unit1.ppt

22/04/2023 Archana tiwari session 2015-2016 1

AMPLITUDE MODULATION

Unit -1Semester :5th

Department:electronics and Instrumentation

Reference books: Communication systems- Taub Schiling Communication systems –Sanjay sharma


INTRODUCTION

• Amplitude Modulation is the simplest and earliest form of transmitters

• AM applications include broadcasting in medium- and high-frequency applications, CB radio, and aircraft communications


Amplitude Modulation

Amplitude Modulation is a process where the amplitude of a carrier signal is altered according to information in a message signal.

The frequency of the carrier signal is usually much greater than the highest frequency of the input message signal.


AM – Basic Definitions

The AM signal

The modulating signal:

The Carrier Signal:

ttmkAts cc cos 1

tm

tAtc cc cos

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-5

0

5

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-1

0

1

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-1

0

1


AM – Basic Definitions(cont.)The information signal

varies the instantaneous amplitude of the carrier


AM – Basic Definitions(cont.)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-5

-4

-3

-2

-1

0

1

2

3

4

5

tmkAts c 1

The Envelope:

The AM Signal

ttmkAts cc cos 1


AMPLITUDE SENSITIVITY

Modulation Index - The ratio between the amplitudes between the amplitudes of the modulating signal and carrier, expressed by the equation:

c

m

EEm =


AM – Percentage Modulation

Under modulated (<100%) 100% modulated

Envelope Detector

Can be used

Envelope Detector

Gives Distorted signal

Over Modulated (>100%)


NEED OF THE MODULATION PROCESS

To reduce the height of the antennaTo avoid mixing signalsTo increase the range of communicationTo improve quality of receptionLastly to increase over all strength


Bandwidth Signal bandwidth is an important

characteristic of any modulation scheme In general, a narrow bandwidth is desirableBandwidth is calculated by:

mFB 2


Power Relationships

Power in a transmitter is important, but the most important power measurement is that of the portion that transmits the information

AM carriers remain unchanged with modulation and therefore are wasteful

Power in an AM transmitter is calculated according to the formula at the right

21

2mPP ct

12

Modulation Index of AM Signal

13

Double-Sideband Suppressed-Carrier AM

• A double-sideband, suppressed-carrier (DSB-SC) AM signal is obtained by multiplying the message signal m(t) with the carrier signal c(t) =

Accos(2fct)

• Amplitude-modulated signal

• An example of the message signal m(t), the carrier c(t), and the modulated signal u (t) are shown in Figure 3.1

• This figure shows that a relatively slowly varying message signal m(t) is changed into a rapidly varying modulated signal u(t), and due to its rapid changes with time, it contains higher frequency components

• At the same time, the modulated signal retains the main characteristics of the message signal; therefore, it can be used to retrieve the message signal at the receiver

)2cos()()()()( tftmAtctmtu cc

14

Double-Sideband Suppressed-Carrier AM

• Figure 3.1 An example of message, carrier, and DSB-SC modulated signals

Single-Sideband AM• The two sidebands of an AM signal are mirror images of one

another• As a result, one of the sidebands is redundant• Using single-sideband suppressed-carrier transmission results

in reduced bandwidth and therefore twice as many signals may be transmitted in the same spectrum allotment

• Typically, a 3dB improvement in signal-to-noise ratio is achieved as a result of SSBSC

16

Single-Sideband AM• A method, illustrated in Figure

3.16, generates a DSB-SC AM

signal and then employs a filter

that selects either the upper

sideband or the lower sideband of

the double-sideband AM signal

.,

.

Figure 3.16 Generation of a single-sideband AM signal by filtering one of the sidebands of a DSB-SC AM signal.

Sideband and carrier power• Carrier term does not carry information, and

hence the carrier power is wasted

• The carrier power is the mean sq. value of which is • The sideband power is the mean sq. value

• of which is

sidebandscarrierttmtAt ccAM cos)(cos)(cP

tA ccos 2/2AsP

ttm ccos)( 2/)(2 tm

Power Efficiency

• The power efficiency

• For the special case of tone modulation

• Hence

tAtm m cos)(

%100)(

)(22

2

tmA

tmPP

P

sc

s

%1002

%1002/

2/2

2

22

2

AA

APP

P

sc

s

2/)( 22 Atm

%33,1 max

19

Single-Sideband AM• A DSB-SC AM signal transmits two sidebands and required

a channel bandwidth of Bc = 2W Hz– However, the two sidebands are redundant

• The transmission of either sideband is sufficient to reconstruct the message signal m(t) at the receiver

• Thus, we reduce the bandwidth of the transmitted signal from 2W to W

• In the appendix 3A, a single-sideband (SSB) AM signal is represented mathematically as

– where is the Hilbert transform of m(t)– The plus sign indicates the lower sideband and the minus sign

indicates the upper sideband)(ˆ tm

)2sin()(ˆ)2cos()()( tftmAtftmAtu cccc

20

Single-Sideband AM• The SSB-AM signal u(t) may be

generated by using the system

configuration as shown in right. (Generation of a lower SSB-AM)

• Another method (“filter method”)

generates a DSB AM signal and

then employs a filter that selects

either the upper sideband or the

lower sideband of the DSB AM.

21

Demodulation of SSB-AM Signals• To recover the message signal m(t) in the received SSB-AM signal, we

require a phase-coherent or synchronous demodulator• For the USSB signal

• By passing the product signal in above equation through an lowpass filter, the double-frequency components are eliminated. Then

– Note that the phase offset not only reduces the amplitude of the desired signal m(t) by cos, but it also results in an undesirable sideband signal due to the presence of in yl(t)

– The latter term was not present in the demodulation of a DSBSC signal– It contributes to the distortion of the demodulated SSB signal

)(ˆ tm

terms.freq. double )sin()(ˆ)cos()()2cos()()2cos()(

21

21

tmAtmAtftutftr

cc

cc

)sin()(ˆ)cos()()( 21

21 tmAtmAty ccl

22

Demodulation of SSB-AM Signals• The transmission of a pilot tone at the carrier frequency is a

very effective for providing a phase-coherent reference signal • However, a portion of the transmitted power must be

allocated to the transmission of the carrier• The spectral efficiency of SSB AM is very attractive in voice

communications over telephone channels• “Filter method”, which selects one of the two signal sidebands

for transmission, is difficult to implement when the message signal m(t) has a large power concentrated around f = 0– In such a case, the sideband filter must have an extremely sharp

cutoff around the carrier in order to reject the sideband– Such filter characteristics are very difficult to implement in

practice

23

Vestigial-Sideband AM• The stringent-frequency response requirements on the

sideband filter in an SSB-AM system can be relaxed by allowing vestige, which is a portion of the unwanted sideband, to appear at the output of the modulator

• Thus, we simplify the design of the sideband filter at the cost of a small increase in the channel bandwidth required to transmit the signal

• The resulting signal is called vestigial-sideband (VSB) AM– This type of modulation is appropriate for signals that have a strong

low-frequency component, such as video signals– That is why this type of modulation is used in standard TV

broadcasting

24

Vestigial-Sideband AM• To generate a VSB-AM signal, we generate a DSB-SC AM

signal and pass it through a sideband filter with the frequency response H( f ), as shown in below

• In the time domain, the VSB signal may be expressed as

– where h(t) is the impulse response of the VSB filter

• In the frequency domain, the corresponding expression is (eq. 1)

)(]2cos)([)( thtftmA tu cc

)()()(2

)( fHffMffMAfU cncnc

Generation of vestigial-sideband AM signal.

25

Vestigial-Sideband AM• To determine the frequency-response characteristics of the

filter, we will consider the demodulation of the VSB signal u(t).• We multiply u(t) by the carrier component cos2fct and pass

the result through an ideal lowpass filter, as shown in below.– Thus, the product signal is or

)(t

tftutv c2cos)()( )()(21)( cc ffUffUfV

Demodulation of VSB signal.

26

Vestigial-Sideband AM• If we substitute U( f ) from eq. (1) into V(f) , we obtain

• The lowpass filter rejects the double-frequency terms and passes only the components in the frequency range | f|W

• Hence, the signal spectrum at the output of the ideal lowpass filter is

• The message signal at the output of the lowpass filter must be undistorted– Hence, the VSB-filter characteristic must satisfy the condition

)()2()(4

)()()2(4

)( ccc

ccc ffHffMfMAffHfMffMAfV

)()()(4

)( ccc

l ffHffHfMAfV

WfffHffH cc || constant )()(

27

Vestigial-Sideband AM

• We note that H(f) selects the upper sideband and a vestige of the lower sideband

• It has odd symmetry about the carrier frequency fc in the frequency range fc - fa < f < fc + fa, where fa is a conveniently selected frequency that is some small fraction of W, i.e., fa << W

• Thus, we obtain an undistorted version of the transmitted signal

VSB-filter characteristics.

28

Vestigial-Sideband AM• The frequency response of a VSB filter that selects the lower

sideband and a vestige of the upper sideband is shown in below

• In practice, the VSB filter is designed to have some specified phase characteristic

• To avoid distortion of the message signal, VSB filter should have a linear phase over its passband fc - fa | f | fc + W

Frequency response of the VSB filter for selecting the lower sideband of the message signals.

Advantages/disadvantagesAdvantages of Amplitude Modulation, AMThere are several advantages of amplitude modulation, and some of these reasons have

meant that it is still in widespread use today:• It is simple to implement• it can be demodulated using a circuit consisting of very few components• AM receivers are very cheap as no specialized components are needed.

Disadvantages of amplitude modulationAmplitude modulation is a very basic form of modulation, and although its simplicity is one of

its major advantages, other more sophisticated systems provide a number of advantages. Accordingly it is worth looking at some of the disadvantages of amplitude modulation.

• It is not efficient in terms of its power usage• It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice

that of the highest audio frequency• It is prone to high levels of noise because most noise is amplitude based and obviously

AM detectors are sensitive to it.

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