Analog Modulation

Topics: Amplitude modulation, Single-sideband modulation, Modulation Pages: 9 (1682 words) Published: July 28, 2013
Amplitude Modulation and Angle Modulation

Abstract— The aim of this document is to improve understanding of amplitude modulation and angle modulation. We have done this by using Lab view. We have shown 4 types of Amplitude Modulation & 2 types of Angle Modulation. Keywords-amplitude modulation (AM), angle modulation, double side band modulation (DSB), double sideband suppressed carrier modulation (DSB-SC), double sideband full carrier modulation (DSB-FC), single side band modulation(SSB), phase modulation (PM), frequency modulation (FM), m(t)-message signal, A(c)-amplitude of carrier, Am- amplitude of message signal, f(m) or f(c)-frequency, Y(t)-modulated signal, Xc(t)-carrier signal, Wc [Fc=Wc/2pi] frequency of carrier signal, Wm[Fm=Wm/2pi] frequency of message signal, Ka-amplitude sensitivity factor, USB-upper side band, LSB-lower side band, BW-band width, VSB- vestigial sideband modulation INTRODUCTION

The process of converting information so it can easily be sent through medium is known as Modulation. Basically required for communication purpose (long range medium). Modulation is usually requires a shift of the range of frequencies contained in the information signal of another frequency for suitable range transmission. Modulation can be of different types, we put emphasized on Amplitude Modulation and Angle Modulation.

1. Amplitude modulation

In AM, information signal or message signal varies the amplitude of carrier signal which is of constant amplitude and high frequency .The product of these two signals will be the modulated signal. 1) REPRESTATION:

The carrier signal is represented by
Xc(t)=A(t)sinWct ; Wc=2piFc
* All terms are defined above.
Message signal is represented by m(t).
m (t)= m(t) A(t)sinWct
Modulated signal Y(t) is given as:

2) MODULATION INDEX:
Modulation index also known as modulating factor is ratio of amplitude of message signal to carrier. It should be a number between 0-1 thus avoid distortion to occur in AM. m= Am/Ac;
* If m >1 – the signal is over modulation
* If m<1 – under modulation
* If m= 1- 100% modulation
3) LAB VIEW SIMULATION:
With 0 phase angle:

LABVIEW block diagram of AM(figure:1.b)

Xc(t) [carrier signal]:

Figure: 1.c
m(t) [message signal]:

Figure: 1.d
Y(t) [modulated signal]:

Figure: 1.e
SPECTRUM

Figure: 1.f

Figure:1.g
Fig:1.g shows that peak value of Xc(t) become zero reference for m(t) so, the value of m(t) amplitude is added and subtracted from the peak value of Xc(t) gives us modulated signal. 4) DIFFERENT CASES:

a) WHEN β=“0”(UNDER MODLATION) :

b) WHEN β=“1” (100% MODULATION):

c) WHEN β >1 (OVER MODULATION):

d) Xc(t) WITH 90° PHASE SHIFT:

e) Xc(t) WITH 180° PHASE SHIFT

f) M(t) WITH 90° PHASE SHIFT:

g) M(t) WITH 180° PHASE SHIFT:

h) EQUAL AMPLITUDE OF Xc(t) & m(t):

i) Ac < Am:

j) Fc = fm:

NOTE:
* Ac > Am, otherwise it cause distortion.
* Carrier signal must be of constant amplitude.
* Wc > Wm is necessary condition for the sidebands not to overlap, if not satisfied Y(t) undergoes frequency distortion due to spectral overlap. * Carrier signals can be sine, cosine or pulse train.

* Simple modulation also known as full amplitude modulation and double side band full carrier modulation.

Spectral overlap

Figure 1.h
Following are types of Amplitude modulation:
2A) Double sideband modulation (DSB):
The modulation in which the upper side band is equals to(fc+fm) and lower side band is equals to(fc-fm) are merged together for the result is called DSB modulation. M(t) & Xc(t) is same as shown in figure 1.d &1.c respectively then y(t) is:

Figure 2.a

Spectrum is shown in the figure 2.b. The part of the spectrum lies above Wc is called upper sideband (usb= fc+fm) and the part of the spectrum lies below the Wc is called lower sideband (lsb=fc-fm). The...


References: * www.mathworks.com.
* www.cnx.org
* Books: simon haykin
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