AM Modulation Demodulation Experiment

AM modulation demodulation experiment 1
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AM modulation Demodulation
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AM modulation demodulation experiment 2
INTRODUCTION OF AM MODULATION-DEMODULATION
Amplitude modulation of a carrier signal entails amplitude variation of the carrier, high
frequency, signal that is in sync with the baseband modulating, low frequency, signal
amplitude (MIT 6.02
DRAFT Lecture Notes, 2012). The low frequency modulating signal
is known as baseband signal or input signal such as speech while the high frequency signal is
referred to as carrier signal. In other words, baseband is translated into a higher frequency
component using carrier signal. The carrier signal embedded with baseband propagates
contained message or information over a longer distance as opposed to when baseband is
used alone (ElProCus ,2019).
It is the simplest method f modulating carrier signal. Amplitude modulation has been mainly
used in AM radio for sending information in audio form to listeners’ radios because humans’
ears can decode sound of frequencies between 20Hz to 16 kHz. Simplicity of AM modulation
technique is the main leverage as compared to other types of modulation (Deutsch Circuit
Design, 2019).
Amplitude modulation only requires the amplitude of carrier signal to be modulated by the
baseband and more importantly, the receiver could be made of a simple envelop based on a
basic diode circuit. As a result, the costs are reduced significantly since it need not complex
demodulator (Electronic Notes, 2019).
However, comparing with other types of modulation, amplitude modulation is the most
vulnerable mode of signal transmission as it is easily affected by noise (Deutsch Circuit
Design, 2019).
Most signal noise in modulation is amplitude based, hence AM demodulator are unable to
sensitively filter to such noise. In addition to the aforementioned drawback, AM modulation
does not efficiently utilize the available bandwidth since the required bandwidth is almost
two times that of the highest frequency of audio baseband signal. Also, AM modulation is not
as efficient as other modulating modes with regard to usage of power (Electronic Notes,
2019).
THEORETICAL BACKGROUND OF AM MODULATION-DEMODULATION
Derivation of AM modulation
At the initial stage, the system is fed with two signals, baseband message signal and carrier,
high frequency signal.
Baseband or modulating signal expression is given by;
V m =V m cos ( ωm t )=V m cos ( 2 π f m t )
Wheref m−frequency of themodulation signal ( Low frequency signal).
V m −isthe modulating signal ' s am plitude (Peak voltage).

AM modulation demodulation experiment 3
The carrier signal, high frequency signal, is expressed as shown below.
V c=V c cos ( ωc t )=V c cos ( 2 π f c t )
Where V c is the amplitude of the carrier signal ( Peak voltage)
f c is the frequency of the carrier signal( high frequency signal).
The general modulated waveform is obtained by the product result of the carrier and the
baseband (modulating) signals as shown below.
V AM (t )= [ A+V m od(t ) ] V carrier (t )
Where V AM (t )is the modulated signal .
A is a constant representing amplitude of thewaveform .
Forms of AM modulated waveforms
Types of AM used determines the form of the modulated signal waveform. There are four
types of AM modulating schemes namely (Bharathuniv, 2019).
a) Double-Side-Band With carrier(DSBWC)
b) Double-Side-Band-Suppressed-Carrier (DSBSC)
c) Single-Side-Band (SSB).
d) Vestigial Sideband (VSB)
Double-side-Band-with Carrier (DSBWC)
DSBWC is a form of AM transmission where by AM modulated waveform consists of the
carrier alongside with two opposite sidebands (Tutorials point, 2019).
Sideband is referrers to as a band of frequencies of the carrier and modulating signal and
contains power.
This type of modulation has three cosine components namely; the frequency of the carrier f c ,
frequencies of the lower sideband (f c−f m ), and the frequency of the upper sideband (f c+ f m).
The modulating signal is superimposed on the sidebands or envelops of the modulating
signal. Modulated signal equation are modelled as shown below.
V AM (t )= [ A+V m(t ) ] V c (t )
V (t )= [ A +mV m cos ( ωm t ) ] cos ( ωc t )
Where mis known as the modulating index.
0 ≤ m≤ 1
Simplifying and rearranging the equation of the modulating signal.

AM modulation demodulation experiment 4
V DSBWC=V C cos ( 2 π f c t )+ 0.5 V m cos {2 πt ( f c+ f m ) }+ 0.5V m cos {2 πt ( f c−f m ) }
These frequencies when plotted, resultant spectrum characteristics as shown below.
Fig 1: Spectrum of DSBWC AM signal
The above equation has;
The carrier +USB + LSB
Double-side-Band-Suppressed Carrier (DSBSC)
The AM modulated signal contains only upper and lower sidebands. The carrier signal, high
frequency component, is been eliminated from the modulated signal (ElProCus, 2019)
DSBSC signal is given by the equation below.
V DSBSC=0.5V m cos {2 πt ( f c +f m ) }+0.5 V m cos {2 πt ( f c−f m ) }
The spectrum of DSBSC signal is as shown below.