required
• Emona Telecoms-Trainer 101 (plus power-pack)
25

• Dual-channel 20MHZ oscilloscope
• Three Emona Telecoms-Trainer 101 oscilloscope leads
• Assorted Emona Telecoms-Trainer 101 patch leads
PROCEDURE:
Generating ASK signal
1. Connect the set-up shown in the Figure 1 below.
Note: Insert the black plugs of the oscilloscope leads into a ground (GND) socket.
Figure 1
This set-up can be represented by the block diagram as in the Figure 2. The sequence
Generator module is used to model a digital signal and its SYNC output is used to
trigger the scope to provide a stable display. The Dual Analog switch module is used to
generate the ASK signal with a 100 kHz carrier.
26

Figure 2
2. Set up the scope.
3. Adjust the following scope controls as indicated:
• Vertical Attenuation control for channel 1 to 2V/div.
• Input Coupling controls for both channels to DC.
• Time base control to the 1ms/div position.
• Trigger Source control to the EXT position.
4. Set the scope’s module control to the DUAL position to observe the signal on the Dual
Analog switch module’s output as well as the digital data message.
Demodulating ASK signal using Product Detection
1. Locate the Tunable Low-pass filter module and turn its gain control fully clockwise.
2. Turn the tunable Low-pass Filter module’s Cut-off Frequency Adjust control fully
clockwise.
3. Modify the set-up as shown in the Figure3.
27

Figure 3
The ASK generation and demodulation parts of the set-up is represented in the Figure 4
The Multiplier and the Tunable Low-pass Filter modules are used to implement a product
detector. The product detector’s local carrier’s is “stolen” from the modulator to ensure
carrier synchronization between the modulator and demodulator.
Figure 4
28

29
4. Compare the original and recovered digital signals.
Result
The ASK signal was demodulated through product detection technique using the
Communication Trainer Kit.
Experiment No.8
FSK DEMODULATION USING ENVELOPE DETECTOR
Aim: To demodulate FSK signal through Envelope detection technique using the Communication
Trainer Kit
Theory
Frequency shift keying (FSK) is the frequency modulation (FM) scheme with a digital data signal
for the message instead of speech and/or music. As such, the advantage of FSK over ASK is the
same as the advantage of FM over AM – noise immunity.
FM/FSK resists noise problem (though is not completely immune from it) because
demodulation is designed to respond to the signal’s frequency variations. That being the case, any
variations in the signal’s amplitude due to noise can be removed by a limiter.

Figure 1
Notice that the FSK signal switches between two sine waves. The sine wave that corresponds
with logic-0s in the digital data is called the space frequency. The sine wave that corresponds
with logic-1s in the digital data is called mark frequency. In the above figure, the mark
frequency is higher than the space frequency but this is not a necessary requirement of FSK –
the relative frequency can be reversed.
Equipment Required
• Three Emona Telecoms-Trainer 101 oscilloscope leads
PROCEDURE:
Generating FSK signal
1. Set the scope’s channel 1 Input Coupling control to the GND position.
2. Use the scope’s Channel 1 Vertical Position control to move the trace so that it lines up
with the horizontal line in the middle of the scope’s screen.
30

3. Set the scope’s Input coupling control for both channels to the DC position.
4. Connect the set-up shown in the below Figure 2.
Note: Insert the black plugs of the oscilloscope leads into a ground(GND) socket.
Figure 2
5. While watching the Variable DCV module’s output on the scope, turn its VDC control
until the DC level is 2V.
6. Locate the VCO module and turn its Gain control fully anti-clockwise.
7. Turn the VCO module’s Frequency Adjust control fully anti-clockwise.
8. Set the VCO module’s range control to the LO position.
9. Disconnect the scope from the Variable DCV module and connect the set-up shown in
the below Figure 3.
31

Figure 3
10. Adjust the VCO module’s Frequency Adjust to obtain a 4 kHz sine wave.
Note: Once done, do not change the VCO module’s Frequency Adjust control unless
otherwise instructed.
11. Connect the set-up as shown in the Figure 4.
32

Figure 4
This set-up can be represented by the block diagram shown in Figure 5. The sequence Generator
module is used to module a digital data signal and its SYNC output is used to trigger the scope to
provide a stable display. The Comparator on the Utilities module, together with a DC voltage from
the Variable DCV module, is used to invert the digital data signal.
33

Figure 5
12. Set the scope’s Mode control to the DUAL position to view the digital data signal and the
comparators output.
13. Adjust the following scope controls as indicated:
• Vertical Attenuation for both channels to 2V/div.
• Time base to 1ms/div.
• Trigger Source to EXT.
14. Compare the signals.
Demodulating FSK signal using Envelope Detection
As FSK is really just FM (with a digital message instead of speech and/or music), it can be
recovered using any of the FM demodulation schemes. However, because the FSK signal switches
back and forth between just two frequencies, we can use a method of demodulating it that cannot
be used to demodulate speech-encoded FM signals.
34
1. Locate the Tunable Low-pass Filter module and turn its Cut-off Frequency Adjust
control fully clockwise.

2. Turn the Tunable Low-pass Filter module’s Gain control fully clockwise.
3. Modify the setup as shown in the Figure 6
Figure 6
The Tunable Low-pass Filter module is used to pick out the lower frequency of the FSK signal’s
“two” sine waves and the DIODE and RC LPF on the Utilities module form the envelope detector
to complete the FSK signal’s demodulation. The set up shown in figure 6 is represented by the
block diagram shown in figure 7
Figure 7
35

4. Slowly reduce the Tunable Low-pass Filter module’s cut-off frequency until the FSK
signal’s higher frequency component has been removed (or at least minimized) while
keeping as much of the lower frequency component as possible.
5. Compare the digital data signal and the filter’s output.
6. Change the Scopes Channel 2 connection as shown in the figure 8
Figure 8
7. Compare the original digital data signal with the recovered digital data signal
Result
The FSK signal was demodulated through envelope detection technique using the
Communication Trainer Kit.
36

Experiment No.9
NOISE GENERATION USING PN SEQUENCE
Aim: To generate Band limited noise using PN Sequence through the Communication Trainer Kit
Theory
The theoretical proposition of “white noise” is impossible in practice. To explain, an infinite
number of sine waves (with or without equal power density) would require an infinite
amount of power. Long PN sequences are rich in harmonics. Moreover, although the
spectrums of PN sequences have lobes of changing amplitude, small portions of its spectrum
are relatively flat. That being the case, it’s possible to isolate a small portion of a PN
sequence’s spectrum using a filter to model band-limited White noise
Procedure
2. Turn the VCO module’s Frequency Adjust control fully clockwise.
3. Set the VCO module’s Range control to the HI position.
4. Connect the set-up as shown in the figure1.
Figure 1
5. Set up the scope per the instructions in the Appendix.
Note: Ensure that the scope’s Trigger Source control is set to the CH1 (or INT)
position.
6. Adjust the scope’s Time base to view two just one cycle of the VCO module’s output.
7. Adjust the VCO module’s Frequency Adjust control to obtain a 150 kHz sine wave.
8. Locate the Tunable Low-pass Filter module and set its Gain control to about the middle
of its travel.
9. Turn the Tunable Low-pass Filter module’s Cut-off Frequency Adjust control fully
clockwise.
Note: This sets the Tunable Low-pass Filter module’s cut-off frequency to 15 kHz.
37

10. Connect the set-up as shown in the figure 2.
Figure 2
The above set-up can now be represented by the block diagram shown in figure 3. The
spectral composition of the Sequence Generator module’s Y output includes 255 sine waves
per lobe. A quick calculation shows that, with a 150 kHz bit-clock, the sine waves are
separated by 588Hz (150 kHz /255 = 588.2 Hz).
Figure 3
11. Set the scope’s Time base control to the 1ms/div position.
12. Observe the signal on the Tunable Low-pass Filter module’s output.
Note: You may need to adjust the scope’s Trigger Level control to obtain a stable
display.
The signal on the Tunable Low-pass Filter module’s output isn’t “White” noise
because it is bandwidth limited. Nor is the signal truly “noise”. This can be demonstrated
using the scope. True noise is non-repetitive. However, the signal on the Tunable Low-pass
Filter modules output repeats itself every 1.7ms. [This figure is calculated using the bit-
clock’s period (1÷150,000 Hz) and multiplying it by the PN sequence’s number of
38

39
bits(255).]. The repetitive nature of the “noise” you have modeled can be observed using
the scope.
13. Look closely at the signal – You should see it repeat itself about 5 times across the
display.
14. Measure the “noise” signal’s period.
Result
The band limited noise was generated from the PN sequence using the Communication
Trainer Kit.

40
Experiment No.10
TIME DIVISION MULTIPLEXING
Aim: To model a TDM system using the Communication Trainer Kit
Theory
Time division multiplexing is method of sharing the channel that is particularly suited to
digital communications. TDM involves giving multiple users exclusive access to the entire channel
(or to a carrier frequency if TDM is used in conjunction with FDM) Importantly , provided the
duration of access is extremely short (much less than a second) and the rate of the access is fast,
the users appear to have simultaneous and continuous access to the channel(or carrier).
TDM (or TDMA for time division multiple access) has been used extensively in
telecommunications with PCM digital data. PCM samples analog signals converting them to
proportional binary numbers. The binary numbers are then transmitted serially in frames usually
containing an additional bit (or bits) for frame synchronization by the PCM decoder. The PCM
encoder’s clock and the frame-size determine the sample rate.
Suppose the PCM decoder only reads and decodes the contents of every alternate frame in
the serial data. The effect of this is the same as halving the sample rate. So, the maximum message
frequency would also be halved. The benefit of reading only every second frame however is, the
unread frames are free to be filled with the PCM data for a second message having a maximum
frequency equal to the first. Of course, for TDM to work with PCM signals, the PCM decoder
must be designed to read the alternate frames as separate sets of data.
Equipment
• Two Emona Telecoms-Trainer 101 oscilloscope leads
• One set of headphones(stereo)
Procedure
1. Set up a single channel PCM communication System
2. Activate the PCM Encoder and Decoder Modules.
3. Transmit two signals to model a TDM system
• Setting up the PCM encoding – decoding scheme
2. Turn the VCO module’s Frequency Adjust control fully anti-clockwise.
3. Set the VCO module’s Range control to the LO position.

4. Connect the set-up shown in figure 1.
Figure 1
5. Set the scope’s Time base control to the 50µs/div position.
6. Adjust the VCO module’s Frequency Adjust control to obtain a 3kHz sine wave.
7. Locate the Tunable Low-pass filter module and set its Gain to about the middle of
its travel.
8. Adjust the Tunable Low-pass Filter module’s Cut-off Frequency control for the
highest cut-off frequency.
9. Set the PCM Encoder module’s Mode switch to the PCM position.
10. Connect the set-up shown in figure 2.
41

Figure 2
This set-up can be represented by the block diagram shown in Figure 3. The VCO module
is used to produce a 3kHz sinewave message signal for the PCM Encoder module. The PCM
Encoder module converts the message signal to a sampled version of the original signal. The
tunable Low-pass Filter module is a reconstruction filter (also known as an anti-alias filter).to
recover the message on the decoder’s output.
Figure 3
42

11. Set the scope’s Mode control to the DUAL position to view the signal on the
reconstruction filters output (that is, the Tunable Low-pass Filter module’s
output) as well as the message.
Time Division Multiplexing
12. Set the PCM Encoder module’s Mode switch to the TDM position.
13. Return the reconstruction filters cut-off frequency to maximum.
14. Set the VCO modules output frequency to 2kHz.
15. Slowly reduce the reconstruction filters cut-off frequency until you obtain an
undistorted copy of the message.
16. Set the VCO module’s output frequency to 1kHz.
17. Modify the setup as shown in figure 4
Figure 4
The above set up can be represented by the block diagram shown in Figure 5
43

Figure 5
18. Compare the original message-2 with the reconstructed version of message-2.
They should be the same.
Result
The Time Division Multiplexing System was studied from the Communication Trainer Kit
44

Comm lab manual_final

More Related Content

What's hot (19)

Similar to Comm lab manual_final (20)

Recently uploaded (20)

Comm lab manual_final