4th edition: chapter 1 - alexu.edu.eg

Lecture 4

Amplitude Modulation

Agenda

Vestigial Sideband (VSB) Modulation

Local Carrier Synchronization for Suppressed Carrier

Signals Transmission

Applications of AM

1-2 Amplitude Modulation

Introduction

It is rather difficult to generate exact SSB signals

Such signals require that the message signal m(t) have a null around DC

The generation of DSB signals is much simpler, however, it requires twice the signal bandwidth

Vestigial Sideband (VSB) modulation (also called asymmetric sideband system) is a compromise between DSB and SSB

Amplitude Modulation 1-3

Introduction VSB inherits the advantages of DSB and SSB

but avoids their disadvantages at a small cost

VSB signals are easy to generate and their bandwidth is only a little greater than that of SSB signals (typically higher with 25% to 33%)

In VSB, instead of rejecting one sideband completely (as in SSB), a gradual cutoff of one sideband is accepted via A nonideal bandpass filter

The baseband signal can be recovered exactly by a synchronous detector in conjunction with an appropriate equalizer filter at the receiver


Introduction

If a large carrier is transmitted along with the VSB signal, the baseband signal can be recovered by an envelope or a rectifier detector

VSB signals are generated using standard AM or DSB-SC modulation, then passing modulated signal through a sideband shaping filter

Demodulation uses either standard AM or DSB-SC demodulation, depending on whether a carrier tone is transmitted

VSB modulation with envelope detection is used to modulate image in analog TV signals. (The audio signal is modulated using FM)


Introduction VSB Example: US TV NTSC

Using VSB instead of DSB saves about 3 MHz and allows a carrier tone.

The shaping filter satisfies Hi(f − fc) + Hi(f + fc) = c, so Ho(f) = 1/c.


Vestigial Sideband (VSB)


VSB: Modulator


If the vestigial shaping filter that produces the VSB from DSB is Hi(f)

Then the resulting VSB signal spectrum is

Vestigial Sideband (VSB)


The VSB shaping filter Hi(f) allows the transmission of one sideband but suppresses the other sideband, not completely, but gradually

For detecting m(t), VSB uses synchronous demodulation at the receiver

The resulting signal is further passed through the low-pass equalizer filter of the transfer function Ho(f)

VSB: Demodulator


The output of the low-pass equalizer filter Ho(f) is required to be m(t)

Using the equation

VSB: Demodulator

By eliminating the spectra at ±4fc we obtain

Hence

Note that because Hi(f) is a bandpass filter, the terms Hi(f ±fc) contains low-pass components


VSB: Demodulation Filter

Ho(f) is not large for small values of f


Complementary VSB Filter and Envelope Detection of VSB+C Signals

Special case of VSB modulator, Hi(f) can be

The output filter is just a simple low-pass filter with transfer function Ho(f)

The resulting VSB signal plus carrier (VSB+C) can be envelope-detected


Complementary VSB Filter and Envelope Detection of VSB+C Signals

As known, SSB+C requires a much larger

carrier than DSB-C (AM) for envelope

detection

Because VSB+C is an in-between case, the

added carrier required in VSB is larger

than that in AM, but smaller than that in

SSB+C


Local Carrier Synchronization

In a suppressed carrier, amplitude-modulated system (DSB-SC, SSB-SC, and VSB-SC), the coherent receiver must generate a local carrier, synchronous with the incoming carrier (frequency and phase)

Any discrepancy in the frequency or phase of the local carrier gives rise to distortion in the detector output



Consider an SSB-SC case where a received signal is

because of propagation delay and Doppler frequency shift

The local carrier remains 2Cos 2πfct

The product of the received signal and the local carrier

1-16


The bandpass component is filtered out by the receiver low-pass filter, leaving the output

In practice, if the radio wave travels a distance of d meters at the speed of light c, then the phase delay

Which can be any value within the interval



There are two ways to recover the incoming carrier at the receiver The transmitter transmits a pilot (sinusoid)

signal that can be either the exact carrier or directly related to the carrier (e.g., a pilot at half the carrier frequency)

No pilot is transmitted. Using nonlinear device to process the received signal to generate a separate carrier component that can be extracted using narrow bandpass filters


Applications of AM

Superheterodyne AM Receiver

Stereophonic Systems

FDM and Telephony Systems

Analog TV Systems


Superheterodyne AM Receiver Frequency mixers down-convert the incoming signal to

an intermediate frequency (IF) for further processing

The Local Oscillator (LO) is tuned to select the different channels: – The output of the mixer shifts the signal to fRF + fLO and

fRF – fLO

– Images (i.e, undesired input frequencies) are to be avoided by carefully selecting fIF


Stereophonic System What is stereo?

Stereo Transmitter (compatible with monophonic systems)


FM

modulation

Frequency Division Multiplexing

Signal multiplexing allows the transmission of several signals on the same channel

In Time Division Multiplexing (TDM), several signals time-share the same channel

In Frequency Division Multiplexing (FDM), several signals share the band of a channel

In FDM, each signal is modulated by a different carrier frequency (subcarriers) avoiding overlap


Frequency Division Multiplexing

Each signal may use a different kind of modulation (e.g., DSB-SC, AM, SSB-SC, VSB-SC, or frequency or phase modulation)

There is a composite signal (considered as a baseband signal) with adding all modulated spectra that further modulation with a radio-frequency (RF) carrier is applied for the purpose of transmission

At the receiver, the incoming signal is first demodulated by the RF carrier to retrieve the composite baseband signal

Then, using bandpass filters to separate all the modulated signals Amplitude Modulation 1-23

FDM in Telephony Systems

FDM Hierarchy in Telephony Systems voice channel group super group master group

jumbo group


Analog TV System VSB-TC is used with video (monochromatic)

and FM with audio

1-25 NTSC Standard

0.5 MHz

Typical TV Reciver

The tuner is a superheterodyne receiver

Audio and video signals are separated

Video signal is separated from the synchronization signals

Sync signals are used to retrace the signal on the CRT


Lecture Summary

Covered material Vestigial Sideband (VSB) Modulation Local Carrier Synchronization for Suppressed

Carrier Signals Transmission Applications of AM

Material to be covered next lecture Noise Analysis in AM

1-27 Amplitude Modulation

4th edition: chapter 1 - alexu.edu.eg

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