2
1. Amplitude Modulation & Demodulation
Aim: 1. To generate amplitude modulated wave and determine the percentage modulation.
2. To Demodulate the modulated wave using envelope detector.
Apparatus Required:
Name of the Specifications/Range Quantity
Component/Equipment
fT = 300 MHz
Transistor(BC 107) 1
Pd = 1W
Ic(max) = 100 mA
Diode(0A79) Max Current 35mA 1
Resistors 1K , 2K , 6.8K , 10K 1 each
Capacitor 0.01 F 1
Inductor 130mH 1
CRO 20MHz 1
Function Generator 1MHz 2
Regulated Power Supply 0-30V, 1A 1
Theory:
Amplitude Modulation is defined as a process in which the amplitude of the carrier wave
c(t) is varied linearly with the instantaneous amplitude of the message signal m(t).The standard
form of an amplitude modulated (AM) wave is defined by
s(t ) = Ac [1 + K a m(t )cos(2pf c t )]
Where,
Ka is a constant called the amplitude sensitivity of the modulator.
s(t) is Modulated Signal
Ac s Carrier amplitude
M(t) is Message signal
The amplitude of Ka m(t) is always less than unity
i.e. |Kam(t)| <1 for all t
It ensures that the function 1+Kam(t) is always positive. When the amplitude
sensitivity Ka of the modulator is large enough to make |Kam(t)| >1 for any t, the carrier
wave becomes over modulated, resulting in carrier phase reversals. Whenever the factor
1+Kam(t) crosses zero.
The demodulation circuit is used to recover the message signal from the incoming
AM wave at the receiver. An envelope detector is a simple and yet highly effective device that is
well suited for the demodulation of AM wave, for which the percentage modulation is less than
100%.Ideally, an envelop detector produces an output signal that follows the envelop of the input
signal wave form exactly; hence, the name. Some version of this circuit is used in almost all
commercial AM radio receivers.
The Modulation Index is defined as,
(Emax - Emin )
m=
(E +E )
max min
where Emax and Emin are the maximum and minimum amplitudes of the modulated wave.
Circuit Diagrams:
For modulation:
Fig.1. AM modulator
For demodulation:
Fig.2. AM demodulator
Procedure:
1. The circuit is connected as per the circuit diagram shown in Fig.1.
2. Switch on + 12 volts VCC supply.
3. Apply sinusoidal signal of 1 KHz frequency and amplitude 2 Vp-p as modulating signal, and
carrier signal of frequency 11 KHz and amplitude 15 Vp-p.
4. Now slowly increase the amplitude of the modulating signal up to 7V and note down values
of Emax and Emin.
5. Calculate modulation index using equation
6. Repeat step 5 by varying frequency of the modulating signal.
7. Plot the graphs: Modulation index vs Amplitude & Frequency
8. Connect the circuit diagram as shown in Fig.2.
9. Feed the AM wave to the demodulator circuit and observe the output
10. Note down frequency and amplitude of the demodulated output waveform.
11. Draw the demodulated wave form .m=1
Observations
Table 1: fm= 1KHz, fc=11KHz, Ac=15 V p-p.
S.No. Vm(Volts) Emax(volts) Emin (Volts) m %m (m x100)
Table 2: Am= 4 Vp-p fc =11KHz, Ac=15 V p-p.
S.No. fm(KHz) Emax(volts) Emin(Volts) m %m (m x100)
5
Waveforms and graphs:
Figure 1: Carrier, Modulating and Modulated wave
MATLAB Program and description:
Program:
A=input('enter the carrier signal peak')
B=input('enter the baseband signal peak')
f1=input('enter the baseband signal frequency')
f2=input('enter the carrier signal frequency')
fs=input('enter the sampling frequency')
t=0:0.001:1;
M=cos(2*pi*f1*t);
N=cos(2*pi*f2*t);
m=B/A;
O=A*(1+m*M).*N;
O1=O;
for i=1:length(t)
if O1(i)<=0
O1(i)=0;
end
end
[den, num]=butter(2,2*pi*f1/fs);
M1=filter(den,num,O1);
M11=filter(den,num,M1);
M12=filter(den,num,M11);
M13=filter(den,num,M12);
subplot(5,1,1)
plot(t,M)
title('Baseband signal')
subplot(5,1,2)
plot(t,N)
title('Carrier signal')
subplot(5,1,3)
plot(t,O);
title('Modulated Carrier')
subplot(5,1,4)
plot(t,O1)
title('Rectified Modulated Signal')
subplot(5,1,5)
plot(t,M13)
title('Demodulated Signal')
enter the carrier signal peak A =5
enter the baseband signal peak B =2
enter the baseband signal frequency f1 =10
enter the carrier signal frequency f2 =100
enter the sampling frequency fs =1000
Figure 2 Matlab Output
Precautions:
1. Check the connections before giving the power supply
2. Observations should be done careful
2. Frequency Modulation And Demodulation
Aim: 1. To generate frequency modulated signal and determine the modulation index and
bandwidth for various values of amplitude and frequency of modulating signal.
3. To demodulate a Frequency Modulated signal using FM detector.
Apparatus required:
Name of the
Specifications/Range Quantity
Component/Equipment
Operating voltage –Max-24 Volts
IC 566 1
Operating current-Max.12.5 mA
Power dissipation – 750mW 1
IC 8038
Supply voltage - ±18V or 36V total
Power dissipation -1400mw 1
IC 565
Supply voltage - ±12V
15 K ,10K ,1.8K , 1,2,1
Resistors
39K ,560 2,2
470 pF, 0.1 F 2,1
Capacitors
100pF , 0.001 F 1,1 each
CRO 100MHz 1
Function Generator 1MHz 2
Regulated Power Supply 0-30 v, 1A 1
Theory: The process, in which the frequency of the carrier is varied in accordance with the
instantaneous amplitude of the modulating signal, is called “Frequency Modulation”. The FM
signal is expressed as
s(t ) = Ac cos(2 fc + sin(2 fm t ))
Where AC is amplitude of the carrier signal,
fC is the carrier frequency
b is the modulation index of the FM wave
Circuit Diagrams:
Fig.1. FM Modulator Using IC 566
Fig.2. FM Demodulator Circuit
Procedure:
Modulation:
1. The circuit is connected as per the circuit diagram shown in Fig.1.
2. Without giving modulating signal observe the carrier signal at pin no.3. Measure amplitude and
frequency of the carrier signal. To obtain carrier signal of desired frequency, find value of R from f
= 1/ (2 Π R C ) taking C=100pF.
3. Apply the sinusoidal modulating signal of frequency 4KHz and amplitude 3Vp-p at pin no.5.
4. Now slowly increase the amplitude of modulating signal and measure f min and maximum frequency
deviation ∆f at each step.
5. Evaluate the modulating index (mf = β) using ∆f / fm where ∆f = |fc - fmin|. Calculate Band width.
BW = 2 (β + 1)fm = 2(∆f + fm)
6. Repeat step 4 by varying frequency of the modulating signal.
Demodulation:
1. Connections are made as per circuit diagram shown in Fig.3
2. Check the functioning of PLL (IC 565) by giving square wave to input and observing the
output
3. Frequency of input signal is varied till input and output are locked.
4. Now modulated signal is fed as input and observe the demodulated signal output on CRO
5. Draw the demodulated wave form
Table: 1 fc = 45KHz
S.No. fm(KHz) Tmax ( sec) fmin(KHz) f(KHz) β BW (KHz)
Table 2: fm = 4 KHz, fc = 45 KHz
T
S.No. Am (Volts) ( sec) fmin(KHz) ∆f (KHz) β BW(KHZ)
Waveforms:
MATLAB Program
Program:
clc;
clear all;
close all;
fs=10000;
ac=1;
am=1;
fm=35;
fc=500;
b=10;
t=(0:0.1*fs)/fs;
wc=2*pi*fc;
wm=2*pi*fm;
mt=am*cos(wm*t);
subplot(4,1,1);
plot(t,mt);
title('modulating signal');
ct=ac*cos(wc*t);
subplot(4,1,2);
plot(t,ct);
title('carrier signal');
st=ac*cos((wc*t)+b*sin(wm*t));
subplot(4,1,3);
plot(t,st);
title('modulated signal');
d=demod(st,fc,fs,'fm');
subplot(4,1,4);
plot(t,d);
title('demodulated signal');
MATLAB Plots
Precautions:
1. Check the connections before giving the power supply
2. observations should be done carefully
